A tA nuclear reactor is a device in which nuclear chain reactions are initiated, controlled, and sustained at a steady rate, as opposed to a nuclear bomb, in which the chain reaction occurs in a fraction of a second and is uncontrolled causing an explosion.

The most significant use of nuclear reactors is as an energy source for the generation of electrical power (see Nuclear power) and for the power in some ships (see Nuclear marine propulsion). This is usually accomplished by methods that involve using heat from the nuclear reaction to power steam turbines. There are also other less common uses as discuControlled nuclear fusion could in principle be used in fusion power plants to produce power without the complexities of handling actinides, but significant scientific and technical obstacles remain. Several fusion reactors have been built, but as yet none has 'produced' more thermal energy than electrical energy consumed. Despite research having started in the 1950s, no commercial fusion reactor is expected before 2050. The ITER project is currently leading the effort to commercialize fusion power.

ssed below.

Reactor

The reactor is used to convert nuclear (inaccurately also known as 'atomic') energy into heat. While a reactor could be one in which heat is produced by fusion or radioactive decay, this description focuses on the basic principles of the fission reactor.

ypical nuclear reactor has a few main parts. Inside the "core" where the nuclear reactions take place are the fuel rods and assemblies, the control rods, the moderator, and the coolant. Outside the core are the turbines, the heat exchanger, and part of the cooling system.

The fuel assemblies are collections of fuel rods. These rods are each about 3.5 meters (11.48 feet) long. They are each about a centimeter in diameter. These are grouped into large bundles of a couple hundred rods called fuel assemblies, which are then placed in the reactor core. Inside each fuel rod are hundreds of pellets of uranium fuel stacked end to end.

Also in the core are control rods. These rods have pellets inside that are made of very efficient neutron capturers. An example of such a material is cadmium. These control rods are connected to machines that can raise or lower them in the core. When they are fully lowered into the core, fission can not occur because they absorb free neutrons. However, when they are pulled out of the reactor, fission can start again anytime a stray neutron strikes a 235U atom, thus releasing more neutrons, and starting a chain reaction.

Another component of the reactor is the moderator. The moderator serves to slow down the high speed neutrons "flying" all around the reactor core. If a neutron is moving too fast, and thus is at a high-energy state, it passes right through the 235U nucleus. It must be slowed down to be captured by the nucleus and to induce fission. The most common moderator is water, but sometimes it can be another material.

The job of the coolant is to absorb the heat from the reaction. The most common coolant used in nuclear power plants today is water. In actuality, in many reactor designs the coolant and the moderator are one and the same. The coolant water is heated by the nuclear reactions going on

inside the core. However, this heated water does not boil because it is kept at an extremely intense pressure, thus raising its boiling point above the normal 100° Celsius.

The key components common to most types of nuclear power plants are:

Nuclear fuel Nuclear reactor core Neutron moderator Neutron poison Coolant (often the Neutron Moderator and the Coolant are the same, usually both

purified water) Control rods Reactor vessel Boiler feedwater pump Steam generators (not in BWRs) Steam turbine Electrical generator Condenser Cooling tower (not always required) Radwaste System (a section of the plant handling radioactive waste)

Nuclear fuel

From Wikipedia, the free encyclopedia

Jump to: navigation, search

Nuclear Fuel Process

A graph comparing nucleon number against binding energy

Close-up of a replica of the core of the research reactor at the Institut Laue-Langevin

Nuclear fuel is any material that can be consumed to derive nuclear energy, by analogy to chemical fuel that is burned to derive energy. By far the most common type of nuclear fuel is heavy fissile elements that can be made to undergo nuclear fission chain reactions in a nuclear fission reactor; nuclear fuel in a nuclear fuel cycle can refer to the material or to physical objects (for example fuel bundles composed of fuel rods) composed of the fuel material, perhaps mixed with structural, neutron moderating, or neutron reflecting materials. The most common fissile nuclear fuels are 235 U and 239 Pu , and the actions of mining, refining, purifying, using, and ultimately disposing of these elements together make up the nuclear fuel cycle, which is important for its relevance to nuclear power generation and nuclear weapons.

Not all nuclear fuels are used in fission chain reactions. For example, 238 Pu and some other elements are used to produce small amounts of nuclear power by radioactive decay in radiothermal generators, and other atomic batteries. Light isotopes such as 3H (tritium) are used as fuel for nuclear fusion. If one looks at binding energy of specific isotopes, there can be an energy gain from fusing most elements with a lower atomic number than iron, and fissioning isotopes with a higher atomic number than iron.

Contents

[hide] 1 Oxide fuel

o 1.1 UOX o 1.2 MOX

2 Metal fuel o 2.1 TRIGA fuel o 2.2 Actinide Fuel

3 Less common chemical forms o 3.1 Ceramic fuels

3.1.1 Uranium nitride 3.1.2 Uranium carbide

o 3.2 Liquid fuels 3.2.1 Molten anhydrous salts 3.2.2 Aqueous solutions of uranyl salts

4 Common physical forms of nuclear fuel o 4.1 PWR fuel o 4.2 BWR fuel o 4.3 CANDU fuel

5 Less common fuel forms o 5.1 TRISO fuel o 5.2 RBMK fuel o 5.3 CerMet fuel o 5.4 Plate type fuel

6 Spent nuclear fuel o 6.1 Oxide fuel under accident conditions

7 Fuel behavior and post irradiation examination (PIE) 8 Radioisotope decay fuels

o 8.1 Radioisotope battery o 8.2 Radioisotope thermoelectric generators o 8.3 Radioisotope heater units (RHU)

9 Fusion fuels o 9.1 First generation fusion fuel o 9.2 Second generation fusion fuel o 9.3 Third generation fusion fuel

10 See also 11 External links and references

o 11.1 PWR fuel o 11.2 BWR fuel o 11.3 CANDU fuel o 11.4 TRISO fuel o 11.5 CERMET fuel o 11.6 Plate type fuel o 11.7 TRIGA fuel o 11.8 Space reactor fuels

o 11.9 Fusion fuel

[edit] Oxide fuel

The thermal conductivity of uranium dioxide is low; it is affected by porosity and burn-up. The burn-up results in fission products being dissolved in the lattice (such as lanthanides), the precipitation of fission products such as palladium, the formation of fission gas bubbles due to fission products such as xenon and krypton and radiation damage of the lattice. The low thermal conductivity can lead to overheating of the center part of the pellets during use. The porosity results in a decrease in both the thermal conductivity of the fuel and the swelling which occurs during use.

According to the International Nuclear Safety Center [1] the thermal conductivity of uranium dioxide can be predicted under different conditions by a series of equations.

The bulk density of the fuel can be related to the thermal conductivity

Where ρ is the bulk density of the fuel and ρtd is the theoretical density of the uranium dioxide.

Then the thermal conductivity of the porous phase (Kf)is related to the conductivity of the perfect phase (Ko, no porosity) by the following equation. Note that s is a term for the shape factor of the holes.

Kf = Ko.(1-p/1+(s-1)p)

Rather than measuring the thermal conductivity using the traditional methods in physics such as lees's disk, the Forbes' method or Searle's bar it is common to use a laser flash method where a small disc of fuel is placed in a furnace. After being heated to the required temperature one side of the disc is illuminated with a laser pulse, the time required for the heat wave to flow through the disc, the density of the disc, and the thickness of the disk can then be used to calculated to give the thermal conductivity.

λ = ρCpα λ thermal conductivity ρ density Cp heat capacity α thermal diffusivity

If t1/2 is defined as the time required for the non illuminated surface to experience half its final temperature rise then.

α = 0.1388 L2 / t1/2

L is the thickness of the disc

For details see [2]

[edit] UOX

The thermal conductivity of zirconium metal and uranium dioxide as a function of temperature

Uranium dioxide is a black semiconductor solid. It can be made by reacting uranyl nitrate with a base (ammonia) to form a solid (ammonium uranate). It is heated (calcined) to form U3O8 that can then be converted by heating in an argon / hydrogen mixture (700 oC) to form UO2. The UO2 is then mixed with an organic binder and pressed into pellets, these pellets are then fired at a much higher temperature (in H2/Ar) to sinter the solid. The aim is to form a dense solid which has few pores.

The thermal conductivity of uranium dioxide is very low compared with that of zirconium metal, and it goes down as the temperature goes up.

It is important to note that the corrosion of uranium dioxide in an aqueous environment is controlled by similar electrochemical processes to the galvanic corrosion of a metal surface.

[edit] MOX

Main article: MOX fuel

Mixed oxide, or MOX fuel, is a blend of plutonium and natural or depleted uranium which behaves similarly (though not identically) to the enriched uranium feed for which most nuclear reactors were designed. MOX fuel is an alternative to low enriched uranium (LEU) fuel used in the light water reactors which predominate nuclear power generation.

Some concern has been expressed that used MOX cores will introduce new disposal challenges, though MOX is itself a means to dispose of surplus plutonium by transmutation.

Currently (March, 2005) reprocessing of commercial nuclear fuel to make MOX is done in England and France, and to a lesser extent in Russia, India and Japan. China plans to develop fast breeder reactors and reprocessing.

The Global Nuclear Energy Partnership, is a U.S. plan to form an international partnership to see spent nuclear fuel reprocessed in a way that renders the plutonium in it usable for nuclear fuel but not for nuclear weapons. Reprocessing of spent commercial-reactor nuclear fuel has not been permitted in the United States due to nonproliferation considerations. All of the other reprocessing nations have long had nuclear weapons from military-focused "research"-reactor fuels except for Japan.

[edit] Metal fuel

Metal fuels have the advantage of a much higher heat conductivity than oxide fuels but cannot survive equally high temperatures.

[edit] TRIGA fuel

TRIGA fuel is used in TRIGA (Training, Research, Isotopes, General Atomics) reactors. The TRIGA reactor uses uranium-zirconium-hydride (UZrH) fuel, which has a prompt negative temperature coefficient, meaning that as the temperature of the core increases, the reactivity decreases - so it is physically impossible for a meltdown to occur. Most cores that use this fuel are "high leakage" cores where the excess leaked neutrons can be utilized for research. TRIGA fuel was originally designed to use highly enriched uranium, however in 1978 the U.S. Department of Energy launched its Reduced Enrichment for Research Test Reactors program, which promoted reactor conversion to low-enriched uranium fuel. A total of 35 TRIGA reactors have been installed at locations across the USA. A further 35 reactors have been installed in other countries.

[edit] Actinide Fuel

In a fast neutron reactor the minor actinides produced by neutron capture of uranium and plutonium can be used as fuel. Metal actinide fuel is typically an alloy of zirconium , uranium, plutonium and the minor actinides. It can be made inherently safe as thermal expansion of the metal alloy will increase neutron leakage.

[edit] Less common chemical forms

[edit] Ceramic fuels

Ceramic fuels other than oxides have the advantage of a high heat conductivities and melting points, but they are more prone to swelling than oxide fuels and are much less well understood.

[edit] Uranium nitride

This is often the fuel of choice for reactor designs that NASA produces, one advantage is that UN has a better thermal conductivity than UO2. Uranium nitride has a very high melting point. This fuel has the disadvantage that unless 15N was used (in place of the more common 14N) that a large amount of 14C would be generated from the nitrogen by the pn reaction. As the nitrogen required for such a fuel would be so expensive it is likely that the fuel would have to be reprocessed by a pyro method to enable to the 15N to be recovered. It is likely that if the fuel was processed and dissolved in nitric acid that the nitrogen enriched with 15N would be diluted with the common 14N.

[edit] Uranium carbideMain article: uranium carbide

Much of what is known about uranium carbide is in the form of pin-type fuel elements for liquid metal fast breeder reactors during their intense study during the 60's and 70's. However, recently there has been a revived interest in uranium carbide in the form of plate fuel and most notably, micro fuel particles (such as TRISO particles).

The high thermal conductivity and high melting point make uranium carbide an attractive fuel. In addition, because of the absence of oxygen in this fuel (during the course of radiation, excess gas pressure can build from the formation O2 or other gases) as well as the ability to compliment a ceramic coating (a ceramic-ceramic interface has structural and chemical advantages), uranium carbide could be the ideal fuel candidate for certain Generation IV reactors such as the gas-cooled fast reactor.

[edit] Liquid fuels

[edit] Molten anhydrous salts

These include fuels where the fuel is dissolved in the coolant. They were used in the molten salt reactor experiment and numerous other liquid core reactor experiments, such as the Liquid fluoride reactor. The liquid fuel for the molten salt reactor was LiF-BeF2-ThF4-UF4 (72-16-12-0.4 mol%), it had a peak operating temperature of 705 °C in the experiment but could have gone to much higher temperatures since the boiling point of the molten salt was in excess of 1400 °C.

[edit] Aqueous solutions of uranyl salts

The Aqueous Homogeneous Reactors uses a solution of uranyl sulfate or other uranium salt in water. This homogenous reactor type has not been used for any large power reactors. One of its disadvantages is that the fuel is in a form which is easy to disperse in the event of an accident.

[edit] Common physical forms of nuclear fuel

For use as nuclear fuel, enriched UF6 is converted into uranium dioxide (UO2) powder that is then processed into pellet form. The pellets are then fired in a high-temperature, sintering furnace to create hard, ceramic pellets of enriched uranium. The cylindrical pellets then undergo a grinding process to achieve a uniform pellet size. The pellets are stacked, according to each nuclear core's design specifications, into tubes of corrosion-resistant metal alloy. The tubes are sealed to contain the fuel pellets: these tubes are called fuel rods. The finished fuel rods are grouped in special fuel assemblies that are then used to build up the nuclear fuel core of a power reactor.

The metal used for the tubes depends on the design of the reactor - stainless steel was used in the past, but most reactors now use a zirconium alloy. For the most common types of reactors (BWRs and PWRs) the tubes are assembled into bundles with the tubes spaced precise distances apart. These bundles are then given a unique identification number, which enables them to be tracked from manufacture through use and into disposal.

NRC Image of pre-irradiated Fuel Pellets.

NRC Image of pre-irradiated Fuel Pellets ready for assembly. NRC picture of New (pre-

irradiated) U.S. fuel being inspected.

[edit] PWR fuel

PWR fuel bundle The fuel bundle is from a pressurized water reactor of the nuclear passenger and cargo ship NS Savannah. Designed and built by the Babcock and Wilcox Company.

Pressurized water reactor (PWR) fuel consists of cylindrical rods put into bundles. A uranium oxide ceramic is formed into pellets and inserted into Zircaloy tubes that are bundled together. The Zircaloy tubes are about 1 cm in diameter, and the fuel cladding gap is filled with helium gas to improve the conduction of heat from the fuel to the cladding. There are about 179-264 fuel rods per fuel bundle and about 121 to 193 fuel bundles are loaded into a reactor core. Generally, the fuel bundles consist of fuel rods bundled 14x14 to 17x17. PWR fuel bundles are about 4 meters in length. In PWR fuel bundles, control rods are inserted through the top directly into the fuel bundle. The fuel bundles usually are enriched several percent in 235U. The uranium oxide is dried before inserting into the tubes to try to eliminate moisture in the ceramic fuel that can lead to corrosion and hydrogen embrittlement. The Zircaloy tubes are pressurized with helium to try to minimize pellet cladding interaction (PCI) which can lead to fuel rod failure over long periods.

[edit] BWR fuel

In boiling water reactors (BWR), the fuel is similar to PWR fuel except that the bundles are "canned"; that is, there is a thin tube surrounding each bundle. This is primarily done to prevent local density variations from effecting neutronics and thermal hydraulics of the nuclear core on a global scale. In modern BWR fuel bundles, there are either 91, 92, or 96 fuel rods per assembly depending on the manufacturer. A range between 368 assemblies for the smallest and 800 assemblies for the largest U.S. BWR forms the reactor core. Each BWR fuel rod is back filled with helium to a pressure of about three atmospheres (300 kPa).

[edit] CANDU fuel

CANDU fuel bundles Two CANDU fuel bundles, each about 50 cm in length, 10 cm in diameter. Photo courtesy of Atomic Energy of Canada Ltd.

CANDU fuel bundles are about a half meter in length and 10 cm in diameter. They consist of sintered (UO2) pellets in Zirconium alloy tubes, welded to Zirconium alloy end plates. Each bundle is roughly 20 kg, and a typical core loading is on the order of 4500-6500 bundles, depending on the design. Modern types typically have 37 identical fuel pins radially arranged about the long axis of the bundle, but in the past several different configurations and numbers of pins have been used. The CANFLEX bundle has 43 fuel elements, with two element sizes. It is also about 10 cm (four inches) in diameter, 0.5 m (20 inches) long and weighs about 20 kg (44 lbs) and replaces the 37-pin standard bundle. It has been designed specifically to increase fuel performance by utilizing two different pin diameters. Current CANDU designs do not need enriched uranium to achieve criticality (due to their more efficient heavy water moderator), however, some newer concepts call for low enrichment to help reduce the size of the reactors.

[edit] Less common fuel forms

Various other nuclear fuel forms find use in specific applications, but lack the widespread use of those found in BWRs, PWRs, and CANDU power plants. Many of these fuel forms are only found in research reactors, or have military applications.

[edit] TRISO fuel

TRISO fuel particle which has been cracked, showing the multiple coating layers

Tristructural-isotropic (TRISO) fuel is a type of micro fuel particle. It consists of a fuel kernel composed of UOX (sometimes UC or UCO) in the center, coated with four layers of three isotropic materials. The four layers are a porous buffer layer made of carbon, followed by a dense inner layer of pyrolytic carbon (PyC), followed by a ceramic layer of SiC to retain fission products at elevated temperatures and to give the TRISO particle more structural integrity, followed by a dense outer layer of PyC. TRISO fuel particles are designed not to crack due to the stresses from processes (such as differential thermal expansion or fission gas pressure) at temperatures beyond 1600°C, and therefore can contain the fuel in the worst of accident scenarios in a properly designed reactor. Two such reactor designs are the pebble bed reactor (PBR), in which thousands of TRISO fuel particles are dispersed into graphite pebbles, and the prismatic-block gas-cooled reactor (such as the GT-MHR), in which the TRISO fuel particles are fabricated into compacts and placed in a graphite block matrix. Both of these reactor designs are very high temperature reactors (VHTR) [formally known as the high-temperature gas-cooled reactors (HTGR)], one of the six classes of reactor designs in the Generation IV initiative.

TRISO fuel particles were originally developed in Germany for high-temperature gas-cooled reactors. The first nuclear reactor to use TRISO fuels was the AVR and the first powerplant was the THTR-300. Currently, TRISO fuel compacts are being used in the experimental reactors, the HTR-10 in China, and the HTTR in Japan.

RBMK reactor fuel rod holder 1 - distancing armature; 2 - fuel rods shell; 3 - fuel tablets.

[edit] RBMK fuel

RBMK reactor fuel was used in Soviet designed and built RBMK type reactors. This is a low enriched uranium oxide fuel. The fuel elements in an RBMK are 3m long each, and two of these sit back-to-back on each fuel channel, pressure tube. Reprocessed uranium from Russian VVER reactor spent fuel is used to fabricate RBMK fuel. Following the Chernobyl accident, the enrichment of fuel was changed from 2% to 2.4%, to help avoid future accidents.[citation needed]

[edit] CerMet fuel

CerMet fuel consists of ceramic fuel particles (usually uranium oxide) embedded in a metal matrix. It is hypothesized that this type of fuel is what is used in US Navy reactors. This fuel has high heat transport characteristics and can withstand a large amount of expansion.

[edit] Plate type fuel

ATR Core The Advanced Test Reactor at Idaho National Laboratory uses plate type fuel in a clover leaf arrangement

Plate type fuel has grown out of favor over the years. Plate type fuel is commonly composed of enriched uranium sandwiched between metal cladding. Plate type fuel is used in several research reactors where a high neutron flux is desired, for uses such as material irradiation studies or isotope production, without the high temperatures seen in ceramic, cylindrical fuel. It is currently used in the Advanced Test Reactor (ATR) at Idaho National Laboratory.

[edit] Spent nuclear fuel

Main article: Spent nuclear fuel

Used nuclear fuel is a complex mixture of the fission products, uranium, plutonium and the transplutonium metals. In fuel which has been used at high temperature in power reactors it is common for the fuel not to be homogenous; often the fuel will contain nanoparticles of platinum group metals such as palladium. Also the fuel may well have cracked, swelled and been used close to its melting point. Despite the fact that the used fuel can be cracked it is very insoluble in water, and is able to retain the vast majority of the actinides and fission products within the uranium dioxide crystal lattice.

[edit] Oxide fuel under accident conditions

Main article: Nuclear fuel response to reactor accidents

Two main modes of release exist, the fission products can be vapourised or small particles of the fuel can be dispersed.

[edit] Fuel behavior and post irradiation examination (PIE)

Materials in a high radiation environment (such as a reactor) can undergo unique behaviors such as swelling[3] and non-thermal creep. If there are nuclear reactions within the material (such as what happens in the fuel), the stoichiometry will also change slowly over time. These behaviors can lead to new material properties, cracking, and fission gas release:

Fission gas release o As the fuel is degraded or heated the more volatile fission products which

are trapped within the uranium dioxide may become free. For example see J.Y. Colle, J.P. Hiernaut, D. Papaioannou, C. Ronchi, A. Sasahara, Journal of Nuclear Materials, 2006, 348, 229.

Fuel cracking o As the fuel expands on heating, the core of the pellet expands more than

the rim which may lead to cracking. Because of the thermal stress thus formed the fuel cracks, the cracks tend to go from the centre to the edge in a star shaped pattern.

In order to better understand and control these changes in materials, these behaviors are studied. A common experiment to do this is post irradiation examination, in which fuel will be examined after it is put through reactor-like conditions [4] [5] [6] [7]. Due to the intensely radioactive nature of the used fuel this is done in a hot cell. A combination of nondestructive and destructive methods of PIE are common.

The PIE is used to check that the fuel is both safe and effective. After major accidents the core (or what is left of it) is normally subject to PIE in order to find out what happened. One site where PIE is done is the ITU which is the EU centre for the study of highly radioactive materials.

In addition to the effects of radiation and the fission products on materials, scientists also need to consider the temperature of materials in a reactor, and in particular, the fuel. Too high a fuel temperature can compromise the fuel, and therefore it is important to control the temperature in order to control the fission chain reaction.

The temperature of the fuel varies as a function of the distance from the centre to the rim. At distance x from the centre the temperature (Tx) is described by the equation where ρ is the power density (W m-3) and Kf is the thermal conductivity.

Tx = TRim + ρ (rpellet2 - x2) (4 Kf)-1

To explain this for a series of fuel pellets being used with a rim temperature of 200 oC (typical for a BWR) with different diameters and power densities of 250 MW m-3 have

been modeled using the above equation. Note that these fuel pellets are rather large; it is normal to use oxide pellets which are about 10 mm in diameter.

Nuclear Fuel Temperature Profiles

Temperature profile for a 20 mm diameter fuel pellet with a power density of 250 MW per cubic meter. Note the central temperature is very different for the different fuel solids.

Temperature profile for a 26 mm diameter fuel pellet with a power density of 250 MW per cubic meter.

Temperature profile for a 32 mm diameter fuel pellet with a power density of 250 MW per cubic meter.

Temperature profile for a 20 mm diameter fuel pellet with a power density of 500 MW per cubic meter. Because the melting point of uranium dioxide is about 3300 K, it is clear that uranium oxide fuel is overheating at the center.

Temperature profile for a 20 mm diameter fuel pellet with a power density of 1000 MW per cubic meter. The fuels other than uranium dioxide are not compromised.

Reference Radiochemistry and Nuclear Chemistry, G. Choppin, J-O Liljenzin and J. Rydberg, 3rd Ed, 2002, Butterworth-Heinemann, ISBN 0-7506-7463-6

[edit] Radioisotope decay fuels

[edit] Radioisotope battery

Main article: atomic battery

The terms atomic battery, nuclear battery and radioisotope battery are used interchangely to describe a device which uses the radioactive decay to generate electricity. These systems use radioisotopes that produce low energy beta particles or sometimes alpha particles of varying energies. Low energy beta particles are needed to prevent the production of high energy penetrating Bremsstrahlung radiation that would require heavy shielding. Radioisotopes such as tritium, nickel-63, promethium-147, and technetium-99 have been tested. Plutonium-238, curium-242, curium-244 and strontium-90 have been used.

There are two main categories of atomic batteries: thermal and non-thermal. The non-thermal atomic batteries, which have many different designs, exploit charged alpha and beta particles. These designs include the direct charging generators, Betavoltaics, the optoelectric nuclear battery, and the radioisotope piezoelectric generator. The thermal atomic batteries on the other hand, convert the heat from the radioactive decay to

electricity. These designs include thermionic converter, thermophotovoltaic cells, alkali-metal thermal to electric converter, and the most common design, the radioisotope thermoelectric generator.

[edit] Radioisotope thermoelectric generators

A radioisotope thermoelectric generator (RTG) is a simple electrical generator which converts heat into electricity from a radioisotope using an array of thermocouples.

238Pu has become the most widely used fuel for RTGs. In the form of plutonium dioxide it has a half-life of 87.7 years, reasonable energy density and exceptionally low gamma and neutron radiation levels. Some Russian terrestrial RTGs have used 90Sr; this isotope has a shorter half-life and a much lower energy density, but is cheaper. Early RTGs, first built in 1958 by the U.S. Atomic Energy Commission, have used 210Po. This fuel provides phenomenally huge energy density, (a single gram of polonium-210 generates 140 watts thermal) but has limited use because of its very short half-life and gamma production and has been phased out of use in this application.

[edit] Radioisotope heater units (RHU)

Photo of a disassembled RHU

Radioisotope heater units normally provide about 1 watt of heat each, derived from the decay of a few grams of Plutonium-238. This heat is given off continuously for several decades.

Their function is to provide highly localised heating of sensitive equipment (such as electronics) in deep space. The Cassini-Huygens orbiter to Saturn contains 82 of these units (in addition to its 3 main RTG's for power generation). The Huygens probe to Titan contains 35 devices.

[edit] Fusion fuels

Most fusion fuels fit in here. They include tritium (3H) and deuterium (2H) as well as helium three (3He). Many other elements can be fused together if they can be forced close enough to each other at high enough temperatures. In general, fusion fuels are expected to have at least three generations based on the ease of fusing light atomic nuclei together.

[edit] First generation fusion fuel

Deuterium and tritium are both considered first-generation fusion fuels; with many permutations in which they can be fused together. The three most commonly cited are;

2H + 3H n (14.07 MeV) + 4He (3.52 MeV) 2H + 2H n (2.45 MeV) + 3He (0.82 MeV) 2H + 2H p (3.02 MeV) + 3H (1.01 MeV)

[edit] Second generation fusion fuel

Second generation fuels require either higher confinement temperatures or longer confinement time than those required of first generation fusion fuels. This group consists of deuterium and helium three. The products of these reactants are all charged particles, but there may be non-beneficial side reactions leading to radioactive activation of fusion reactor components.

2H + 3He p (14.68 MeV) + 4He (3.67 MeV)

[edit] Third generation fusion fuel

Main article: Aneutronic fusion

There are several potential third generation fusion fuels. Third generation fusion fuels produce only charged particles in the primary reactions and any side reactions are relatively unimportant. Therefore, there would be little radioactive activation of the fusion reactor. This is often seen as the end goal of fusion research. 3He has the highest Maxwellian reactivity of any 3rd generation fusion fuel, but there are no significant natural sources of this substance on Earth.

3He + 3He 2p + 4He (12.86 MeV)

Another potential aneutronic fusion reaction is the proton-boron reaction:

p + 11B → 34He

Under reasonable assumptions, side reactions will result in about 0.1% of the fusion power being carried by neutrons. With 123 keV, the optimum temperature for this reaction is nearly ten times higher than that for the pure hydrogen reactions, the energy confinement must be 500 times better than that required for the D-T reaction, and the power density will be 2500 times lower than for D-T.

Nuclear reactor core

From Wikipedia, the free encyclopedia

Jump to: navigation, search

Example of the core of a nuclear power plant, a VVER design.

A nuclear reactor core is that portion of a nuclear reactor containing the nuclear fuel components where the nuclear reactions take place.

Contents

[hide] 1 Description 2 Water-moderated reactors 3 Graphite-moderated reactors 4 Experimental & Developmental Reactors 5 See also

6 References

[edit] Description

The nuclear reactor core is a lead housing for fuel rods and control rods.

[edit] Water-moderated reactors

Inside the core of a typical pressurized water reactor or boiling water reactor. are pencil-thin nuclear fuel rods, each about 12 feet (3.7 m) long, which are grouped by the hundreds in bundles called "fuel assemblies". Inside each fuel rod, pellets of uranium, or more commonly uranium oxide, are stacked end to end. Also inside the core are control rods, filled with pellets of substances like hafnium or cadmium that readily capture neutrons. When the control rods are lowered into the core, they absorb neutrons, which thus cannot take part in the chain reaction. On the converse, when the control rods are lifted out of the way, more neutrons strike the fissile uranium-235 (U-235) or plutonium-239 (Pu-239) nuclei in nearby fuel rods, and the chain reaction intensifies.

The heat of the fission reaction is removed by the water, which also acts to moderate the neutron reactions. An alternative form of nuclear fuel would be fissile uranium-233 (U-233) made by the neutron-bombardment of the common thorium-232. Also, fissile uranium-234 (U-234) is found as a trace addition to U-235 wherever U-235 is found. They are both good nuclear "fuels".

[edit] Graphite-moderated reactors

There are also Graphite moderated reactors in use.

One type uses solid graphite for the neutron moderator and ordinary water for the coolant. See the Soviet-made RBMK nuclear-power reactor. These were the reactors that were the cause of the Chernobyl disaster.

In a so-called advanced gas-cooled reactor, a British design, the core is made of a graphite neutron moderator where the fuel assemblies are located. Carbon dioxide gas acts as a coolant and it circulates through the graphite removing heat.

[edit] Experimental & Developmental Reactors

Several merely experimental or hypothetical nuclear reactor cores are mentioned below.

There have been developmental graphite-moderated nuclear power reactors that were cooled by helium gas. These are no longer in service.

The core of a molten salt reactor is a block of graphite through which holes are bored in which molten salt circulates. The graphite serves as a neutron moderator, it is the solid structure of the reactor. The molten salt that circulates in the channels is both the fuel and the coolant, it contains the fissionable material needed to sustain the chain reaction.

A set of compact nuclear reactors were developed by the United States under the Systems Nuclear Auxiliary Power Program (SNAP). One SNAP reactor, the SNAP-10A was launched into space and was successfully operated for 43 days in 1965.

Aqueous homogeneous reactors cores employ water in which soluble nuclear salts (usually uranyl sulfate or uranyl nitrate) have been dissolved. As the water serves as the solvent for the uranium salts, it serves as the fuel. As it is water, it serves to cool the reactor as well- hence the name 'homogeneous' (as coolant and fuel are one homogeneous substance). The water can be either heavy water or ordinary light water.

In a gaseous fission reactor the reaction takes place in a core which is bounded and created by magnetic field. The fuel is supplied and fission occurs in the gas phase.

Neutron moderator

From Wikipedia, the free encyclopedia

Jump to: navigation, search

This article needs additional citations for verification.Please help improve this article by adding reliable references. Unsourced material may be challenged and removed. (February 2008)

In nuclear engineering, a neutron moderator is a medium which reduces the velocity of fast neutrons, thereby turning them into thermal neutrons capable of sustaining a nuclear chain reaction involving uranium-235.

Commonly used moderators include regular (light) water (75% of the world's reactors), solid graphite (20% of reactors) and heavy water (5% of reactors).[1] Beryllium has also been used in some experimental types, and hydrocarbons have been suggested as another possibility.

Contents

[hide] 1 Explanation 2 Form and location 3 Moderator impurities 4 Non graphite moderators 5 Materials used 6 References

7 See also

[edit] Explanation

In a thermal nuclear reactor, the nucleus of a heavy fuel element such as uranium absorbs a slow-moving free neutron, becomes unstable, and then splits ("fissions") into two smaller atoms ("fission products"). The fission process for uranium atoms yields two fission products, two to three fast-moving free neutrons, plus an amount of energy primarily manifested in the kinetic energy of the recoiling fission products. Because more free neutrons are released from a uranium fission event than thermal neutrons are required to initiate the event, the reaction can become self sustaining — a chain reaction — under controlled conditions, thus liberating a tremendous amount of energy. However, the probability of further fission events occurring is dependent upon the speed (energy) of the incident neutrons. Faster neutrons are much less likely to cause further fission. (Note: It is not impossible for fast neutrons to cause fission, just much less likely.) The newly-released fast neutrons, moving at roughly 10% of the speed of light, must be slowed down or "moderated", typically to speeds of a few kilometers per second, if they are to be likely to cause further fission in neighbouring uranium nuclei and hence continue the chain reaction.

A good neutron moderator is a material full of atoms with light nuclei which do not easily absorb neutrons. The neutrons strike the nuclei and bounce off. In this process, some energy is transferred between the nucleus and the neutron. More energy is transferred per collision if the nucleus is lighter; see elastic collision. After sufficiently many such impacts, the velocity of the neutron will be comparable to the thermal velocities of the nuclei; this neutron is then called a thermal neutron.

A fast reactor uses no moderator, but relies on fission produced by unmoderated fast neutrons to sustain the chain reaction.

In all moderated reactors, some neutrons of all energy levels will produce fission, including fast neutrons. Some reactors are more fully thermalised than others; For example in a CANDU reactor nearly all fission reactions are produced by thermal neutrons, while in a pressurized water reactor (PWR) a considerable portion of the fissions are produced by higher-energy neutrons. In the proposed water-cooled supercritical water reactor (SCWR), the proportion of fast fissions may exceed 50%, making it technically a fast neutron reactor.

Moderators are also used in non-reactor neutron sources, such as plutonium-beryllium and spallation sources.

[edit] Form and location

The form and location of the moderator can greatly influence the cost and safety of a reactor. Classically, moderators were precision-machined blocks with embedded ducting to carry away heat. Also, they were in the hottest part of the reactor, and therefore subject to corrosion and ablation. In some materials, notably graphite, the impact of the neutrons with the moderator can cause the moderator to accumulate dangerous amounts of Wigner energy. At Windscale, this problem led to the infamous Windscale fire.

Some pebble-bed reactors' moderators are not only simple, but also inexpensive: the nuclear fuel is embedded in spheres of reactor-grade pyrolytic carbon, roughly of the size of tennis balls. The spaces between the balls serve as ducting. The reactor is operated above the Wigner annealing temperature so that the graphite does not accumulate dangerous amounts of Wigner energy.

[edit] Moderator impurities

Good moderators are also free of neutron-absorbing impurities such as boron. In commercial nuclear power plants the moderator typically contains dissolved boron. The boron concentration of the reactor coolant can be changed by the operators by adding boric acid or by diluting with water to manipulate reactor power. The German World War II nuclear program suffered a substantial setback when its inexpensive graphite moderators failed to work. At that time, most graphites were deposited on boron electrodes, and the German commercial graphite contained too much boron. Since the war-time German program never discovered this problem, they were forced to use far more expensive heavy water moderators. In the U.S., Leo Szilard, a former chemical engineer, discovered the problem.

[edit] Non graphite moderators

Some moderators are quite expensive, for example beryllium, and reactor grade heavy water. Reactor-grade heavy water must be 99.75% pure to enable reactions with unenriched uranium. This is difficult to prepare because heavy water and regular water form the same chemical bonds in almost the same ways, at only slightly different speeds.

The much cheaper light water moderator ( essentially very pure regular water ) absorbs too many neutrons to be used with unenriched natural uranium, and therefore uranium enrichment or nuclear reprocessing becomes necessary to operate such reactors, increasing overall costs. Both enrichment and reprocessing are expensive and technologically challenging processes, and additionally both enrichment and several types of reprocessing can be used to create weapons-usable material, causing proliferation concerns. Reprocessing schemes that are more resistant to proliferation are currently under development.

The CANDU reactor's moderator doubles as a safety feature. A large tank of low-temperature, low-pressure heavy water moderates the neutrons and also acts as a heat sink in extreme loss-of-coolant accident conditions. It is separated from the fuel rods that actually generate the heat. Heavy water is very effective at slowing down (moderating) neutrons, giving CANDU reactors their important and defining characteristic of high "neutron economy".

[edit] Materials used

Hydrogen , as in ordinary water ("light water"), in light water reactors. The reactors require enriched uranium to operate.

o There are also proposals to use the compound formed by the chemical reaction of metallic uranium and hydrogen (uranium hydride--UH3) as a combination fuel and moderator in a new type of reactor.

o Hydrogen is also used in the form of cryogenic liquid methane and sometimes liquid hydrogen as a cold neutron source in some research reactors: yielding a Maxwell–Boltzmann distribution for the neutrons whose maximum is shifted to much lower energies.

Deuterium , in the form of heavy water, in heavy water reactors, e.g. CANDU. Reactors moderated with heavy water can use unenriched natural uranium.

Carbon , in the form of reactor-grade graphite or pyrolytic carbon, used in e.g. RBMK and pebble-bed reactors, or in compounds, e.g. carbon dioxide [1]. Lower-temperature reactors are susceptible to buildup of Wigner energy in the material. Like deuterium-moderated reactors, some of these reactors can use unenriched natural uranium.

o Graphite is also deliberately allowed to be heated to around 2000 K or higher in some research reactors to produce a hot neutron source: giving a Maxwell–Boltzmann distribution whose maximum is spread out to generate higher energy neutrons.

Beryllium , in the form of metal. Beryllium is expensive and toxic, so its use is limited.

Lithium -7, in the form of a fluoride salt, typically in conjunction with beryllium fluoride salt (FLiBe). This is the most common type of moderator in a Molten Salt Reactor.

Other light-nuclei materials are unsuitable for various reasons. Helium is a gas and is not possible to achieve its sufficient density, lithium-6 and boron absorb neutrons.

[edit] References

(January 1993) DOE Fundamentals Handbook: Nuclear Physics and Reactor Theory. U.S. Department of Energy. Retrieved on 2007-09-26.

1. ̂ Miller, Jr., George Tyler (2002). Living in the Environment: Principles,

Nuclear poison

From Wikipedia, the free encyclopedia

  (Redirected from Neutron poison)Jump to: navigation, searchFor information on radioactive toxins, see Radiation poisoning.

A nuclear poison, also called a neutron poison is a substance with a large neutron absorption cross-section in applications, such as nuclear reactors, when absorbing neutrons is an undesirable effect. However neutron-absorbing materials, also called poisons, are intentionally inserted into some types of reactors in order to lower the high reactivity of their initial fresh fuel load. Some of these poisons deplete as they absorb neutrons during reactor operation, while others remain relatively constant.

Contents

[hide] 1 Transient fission product poisons 2 Accumulating fission product poisons 3 Decay poisons 4 Control poisons

o 4.1 Burnable poisons o 4.2 Non-burnable poison o 4.3 Soluble poisons

5 References

[edit] Transient fission product poisons

Some of the fission products generated during a nuclear reaction have a high neutron absorption capacity, such as xenon-135 (Xe-135) and samarium-149 (Sm-149). Because these two fission product poisons remove neutrons from the reactor, they will have an impact on the thermal utilization factor and thus the reactivity. The poisoning of a reactor core by these fission products may become so serious that the chain reaction comes to a standstill.

Xe-135 in particular has a tremendous impact on the operation of a nuclear reactor. The inability of a reactor to be started due to the effects of Xe-135 is sometimes referred to as xenon precluded start-up. The period of time where the reactor is unable to override the effects of Xe-135 is called the xenon dead time. During periods of steady state operation, at a constant neutron flux level, the Xe-135 concentration builds up to its equilibrium value for that reactor power in about 40 to 50 hours. When the reactor power is increased, Xe-135 concentration initially decreases because the burn up is increased at the new higher power level. Because 95% of the Xe-135 production is from iodine-135 decay, which has a 6 to 7 hour half-life, the production of Xe-135 remains constant, at this point, the Xe-135 concentration reaches a minimum. The concentration then increases to the new equilibrium level for the new power level in again roughly 40 to 50 hours. The magnitude and the rate of change of concentration during the initial 4 to 6 hours following the power change is dependent upon the initial power level and on the amount of change in power level; the Xe-135 concentration change is greater for a larger change in power level. When reactor power is decreased, the process is reversed.[1]

Because Sm-149 is not radioactive and is not removed by decay, it presents problems somewhat different from those encountered with Xe-135. The equilibrium concentration and (thus the poisoning effect) builds to an equilibrium value during reactor operation in about 500 hours, and since Sm-149 is stable, the concentration remains essentially constant during reactor operation.[2]

[edit] Accumulating fission product poisons

There are numerous other fission products that, as a result of their concentration and thermal neutron absorption cross section, have a poisoning effect on reactor operation. Individually, they are of little consequence, but taken together they have a significant impact. These are often characterized as lumped fission product poisons and accumulate at an average rate of 50 barns per fission event in the reactor. The buildup of fission product poisons in the fuel eventually leads to loss of efficiency, and in some cases to instability. In practice, buildup of reactor poisons in nuclear fuel is what determines the lifetime of nuclear fuel in a reactor: long before all possible fissions have taken place, buildup of long-lived neutron-absorbing fission products damps out the chain reaction. This is the reason that nuclear reprocessing is a useful activity: solid spent nuclear fuel contains about 99% of the original fissionable material present in newly manufactured nuclear fuel. Chemical separation of the fission products restores the fuel so that it can be used again.

Other potential approaches to fission product removal include solid but porous fuel which allows escape of fission products[3] and liquid or gaseous fuel (Molten salt reactor, Aqueous homogeneous reactor). These ease the problem of fission product accumulation in the fuel, but pose the additional problem of safely removing and storing the fission products.

Other fission products with relatively high absorption cross sections include 83Kr, 95Mo, 143Nd, 147Pm.[4] Above this mass, even many even-mass number isotopes have large

absorption cross sections, allowing one nucleus to serially absorb multiple neutrons. Fission of heavier actinides produces more of the heavier fission products in the lanthanide range, so the total neutron absorption cross section of fission products is higher. [5]

In a fast reactor the fission product poison situation may differ significantly because neutron absorption cross sections can differ for thermal neutrons and fast neutrons. In the RBEC-M Lead-Bismuth Cooled Fast Reactor, the fission products with neutron capture more than 5% of total fission products capture are, in order, Cs-133, Ru-101, Rh-103, Tc-99, Pd-105, Pd-107 in the core, with Sm-149 replacing Pd-107 for 6th place in the breeding blanket.[6]

[edit] Decay poisons

In addition to fission product poisons, other materials in the reactor decay to materials that act as neutron poisons. An example of this is the decay of tritium to helium-3 (He-3). Since tritium has a half-life of 12.3 years, normally this decay does not significantly affect reactor operations because the rate of decay of tritium is so slow. However, if tritium is produced in a reactor and then allowed to remain in the reactor during a prolonged shutdown of several months, a sufficient amount of tritium may decay to He-3 to add a significant amount of negative reactivity. Any He-3 produced in the reactor during a shutdown period will be removed during subsequent operation by a neutron-proton reaction.

[edit] Control poisons

During operation of a reactor the amount of fuel contained in the core constantly decreases. If the reactor is to operate for a long period of time, fuel in excess of that needed for exact criticality must be added when the reactor is built. The positive reactivity due to the excess fuel must be balanced with negative reactivity from neutron-absorbing material. Movable control rods containing neutron-absorbing material is one method, but control rods alone to balance the excess reactivity may be impractical for a particular core design as there may be insufficient room for the rods or their mechanisms.

[edit] Burnable poisons

To control large amounts of excess fuel without control rods, burnable poisons are loaded into the core. Burnable poisons are materials that have a high neutron absorption cross section that are converted into materials of relatively low absorption cross section as the result of neutron absorption. Due to the burn-up of the poison material, the negative reactivity of the burnable poison decreases over core life. Ideally, these poisons should decrease their negative reactivity at the same rate the fuel's excess positive reactivity is depleted. Fixed burnable poisons are generally used in the form of compounds of boron or gadolinium that are shaped into separate lattice pins or plates, or introduced as additives to the fuel. Since they can usually be distributed more uniformly than control

rods, these poisons are less disruptive to the core's power distribution. Fixed burnable poisons may also be discretely loaded in specific locations in the core in order to shape or control flux profiles to prevent excessive flux and power peaking near certain regions of the reactor. Current practice however is to use fixed non-burnable poisons in this service.[7]

[edit] Non-burnable poison

A non-burnable poison is one that maintains a constant negative reactivity worth over the life of the core. While no neutron poison is strictly non-burnable, certain materials can be treated as non-burnable poisons under certain conditions. One example is hafnium. The removal (by absorption of neutrons) of one isotope of hafnium leads to the production of another neutron absorber, and continues through a chain of five absorbers. This absorption chain results in a long-lived burnable poison which approximates non-burnable characteristics.[8]

[edit] Soluble poisons

Soluble poisons, also called chemical shim, produce a spatially uniform neutron absorption when dissolved in the water coolant. The most common soluble poison in commercial pressurized water reactors (PWR) is boric acid, which is often referred to as soluble boron, or simply solbor. The boric acid in the coolant decreases the thermal utilization factor, causing a decrease in reactivity. By varying the concentration of boric acid in the coolant, a process referred to as boration and dilution, the reactivity of the core can be easily varied. If the boron concentration is increased, the coolant/moderator absorbs more neutrons, adding negative reactivity. If the boron concentration is reduced (dilution), positive reactivity is added. The changing of boron concentration in a PWR is a slow process and is used primarily to compensate for fuel burnout or poison buildup. The variation in boron concentration allows control rod use to be minimized, which results in a flatter flux profile over the core than can be produced by rod insertion. The flatter flux profile occurs because there are no regions of depressed flux like those that would be produced in the vicinity of inserted control rods. This system is not in widespread use because the chemicals make the moderator temperature reactivity coefficient less negative.[7]

Soluble poisons are also used in emergency shutdown systems. During SCRAM the operators can inject solutions containing neutron poisons directly into the reactor coolant. Various solutions, including sodium polyborate and gadolinium nitrate (Gd(NO3)3 •x H2O), are used.[7]

Coolant

From Wikipedia, the free encyclopedia

Jump to: navigation, search

A coolant is a fluid which flows through a device in order to prevent its overheating, transferring the heat produced by the device to other devices that utilize or dissipate it. An ideal coolant has high thermal capacity, low viscosity, is low-cost, and is chemically inert, neither causing nor promoting corrosion of the cooling system. Some applications also require the coolant to be an electrical insulator.

While the term coolant is commonly used in automotive, residential and commercial temperature-control applications, in industrial processing, heat transfer fluid is one technical term more often used, in high temperature as well as low temperature manufacturing applications[1].

The coolant can either keep its phase and stay liquid or gaseous, or can undergo a phase change, with the latent heat adding to the cooling efficiency. The latter, when used to achieve low temperatures, is more commonly known as refrigerant.

Contents

[hide] 1 Gases 2 Liquids 3 Notes 4 See also

5 External links

[edit] Gases

Air is a common form of a coolant. Air cooling uses either convective airflow (passive cooling), or a forced circulation using fans.

Inert gases are frequently used as coolants in gas-cooled nuclear reactors. Helium is the most favored coolant due to its low tendency to absorb neutrons and become radioactive. Nitrogen and carbon dioxide are frequently used as well.

Sulfur hexafluoride is used for cooling and insulating of some high-voltage power systems (circuit breakers, switches, some transformers, etc.).

Steam can be used where high specific heat capacity is required in gaseous form and the corrosive properties of hot water are accounted for.

[edit] Liquids

The most common coolant is water. Its high heat capacity and low cost makes it a suitable heat-transfer medium. It is usually used with additives, like corrosion inhibitors and antifreezes. Antifreeze, a solution of a suitable organic chemical (most often ethylene glycol, diethylene glycol, or propylene glycol) in water, is used when the water-based coolant has to withstand temperatures below 0 °C, or when its boiling point has to be raised.

Very pure deionized water, due to its relatively low electrical conductivity, is used to cool some electrical equipment, often high-power transmitters.

Heavy water is used in some nuclear reactors; it also serves as a neutron moderator.

Oils are used for applications where water is unsuitable. With higher boiling points than water, oils can be raised to considerably higher temperatures (above 100 degrees Celsius) without introducing high pressures within the container or loop system in question[2].

Mineral oils serve as both coolants and lubricants in many mechanical gears. Castor oil is also used. Due to their high boiling points, mineral oils are used in portable electric radiator-style space heaters in residential applications, and in closed-loop systems for industrial process heating and cooling.

Silicone oils are favored for their wide range of operating temperatures. However their high cost limits their applications.

Fluorocarbon oils are used for the same reasons. High-power electric transformers use transformer oil for cooling and additional

electric insulation.

Cutting fluid is a coolant that also serves as a lubricant for metal-shaping machine tools.

Liquid fusible alloys can be used as coolants in applications where high temperature stability is required, eg. some fast breeder nuclear reactors. Sodium or sodium-potassium alloy NaK are frequently used; in special cases lithium can be employed. Another liquid metal used as a coolant is lead, in eg. lead cooled fast reactors, or a lead-bismuth alloy. Some early fast neutron reactors used mercury.

For very high temperature applications, eg. molten salt reactors or very high temperature reactors, molten salts can be used as coolants. One of the possible combinations is the mix of sodium fluoride and sodium tetrafluoroborate (NaF-NaBF4).

Freons were frequently used for immersive cooling of eg. electronics.

Refrigerants are coolants used for reaching low temperatures by undergoing phase change between liquid and gas. Halomethanes were frequently used, most often R-12 and R-22, but due to environmental concerns are being phased out, often with liquified propane or other haloalkanes like R-134a. Anhydrous ammonia is frequently used in

large commercial systems, and sulfur dioxide was used in early mechanical refrigerators. Carbon dioxide (R-744) is used as a working fluid in climate control systems for cars, residential air conditioning, commercial refrigeration, and vending machines.

Heat pipes are a special application of refrigerants.

Liquid gases are used as coolants for cryogenic applications, including cryo-electron microscopy, overclocking of computer processors, applications using superconductors, or extremely sensitive sensors and very low-noise amplifiers. The most common and least expensive coolant in use is liquid nitrogen which boils at about -196 C (77K). Liquid air is used to lower degree, due to its oxygen content which makes it prone to cause fire or explosions when in contact with combustible materials. Lower temperatures can be reached using liquefied neon which boils at about -246 C. The lowest temperatures, used for the most powerful superconducting magnets, are reached using liquid helium.

Fuels are frequently used as coolants for engines. A cold fuel flows over some parts of the engine, absorbing its waste heat and being preheated before combustion. Kerosene and other jet fuels frequently serve in this role in aviation engines and liquid hydrogen is used both as a fuel and as a coolant to cool nozzles and chambers of rocket engines.

[edit] Notes

1. ̂ Globalspec.com 2. ̂ Paratherm Corporation

Neutron moderator

From Wikipedia, the free encyclopedia

Jump to: navigation, search

This article needs additional citations for verification.Please help improve this article by adding reliable references. Unsourced material may be challenged and removed. (February 2008)

In nuclear engineering, a neutron moderator is a medium which reduces the velocity of fast neutrons, thereby turning them into thermal neutrons capable of sustaining a nuclear chain reaction involving uranium-235.

Commonly used moderators include regular (light) water (75% of the world's reactors), solid graphite (20% of reactors) and heavy water (5% of reactors).[1] Beryllium has also been used in some experimental types, and hydrocarbons have been suggested as another possibility.

Contents

[hide] 1 Explanation 2 Form and location 3 Moderator impurities 4 Non graphite moderators 5 Materials used 6 References

7 See also

[edit] Explanation

In a thermal nuclear reactor, the nucleus of a heavy fuel element such as uranium absorbs a slow-moving free neutron, becomes unstable, and then splits ("fissions") into two smaller atoms ("fission products"). The fission process for uranium atoms yields two fission products, two to three fast-moving free neutrons, plus an amount of energy primarily manifested in the kinetic energy of the recoiling fission products. Because more free neutrons are released from a uranium fission event than thermal neutrons are required to initiate the event, the reaction can become self sustaining — a chain reaction — under controlled conditions, thus liberating a tremendous amount of energy. However, the probability of further fission events occurring is dependent upon the speed (energy) of the incident neutrons. Faster neutrons are much less likely to cause further fission. (Note: It is not impossible for fast neutrons to cause fission, just much less likely.) The newly-released fast neutrons, moving at roughly 10% of the speed of light, must be slowed down or "moderated", typically to speeds of a few kilometers per second, if they are to be likely to cause further fission in neighbouring uranium nuclei and hence continue the chain reaction.

A good neutron moderator is a material full of atoms with light nuclei which do not easily absorb neutrons. The neutrons strike the nuclei and bounce off. In this process, some energy is transferred between the nucleus and the neutron. More energy is transferred per collision if the nucleus is lighter; see elastic collision. After sufficiently many such impacts, the velocity of the neutron will be comparable to the thermal velocities of the nuclei; this neutron is then called a thermal neutron.

A fast reactor uses no moderator, but relies on fission produced by unmoderated fast neutrons to sustain the chain reaction.

In all moderated reactors, some neutrons of all energy levels will produce fission, including fast neutrons. Some reactors are more fully thermalised than others; For example in a CANDU reactor nearly all fission reactions are produced by thermal neutrons, while in a pressurized water reactor (PWR) a considerable portion of the fissions are produced by higher-energy neutrons. In the proposed water-cooled

supercritical water reactor (SCWR), the proportion of fast fissions may exceed 50%, making it technically a fast neutron reactor.

Moderators are also used in non-reactor neutron sources, such as plutonium-beryllium and spallation sources.

[edit] Form and location

The form and location of the moderator can greatly influence the cost and safety of a reactor. Classically, moderators were precision-machined blocks with embedded ducting to carry away heat. Also, they were in the hottest part of the reactor, and therefore subject to corrosion and ablation. In some materials, notably graphite, the impact of the neutrons with the moderator can cause the moderator to accumulate dangerous amounts of Wigner energy. At Windscale, this problem led to the infamous Windscale fire.

Some pebble-bed reactors' moderators are not only simple, but also inexpensive: the nuclear fuel is embedded in spheres of reactor-grade pyrolytic carbon, roughly of the size of tennis balls. The spaces between the balls serve as ducting. The reactor is operated above the Wigner annealing temperature so that the graphite does not accumulate dangerous amounts of Wigner energy.

[edit] Moderator impurities

Good moderators are also free of neutron-absorbing impurities such as boron. In commercial nuclear power plants the moderator typically contains dissolved boron. The boron concentration of the reactor coolant can be changed by the operators by adding boric acid or by diluting with water to manipulate reactor power. The German World War II nuclear program suffered a substantial setback when its inexpensive graphite moderators failed to work. At that time, most graphites were deposited on boron electrodes, and the German commercial graphite contained too much boron. Since the war-time German program never discovered this problem, they were forced to use far more expensive heavy water moderators. In the U.S., Leo Szilard, a former chemical engineer, discovered the problem.

[edit] Non graphite moderators

Some moderators are quite expensive, for example beryllium, and reactor grade heavy water. Reactor-grade heavy water must be 99.75% pure to enable reactions with unenriched uranium. This is difficult to prepare because heavy water and regular water form the same chemical bonds in almost the same ways, at only slightly different speeds.

The much cheaper light water moderator ( essentially very pure regular water ) absorbs too many neutrons to be used with unenriched natural uranium, and therefore uranium enrichment or nuclear reprocessing becomes necessary to operate such reactors, increasing overall costs. Both enrichment and reprocessing are expensive and

technologically challenging processes, and additionally both enrichment and several types of reprocessing can be used to create weapons-usable material, causing proliferation concerns. Reprocessing schemes that are more resistant to proliferation are currently under development.

The CANDU reactor's moderator doubles as a safety feature. A large tank of low-temperature, low-pressure heavy water moderates the neutrons and also acts as a heat sink in extreme loss-of-coolant accident conditions. It is separated from the fuel rods that actually generate the heat. Heavy water is very effective at slowing down (moderating) neutrons, giving CANDU reactors their important and defining characteristic of high "neutron economy".

[edit] Materials used

Hydrogen , as in ordinary water ("light water"), in light water reactors. The reactors require enriched uranium to operate.

o There are also proposals to use the compound formed by the chemical reaction of metallic uranium and hydrogen (uranium hydride--UH3) as a combination fuel and moderator in a new type of reactor.

o Hydrogen is also used in the form of cryogenic liquid methane and sometimes liquid hydrogen as a cold neutron source in some research reactors: yielding a Maxwell–Boltzmann distribution for the neutrons whose maximum is shifted to much lower energies.

Deuterium , in the form of heavy water, in heavy water reactors, e.g. CANDU. Reactors moderated with heavy water can use unenriched natural uranium.

Carbon , in the form of reactor-grade graphite or pyrolytic carbon, used in e.g. RBMK and pebble-bed reactors, or in compounds, e.g. carbon dioxide [1]. Lower-temperature reactors are susceptible to buildup of Wigner energy in the material. Like deuterium-moderated reactors, some of these reactors can use unenriched natural uranium.

o Graphite is also deliberately allowed to be heated to around 2000 K or higher in some research reactors to produce a hot neutron source: giving a Maxwell–Boltzmann distribution whose maximum is spread out to generate higher energy neutrons.

Beryllium , in the form of metal. Beryllium is expensive and toxic, so its use is limited.

Lithium -7, in the form of a fluoride salt, typically in conjunction with beryllium fluoride salt (FLiBe). This is the most common type of moderator in a Molten Salt Reactor.

Other light-nuclei materials are unsuitable for various reasons. Helium is a gas and is not possible to achieve its sufficient density, lithium-6 and boron absorb neutrons.

A control rod is a rod made of chemical elements capable of absorbing many neutrons without fissioning themselves. They are used in nuclear reactors to control the rate of fission of uranium and plutonium. Because these elements have different capture cross sections for neutrons of varying energies, the compositions of the control rods must be designed for the neutron spectrum of the reactor it is supposed to control. Light water reactors (BWR, PWR) operate with "thermal" neutrons, breeder reactors with "fast" neutrons.

Operation principle

Control rods are usually combined into control rod assemblies — typically 20 rods for a commercial Pressurized Water Reactor (PWR) assembly — and inserted into guide tubes within a fuel element. A control rod is removed from or inserted into the central core of a nuclear reactor in order to control the neutron flux — increase or decrease the number of neutrons which will split further uranium atoms. This in turn affects the thermal power of the reactor, the amount of steam generated, and hence the electricity produced.

Control rods often stand vertically within the core. In pressurised water reactors (PWR), they are inserted from above, the control rod drive mechanisms being mounted on the reactor pressure vessel head. Due to the necessity of a steam dryer above the core of a boiling water reactor (BWR) this design requires insertion of the control rods from un derneath the core. The control rods are partially removed from the core to allow a chain reaction to occur. The number of control rods inserted and the distance by which they are inserted can be varied to control the reactivity of the reactor.

[edit] Materials used

Chemical elements with a sufficiently high capture cross section for neutrons include silver, indium and cadmium. Other elements that can be used include boron, cobalt, hafnium, dysprosium, gadolinium, samarium, erbium, and europium, or their alloys and compounds, e.g. high-boron steel, silver-indium-cadmium alloy, boron carbide, zirconium diboride, titanium diboride, hafnium diboride, gadolinium titanate, and dysprosium titanate. The choice of materials is influenced by the energy of neutrons in the reactor, their resistance to neutron-induced swelling, and the required mechanical and lifetime properties. The rods may have the form of stainless steel tubes filled with neutron absorbing pellets or powder. The swelling of the material in the neutron flux can cause deformation of the rod, leading to its premature replacement. The burnup of the absorbing isotopes is another limiting lifetime factor.

Silver-indium-cadmium alloys, generally 80% Ag, 15% In, and 5% Cd, are a common control rod material for pressurized water reactors. The somewhat different energy absorption regions of the materials make the alloy an excellent neutron absorber. It has good mechanical strength and can be easily fabricated. It has to be encased in stainless steel to prevent corrosion in hot water.

Boron is another common neutron absorber. Due to different cross sections of 10B and 11B, boron containing materials enriched in 10B by isotopic separation are frequently used. The wide absorption spectrum of boron makes it suitable also as a neutron shield. Mechanical properties of boron in its elementary form are unfavorable, therefore alloys or compounds have to be used instead. Common choices are high-boron steel and boron carbide. Boron carbide is used as a control rod material in both pressurized water reactors and boiling water reactors.

Hafnium has excellent properties for reactors using water for both moderation and cooling. It has good mechanical strength, can be easily fabricated, and is resistant to corrosion in hot water. [1] Hafnium can be alloyed with small amounts of other elements; e.g. tin and oxygen to increase tensile and creep strength, iron, chromium and niobium for corrosion resistance, and molybdenum for wear resistance, hardness, and machineability. Some such alloys are designated as Hafaloy, Hafaloy-M, Hafaloy-N, and Hafaloy-NM. [1][] Its high cost and low availability limit its use in civilian reactors, though it is used in some US Navy reactors.

Dysprosium titanate is a new material currently undergoing evaluation for pressurized water control rods. Dysprosium titanate is a promising replacement for Ag-In-Cd alloys due to its much higher melting point, no tendency to react with cladding materials, simple fabrication, non-radioactive waste, no swelling, and no outgassing. It was developed in Russia, and is recommended by some for VVER and RBMK reactors. [2]

Hafnium diboride is another such new material. It can be used standalone or prepared in a sintered mixture of hafnium and boron carbide powders. [3]

[edit] Additional means of reactivity regulation

Usually there are also other means of controlling reactivity: In the PWR design a soluble neutron absorber (boric acid) is added to the reactor coolant allowing the complete extraction of the control rods during stationary power operation ensuring an even power and flux distribution over the entire core. This chemical shim, along with the use of burnable neutron poisons within the fuel pellets, is used to assist regulation of the long term reactivity of the core,[4] while the control rods are used for rapid changes to the reactor power (e.g. shutdown and start up). Operators of BWRs use the coolant flow through the core to control reactivity by varying the speed of the reactor recirculation pumps (an increase in coolant flow through the core improves the removal of steam bubbles, thus increasing the density of the coolant/moderator with the result of increasing power).

[edit] Safety

In most reactor designs, as a safety measure, control rods are attached to the lifting machinery by electromagnets, rather than direct mechanical linkage. This means that automatically in the event of power failure, or if manually invoked due to failure of the lifting machinery, the control rods will fall, under gravity, fully into the pile to stop the

reaction. A notable exception to this fail-safe mode of operation is the BWR which requires the hydraulical insertion of control rods in the event of an emergency shut-down, using water from a special tank that is under high nitrogen pressure. Quickly shutting down a reactor in this way is called Scramming the reactor.

Urban legend has it that the control rods hung above the reactor, suspended by a rope. In an emergency a person assigned to the job would take a fire axe and cut the rope, allowing the rods to fall into the reactor and stop the fission. At some point the title of the person assigned this duty was given as SCRAM, or Safety Control Rod Ax Man (although this may be a backronym). This term continues to be in use today for shutting down a reactor by dropping the control rods.

[edit] Criticality accident prevention

Mismanagement or control rod failure was often the cause or aggravating factor for nuclear accidents, including the SL-1 explosion and the Chernobyl disaster.

The absorption cross section for 10B (top) and 11B (bottom) as a function of energy

Homogeneous neutron absorbers have often been used to manage criticality accidents which involve aqueous solutions of fissile metals, in several such accidents either borax (sodium borate) or a cadmium compound has been added to the system. The cadmium can be added as a metal to nitric acid solutions of fissile material, the corrosion of the cadmium in the acid will then generate cadmium nitrate in situ.

In carbon dioxide-cooled reactors such as the AGR, if the solid control rods were to fail to arrest the nuclear reaction nitrogen gas can be injected into the primary coolant cycle. This is because nitrogen has a larger absorption cross-section for neutrons than carbon or oxygen, hence the core would then become less reactive.

As the neutron energy increases the neutron cross section of most isotopes decreases. The boron isotope 10B is responsible for the majority of the neutron absorption. Boron containing materials can be used as neutron shields to reduce the activation of objects close to a reactor core.

Reactor vessel

In a nuclear power plant, the reactor vessel is a pressure vessel containing the coolant and reactor core.

Not all power reactors have a reactor vessel. Power reactors are generally classified by the type of coolant rather than the by the configuration of the reactor vessel used to contain the coolant. The classifications are:

Light water reactor - Includes the PWR, BWR. Vast majority of nuclear power reactors are of this type.

Graphite moderated reactor - Includes the Chernobyl Reactor RBMK that has a highly unusual reactor configuration compared to the vast majority of nuclear powerplants in Russia or around the world.

Gas cooled thermal reactor - Includes the AGR,the Gas Cooled Fast Breeder Reactor or GCFR, and the HTGR. An example of a Gas Cooled Reactor is the British Magnox.

Heavy water reactor - Utilize heavy water, or water with a higher than normal proportion of the hydrogen isotope deuterium in some manner, however it should be noted that D2O (heavy water) is more expensive and may be used as a main component, but not necessarily as a coolant in this case. An example of a heavy water reactor is Canada's CANDU reactor.

Liquid metal cooled reactor - Utilize a liquid metal, such as sodium or a lead-bismuth alloy to cool the reactor core.

Molten Salt Reactors - Special organic coolants are used in this unique design, such as the MSBR.

Of the main classes of reactor with a pressure vessel, the PWR is unique in that the pressure vessel suffers significant neutron irradiation (called fluence) during operation, and may become brittle over time as a result. In particular, the larger pressure vessel of the BWR is better shielded from the neutron flux, so although more expensive to manufacture in the first place because of this extra size, it has an advantage in not needing annealing to extend its life.

Annealing of PWR reactor vessels to extend their working life is a complex and high-value technology being actively developed by both nuclear service providers (AREVA) and operators of PWRs.

Boiler feedwater pump

From Wikipedia, the free encyclopedia

Jump to: navigation, search

A boiler feedwater pump is a specific type of pump used to pump feedwater into a steam boiler. The water may be freshly supplied or returning condensate produced as a result of the condensation of the steam produced by the boiler. These pumps are normally high pressure units that use suction from a condensate return system and can be of the centrifugal pump type or positive displacement type.

[edit] Construction and operation

Feedwater pumps range in size up to many horsepower and the electric motor is usually separated from the pump body by some form of mechanical coupling. Large industrial condensate pumps may also serve as the feedwater pump. In either case, to force the

water into the boiler, the pump must generate sufficient pressure to overcome the steam pressure developed by the boiler. This is usually accomplished through the use of a centrifugal pump.

Feedwater pumps usually run intermittently and are controlled by a float switch or other similar level-sensing device energizing the pump when it detects a lowered liquid level in the boiler. The pump then runs until the level of liquid in the boiler is substantially increased. Some pumps contain a two-stage switch. As liquid lowers to the trigger point of the first stage, the pump is activated. If the liquid continues to drop (perhaps because the pump has failed, its supply has been cut off or exhausted, or its discharge is blocked), the second stage will be triggered. This stage may switch off the boiler equipment (preventing the boiler from running dry and overheating), trigger an alarm, or both.

[edit] Steam-powered pumps

Steam locomotives and the steam engines used on ships and stationary applications such as power plants also required feedwater pumps. In this situation, though, the pump was often powered using a small steam engine that ran using the steam produced by the boiler. A means had to be provided, of course, to put the initial charge of water into the boiler (before steam power was available to operate the steam-powered feedwater pump). The pump was often a positive displacement pump that had steam valves and cylinders at one end and feedwater cylinders at the other end; no crankshaft was required.

Steam generator (nuclear power)

From Wikipedia, the free encyclopedia

Jump to: navigation, searchThis is an article about nuclear power plant equipment. For other uses, see steam generator.

The bend at the top of an old nuclear power plant steam generator, image courtesy of the NRC.

Steam generators are heat exchangers used to convert water into steam from heat produced in a nuclear reactor core. They are used in pressurized water reactors between the primary and secondary coolant loops.

In commercial power plants steam generators can measure up to 70 feet in height and weigh as much as 800 tons. Each steam generator can contain anywhere from 3,000 to 16,000 tubes, each about three-quarters of an inch in diameter. The coolant is pumped, at high pressure to prevent boiling, from the reactor coolant pump, through the nuclear reactor core, and through the tube side of the steam generators before returning to the pump. This is referred to as the primary loop. That water flowing through the steam generator boils water on the shell side to produce steam in the secondary loop that is delivered to the turbines to make electricity. The steam is subsequently condensed via cooled water from the tertiary loop and returned to the steam generator to be heated once again. The tertiary cooling water may be recirculated to cooling towers where it sheds waste heat before returning to condense more steam. Once through tertiary cooling may otherwise be provided by a river, lake, ocean. This primary, secondary, tertiary cooling scheme is the most common way to extract usable energy from a controlled nuclear reaction.

These loops also have an important safety role because they constitute one of the primary barriers between the radioactive and non-radioactive sides of the plant as the primary coolant becomes radioactive from its exposure to the core. For this reason, the integrity of the tubing is essential in minimizing the leakage of water between the two sides of the plant. There is the potential that if a tube bursts while a plant is operating; contaminated steam could escape directly to the secondary cooling loop. Thus during scheduled maintenance outages or shutdowns, some or all of the steam generator tubes are inspected by eddy-current testing.

In other types of reactors, such as the pressurised heavy water reactors of the CANDU design, the primary fluid is heavy water. Liquid metal cooled reactors such as the in Russian BN-600 reactor also use heat exchangers between primary metal coolant and at the secondary water coolant.

Boiling water reactors do not use steam generators, as steam is produced in the pressure vessel.

[edit] Types

Westinghouse and Combustion Engineering designs have vertical U-tubes with inverted tubes for the primary water. Canadian, Japanese, French, and German PWR suppliers use the vertical configuration as well. Russian VVER reactor designs use horizontal steam generators, which have the tubes mounted horizontally. Babcock and Wilcox plants (e.g., Three Mile Island) have smaller steam generators that force water through the top of the OTSGs (once-through steam generators; counter-flow to the feedwater) and out the bottom to be recirculated by the reactor coolant pumps. The horizontal design has proven to be less susceptible to degradation than the vertical U-tube design.

[edit] Tube material

Various high-performance alloys and superalloys have been used for steam generator tubing, including type 316 stainless steel, Alloy 400, Alloy 600MA (mill annealed), Alloy 600TT (thermally treated), Alloy 690TT, and Alloy 800Mod.

Steam turbine

A steam turbine is a mechanical device that extracts thermal energy from pressurized steam, and converts it into useful mechanical work.

It has almost completely replaced the reciprocating piston steam engine (invented by Thomas Newcomen and greatly improved by James Watt) primarily because of its greater thermal efficiency and higher power-to-weight ratio. Because the turbine generates rotary motion, it is particularly suited to be used to drive an electrical generator - about 80% of all electric generation in the world is by use of steam turbines. The steam turbine is a form of heat engine that derives much of its improvement in thermodynamic efficiency through the use of multiple stages in the expansion of the steam (as opposed to the one stage in the Watt engine), which results in a closer approach to the ideal reversible process.

Contents

[hide] 1 History 2 Types

o 2.1 Steam Supply and Exhaust Conditions o 2.2 Casing or Shaft Arrangements

3 Principle of Operation and Design o 3.1 Turbine Efficiency

3.1.1 Impulse Turbines 3.1.2 Reaction Turbines

o 3.2 Operation and Maintenance o 3.3 Speed regulation

4 Direct drive 5 Speed reduction

6 References

[edit] History

The first device that may be classified as a reaction steam turbine was little more than a toy, the classic Aeolipile, described in the 1st century by Hero of Alexandria in Roman

Egypt.[1][2][3] A thousand years later, the first impact steam turbine with practical applications was invented in 1551 by Taqi al-Din in Ottoman Egypt, who described it as a prime mover for rotating a spit. Similar smoke jacks were later described by John Wilkins in 1648 and Samuel Pepys in 1660. Another steam turbine device was created by Italian Giovanni Branca in 1629.[4]

The modern steam turbine was invented in 1884 by the Englishman Charles A. Parsons, whose first model was connected to a dynamo that generated 7.5 kW of electricity. His patent was licensed and the turbine scaled-up shortly after by an American, George Westinghouse. A number of other variations of turbines have been developed that work effectively with steam. The de Laval turbine (invented by Gustaf de Laval) accelerated the steam to full speed before running it against a turbine blade. This was good, because the turbine is simpler, less expensive and does not need to be pressure-proof. It can operate with any pressure of steam. It is also, however, considerably less efficient. The Parson's turbine also turned out to be relatively easy to scale-up. Within Parson's lifetime the generating capacity of a unit was scaled-up by about 10,000 times. [5]

Parsons turbine from the Polish destroyer ORP Wicher II

[edit] Types

Steam turbines are made in a variety of sizes ranging from small 1 hp (0.75 kW) units (rare) used as mechanical drives for pumps, compressors and other shaft driven equipment, to 2,000,000 hp (1,500,000 kW) turbines used to generate electricity. There are several classifications for modern steam turbines.

[edit] Steam Supply and Exhaust Conditions

These types include condensing, noncondensing, reheat, extraction and induction.

Noncondensing or backpressure turbines are most widely used for process steam applications. The exhaust pressure is controlled by a regulating valve to suit the needs of the process steam pressure. These are commonly found at refineries, district heating

units, pulp and paper plants, and desalination facilities where large amounts of low pressure process steam are available.

Condensing turbines are most commonly found in electrical power plants. These turbines exhaust steam in a partially condensed state, typically of a quality near 90%, at a pressure well below atmospheric to a condenser.

Reheat turbines are also used almost exclusively in electrical power plants. In a reheat turbine, steam flow exits from a high pressure section of the turbine and is returned to the boiler where additional superheat is added. The steam then goes back into an intermediate pressure section of the turbine and continues its expansion.

Extracting type turbines are common in all applications. In an extracting type turbine, steam is released from various stages of the turbine, and used for industrial process needs or sent to boiler feedwater heaters to improve overall cycle efficiency. Extraction flows may be controlled with a valve, or left uncontrolled. Induction turbines introduce low pressure steam at an intermediate stage to produce additional power.

[edit] Casing or Shaft Arrangements

These arrangements include single casing, tandem compound and cross compound turbines. Single casing units are the most basic style where a single casing and shaft are coupled to a generator. Tandem compound are used where two or more casings are directly coupled together to drive a single generator. A cross compound turbine arrangement features two or more shafts not in line driving two or more generators that often operate at different speeds. A cross compound turbine is typically used for many large applications.

[edit] Principle of Operation and Design

An ideal steam turbine is considered to be an isentropic process, or constant entropy process, in which the entropy of the steam entering the turbine is equal to the entropy of the steam leaving the turbine. No steam turbine is truly “isentropic”, however, with typical isentropic efficiencies ranging from 20%-90% based on the application of the turbine. The interior of a turbine comprises several sets of blades, or “buckets” as they are more commonly referred to. One set of stationary blades is connected to the casing and one set of rotating blades is connected to the shaft. The sets intermesh with certain minimum clearances, with the size and configuration of sets varying to efficiently exploit the expansion of steam at each stage.

[edit] Turbine Efficiency

Schematic diagram outlining the difference between an impulse and a reaction turbine

To maximize turbine efficiency, the steam is expanded, generating work, in a number of stages. These stages are characterized by how the energy is extracted from them and are known as impulse or reaction turbines. Most modern steam turbines are a combination of the reaction and impulse design. Typically, higher pressure sections are impulse type and lower pressure stages are reaction type.

[edit] Impulse Turbines

An impulse turbine has fixed nozzles that orient the steam flow into high speed jets. These jets contain significant kinetic energy, which the rotor blades, shaped like buckets, convert into shaft rotation as the steam jet changes direction. A pressure drop occurs across only the stationary blades, with a net increase in steam velocity across the stage.

As the steam flows through the nozzle its pressure falls from steam chest pressure to condenser pressure (or atmosphere pressure). Due to this relatively higher ratio of expansion of steam in the nozzle the steam leaves the nozzle with a very high velocity. The steam leaving the moving blades is a large portion of the maximum velocity of the steam when leaving the nozzle. The loss of energy due to this higher exit velocity is commonly called the "carry over velocity" or "leaving loss".

[edit] Reaction Turbines

In the reaction turbine, the rotor blades themselves are arranged to form convergent nozzles. This type of turbine makes use of the reaction force produced as the steam accelerates through the nozzles formed by the rotor. Steam is directed onto the rotor by the fixed vanes of the stator. It leaves the stator as a jet that fills the entire circumference of the rotor. The steam then changes direction and increases its speed relative to the speed of the blades. A pressure drop occurs across both the stator and the rotor, with steam accelerating through the stator and decelerating through the rotor, with no net change in steam velocity across the stage but with a decrease in both pressure and temperature, reflecting the work performed in the driving of the rotor.

[edit] Operation and Maintenance

When warming up a steam turbine for use, the main steam stop valves (after the boiler) have a bypass line to allow superheated steam to slowly bypass the valve and proceed to heat up the lines in the system along with the steam turbine. Also a turning gear is engaged when there is no steam to the turbine to slowly rotate the turbine to ensure even heating to prevent uneven expansion. After first rotating the turbine by the turning gear, allowing time for the rotor to assume a straight plane (no bowing), then the turning gear is disengaged and steam is admitted to the turbine, first to the astern blades then to the ahead blades slowly rotating the turbine at 10 to 15 RPM to slowly warm the turbine.

Problems with turbines are now rare and maintenance requirements are relatively small. Any imbalance of the rotor can lead to vibration, which in extreme cases can lead to a blade letting go and punching straight through the casing. It is, however, essential that the

turbine be turned with dry steam. If water gets into the steam and is blasted onto the blades (moisture carryover) rapid impingement and erosion of the blades can occur, possibly leading to imbalance and catastrophic failure. Also, water entering the blades will likely result in the destruction of the thrust bearing for the turbine shaft. To prevent this, along with controls and baffles in the boilers to ensure high quality steam, condensate drains are installed in the steam piping leading to the turbine.

[edit] Speed regulation

The control of a turbine with a governor is essential, as turbines need to be run up slowly, to prevent damage while some applications (such as the generation of alternating current electricity) require precise speed control. Uncontrolled acceleration of the turbine rotor can lead to an overspeed trip, which causes the nozzle valves that control the flow of steam to the turbine to close. If this fails then the turbine may continue accelerating until it breaks apart, often spectacularly. Turbines are expensive to make, requiring precision manufacture and special quality materials.

[edit] Direct drive

Electrical power stations use large steam turbines driving electric generators to produce most (about 80%) of the world's electricity. Most of these centralised stations are of two types: fossil fuel power plants and nuclear power plants. The turbines used for electric power generation are most often directly coupled to their generators. As the generators must rotate at constant synchronous speeds according to the frequency of the electric power system, the most common speeds are 3000 r/min for 50 Hz systems, and 3600 r/min for 60 Hz systems. In installations with high steam output, as may be found in nuclear power stations, the generator sets may be arranged to operate at half these speeds, but with four-pole generators.[6]

[edit] Speed reduction

The Turbinia - the first steam turbine-powered ship

Another use of steam turbines is in ships; their small size, low maintenance, light weight, and low vibration are compelling advantages. (Steam turbine locomotives were also tested, but with limited success.) A steam turbine is only efficient when operating in the thousands of RPM range while application of the power in propulsion applications may

be only in the hundreds of RPM and so requiring that expensive and precise reduction gears must be used, although several ships, such as Turbinia, had direct drive from the steam turbine to the propeller shafts. This purchase cost is offset by much lower fuel and maintenance requirements and the small size of a turbine when compared to a reciprocating engine having an equivalent power.

Electrical generator

From Wikipedia, the free encyclopedia

Jump to: navigation, search

NRC image of Modern Steam Turbine Generator.

In electricity generation, an electrical generator is a device that converts mechanical energy to electrical energy, generally using electromagnetic induction. The reverse conversion of electrical energy into mechanical energy is done by a motor, motors and generators have many similarities. A generator forces electric charges to move through an external electrical circuit, but it does not create electricity or charge, which is already present in the wire of its windings. It is somewhat analogous to a water pump, which creates a flow of water but does not create the water inside. The source of mechanical energy may be a reciprocating or turbine steam engine, water falling through a turbine or waterwheel, an internal combustion engine, a wind turbine, a hand crank, the sun or solar energy, compressed air or any other source of mechanical energy.

Early 20th century alternator made in Budapest, Hungary, in the power generating hall of a hydroelectric station

Generator in Zwevegem, West Flanders, Belgium

Contents

[hide] 1 Historic developments

o 1.1 Jedlik's Dynamo o 1.2 Faraday's disk o 1.3 Dynamo o 1.4 Other rotating electromagnetic generators o 1.5 MHD generator

2 Terminology 3 Excitation 4 Equivalent circuit 5 Vehicle-mounted generators 6 Engine-generator 7 Human powered electrical generators 8 Patents 9 References 10 See also

11 External links

[edit] Historic developments

Before the connection between magnetism and electricity was discovered, electrostatic generators were invented that used electrostatic principles. These generated very high voltages and low currents. They operated by using moving electrically charged belts, plates and disks to carry charge to a high potential electrode. The charge was generated using either of two mechanisms:

Electrostatic induction The triboelectric effect, where the contact between two insulators leaves them

charged.

Because of their inefficiency and the difficulty of insulating machines producing very high voltages, electrostatic generators had low power ratings and were never used for

generation of commercially-significant quantities of electric power. The Wimshurst machine and Van de Graaff generator are examples of these machines that have survived.

[edit] Jedlik's Dynamo

Ányos Jedlik's single pole electric starter (dynamo) (1861)Main article: Jedlik's dynamo

Ányos Jedlik's well functioning electric car model in 1828.

In 1827, Hungarian Anyos Jedlik started experimenting with electromagnetic rotating devices which he called electromagnetic self-rotors. In the prototype of the single-pole electric starter (finished between 1852 and 1854) both the stationary and the revolving parts were electromagnetic. He formulated the concept of the dynamo at least 6 years before Siemens and Wheatstone but didn't patented it as he thought he wasn't the first to realize this. In essence the concept is that instead of permanent magnets, two electromagnets opposite to each other induce the magnetic field around the rotor. Jedlik's invention was decades ahead of its time.

[edit] Faraday's disk

Faraday disk

In 1831-1832 Michael Faraday discovered the operating principle of electromagnetic generators. The principle, later called Faraday's law, is that a potential difference is generated between the ends of an electrical conductor that moves perpendicular to a magnetic field. He also built the first electromagnetic generator, called the 'Faraday disc', a type of homopolar generator, using a copper disc rotating between the poles of a horseshoe magnet. It produced a small DC voltage, and large amounts of current.

This design was inefficient due to self-cancelling counterflows of current in regions not under the influence of the magnetic field. While current flow was induced directly underneath the magnet, the current would circulate backwards in regions outside the influence of the magnetic field. This counterflow limits the power output to the pickup wires, and induces waste heating of the copper disc. Later homopolar generators would solve this problem by using an array of magnets arranged around the disc perimeter to maintain a steady field effect in one current-flow direction.

Another disadvantage was that the output voltage was very low, due to the single current path through the magnetic flux. Experimenters found that using multiple turns of wire in a coil could produce higher more useful voltages. Since the output voltage is proportional to the number of turns, generators could be easily designed to produce any desired voltage by varying the number of turns. Wire windings became a basic feature of all subsequent generator designs.

However, recent advances (rare earth magnets) have made possible homo-polar motors with the magnets on the rotor, which should offer many advantages to older designs.

[edit] Dynamo

Main article Dynamo

Dynamos are no longer used for power generation due to the size and complexity of the commutator needed for high power applications. This large belt-driven high-current dynamo produced 310 amperes at 7 volts, or 2,170 watts, when spinning at 1400 RPM.

Dynamo Electric Machine [End View, Partly Section] (U.S. Patent 284,110   )

The Dynamo was the first electrical generator capable of delivering power for industry. The dynamo uses electromagnetic principles to convert mechanical rotation into a pulsing direct electric current through the use of a commutator. The first dynamo was built by Hippolyte Pixii in 1832.

Through a series of accidental discoveries, the dynamo became the source of many later inventions, including the DC electric motor, the AC alternator, the AC synchronous motor, and the rotary converter.

A dynamo machine consists of a stationary structure, which provides a constant magnetic field, and a set of rotating windings which turn within that field. On small machines the constant magnetic field may be provided by one or more permanent magnets; larger machines have the constant magnetic field provided by one or more electromagnets, which are usually called field coils.

Large power generation dynamos are now rarely seen due to the now nearly universal use of alternating current for power distribution and solid state electronic AC to DC power conversion. But before the principles of AC were discovered, very large direct-current dynamos were the only means of power generation and distribution. Now power generation dynamos are mostly a curiosity.

[edit] Other rotating electromagnetic generators

Without a commutator, the dynamo is an example of an alternator, which is a synchronous singly-fed generator. With an electromechanical commutator, the dynamo is

a classical direct current (DC) generator. The alternator must always operate at a constant speed that is precisely synchronized to the electrical frequency of the power grid for non-destructive operation. The DC generator can operate at any speed within mechanical limits but always outputs a direct current waveform.

Other types of generators, such as the asynchronous or induction singly-fed generator, the doubly-fed generator, or the brushless wound-rotor doubly-fed generator, do not incorporate permanent magnets or field windings (i.e, electromagnets) that establish a constant magnetic field, and as a result, are seeing success in variable speed constant frequency applications, such as wind turbines or other renewable energy technologies.

The full output performance of any generator can be optimized with electronic control but only the doubly-fed generators or the brushless wound-rotor doubly-fed generator incorporate electronic control with power ratings that are substantially less than the power output of the generator under control, which by itself offer cost, reliability and efficiency benefits.

[edit] MHD generator

A magnetohydrodynamic generator directly extracts electric power from moving hot gases through a magnetic field, without the use of rotating electromagnetic machinery. MHD generators were originally developed because the output of a plasma MHD generator is a flame, well able to heat the boilers of a steam power plant. The first practical design was the AVCO Mk. 25, developed in 1965. The U.S. government funded substantial development, culminating in a 25Mw demonstration plant in 1987. In the Soviet Union from 1972 until the late 1980's, the MHD plant U 25 was in regular commercial operation on the Moscow power system with a rating of 25 MW, the largest MHD plant rating in the world at that time. [1] MHD generators operated as a topping cycle are currently (2007) less efficient than combined-cycle gas turbines.

[edit] Terminology

The two main parts of a generator or motor can be described in either mechanical or electrical terms:

Mechanical:

Rotor: The rotating part of an alternator, generator, dynamo or motor. Stator: The stationary part of an alternator, generator, dynamo or motor.

Electrical:

Armature: The power-producing component of an alternator, generator, dynamo or motor. In a generator, alternator, or dynamo the armature windings generate the electrical current. The armature can be on either the rotor or the stator.

Field: The magnetic field component of an alternator, generator, dynamo or motor. The magnetic field of the dynamo or alternator can be provided by either electromagnets or permanent magnets mounted on either the rotor or the stator. (For a more technical discussion, refer to the Field coil article.)

Because power transferred into the field circuit is much less than in the armature circuit, AC generators nearly always have the field winding on the rotor and the stator as the armature winding. Only a small amount of field current must be transferred to the moving rotor, using slip rings. Direct current machines necessarily have the commutator on the rotating shaft, so the armature winding is on the rotor of the machine.

[edit] Excitation

A small early 1900s 75 KVA direct-driven power station AC alternator, with a separate belt-driven exciter generator.

Main article Excitation (magnetic)

An electric generator or electric motor that uses field coils rather than permanent magnets will require a current flow to be present in the field coils for the device to be able to work. If the field coils are not powered, the rotor in a generator can spin without producing any usable electrical energy, while the rotor of a motor may not spin at all. Very large power station generators often utilize a separate smaller generator to excite the field coils of the larger.

In the event of a severe widespread power outage where islanding of power stations has occurred, the stations may need to perform a black start to excite the fields of their largest generators, in order to restore customer power service.

[edit] Equivalent circuit

Equivalent circuit of generator and load.G = generatorVG=generator open-circuit voltageRG=generator internal resistanceVL=generator on-load voltageRL=load resistance

The equivalent circuit of a generator and load is shown in the diagram to the right. To determine the generator's VG and RG parameters, follow this procedure: -

Before starting the generator, measure the resistance across its terminals using an ohmmeter. This is its DC internal resistance RGDC.

Start the generator. Before connecting the load RL, measure the voltage across the generator's terminals. This is the open-circuit voltage VG.

Connect the load as shown in the diagram, and measure the voltage across it with the generator running. This is the on-load voltage VL.

Measure the load resistance RL, if you don't already know it. Calculate the generator's AC internal resistance RGAC from the following formula:

Note 1: The AC internal resistance of the generator when running is generally slightly higher than its DC resistance when idle. The above procedure allows you to measure both values. For rough calculations, you can omit the measurement of RGAC and assume that RGAC and RGDC are equal.

Note 2: If the generator is an AC type, use an AC voltmeter for the voltage measurements.

The maximum power theorem states that the maximum power can be obtained from the generator by making the resistance of the load equal to that of the generator. This is inefficient since half the power is wasted in the generator's internal resistance; practical electric power generators operate with load resistance much higher than internal resistance, so the efficiency is greater.

[edit] Vehicle-mounted generators

Early motor vehicles until about the 1960s tended to use DC generators with electromechanical regulators. These have now been replaced by alternators with built-in rectifier circuits, which are less costly and lighter for equivalent output. Automotive alternators power the electrical systems on the vehicle and recharge the battery after starting. Rated output will typically be in the range 50-100 A at 12 V, depending on the designed electrical load within the vehicle. Some cars now have electrically-powered steering assistance and air conditioning, which places a high load on the electrical system. Large commercial vehicles are more likely to use 24 V to give sufficient power at the starter motor to turn over a large diesel engine. Vehicle alternators do not use permanent magnets and are typically only 50-60% efficient over a wide speed range.[2]

Motorcycle alternators often use permanent magnet stators made with rare earth magnets, since they can be made smaller and lighter than other types. See also hybrid vehicle.

Some of the smallest generators commonly found power bicycle lights. These tend to be 0.5 ampere, permanent-magnet alternators supplying 3-6 W at 6 V or 12 V. Being powered by the rider, efficiency is at a premium, so these may incorporate rare-earth magnets and are designed and manufactured with great precision. Nevertheless, the maximum efficiency is only around 60% for the best of these generators - 40% is more typical - due to the use of permanent magnets. A battery would be required in order to use a controllable electromagnetic field instead, and this is unacceptable due to its weight and bulk.

Sailing yachts may use a water or wind powered generator to trickle-charge the batteries. A small propeller, wind turbine or impeller is connected to a low-power alternator and rectifier to supply currents of up to 12 A at typical cruising speeds.

[edit] Engine-generator

Main article: Engine-generator

An engine-generator is the combination of an electrical generator and an engine (prime mover) mounted together to form a single piece of self-contained equipment. The engines used are usually piston engines, but gas turbines can also be used. Many different versions are available - ranging from very small portable petrol powered sets to large turbine installations.

[edit] Human powered electrical generators

Main article: Self-powered equipment

A generator can also be driven by human muscle power (for instance, in field radio station equipment).

Human powered direct current generators are commercially available, and have been the project of some DIY enthusiasts. Typically operated by means of pedal power, a converted bicycle trainer, or a foot pump, such generators can be practically used to charge batteries, and in some cases are designed with an integral inverter. The average adult could generate about 125-200 watts on a pedal powered generator. Portable radio receivers with a crank are made to reduce battery purchase requirements, see clockwork radio.

[edit] Patents

U.S. Patent 222,881   -- Magneto-Electric Machines : Thomas Edison's main continuous current dynamo. The device's nickname was the "long-legged Mary-Ann". This device has large bipolar magnets. It is inefficient.

U.S. Patent 373,584   -- Dynamo-Electric Machine - Edison's improved dynamo which includes an extra coil and utilizes a field of force.

U.S. Patent 359,748   -- Dynamo Electric Machine - Nikola Tesla's construction of the alternating current induction motor / generator.

U.S. Patent 406,968   -- Dynamo Electric Machine - Tesla's "Unipolar" machine (i.e., a disk or cylindrical conductor is mounted in between magnetic poles adapted to produce a uniform magnetic field).

U.S. Patent 417,794   -- Armature for Electric Machines -Tesla's construction principles of the armature for electrical generators and motors. (Related to patents numbers US327797, US292077, and GB9013.)

U.S. Patent 447,920   -- Method of Operating Arc-Lamps - Tesla's alternating current generator of high frequency alternations (or pulsations) above the auditory level.

U.S. Patent 447,921   -- Alternating Electric Current Generator - Tesla's generator that produces alternations of 15000 per second or more.

Surface condenser

From Wikipedia, the free encyclopedia

  (Redirected from Condenser (steam turbine))Jump to: navigation, search

Surface condenser is the commonly used term for a water cooled shell and tube heat exchanger installed on the exhaust steam from a steam turbine in thermal power stations.[1][2][3] These condensers are heat exchangers which convert steam from its gaseous to its liquid state at a pressure below atmospheric pressure. Where cooling water is in short supply, an air-cooled condenser is often used. An air-cooled condenser is however significantly more expensive and cannot achieve as low a steam turbine exhaust pressure as a surface condenser.

Surface condensers are also used in applications and industries other than the condensing of steam turbine exhaust in power plants.

Contents

[hide] 1 Purpose 2 Why is it required? 3 Diagram of water-cooled surface condenser

o 3.1 Shell o 3.2 Vacuum system

o 3.3 Tube sheets o 3.4 Tubes o 3.5 Waterboxes

4 Corrosion o 4.1 Effects of corrosion o 4.2 Protection from corrosion

5 Other applications of surface condensers 6 See also 7 References

8 External links

[edit] Purpose

In thermal power plants, the primary purpose of a surface condenser is to condense the exhaust steam from a steam turbine to obtain maximum efficiency and also to convert the turbine exhaust steam into pure water (referred to as steam condensate) so that it may be reused in the steam generator or boiler as boiler feed water.

[edit] Why is it required?

The steam turbine itself is a device to convert the heat in steam to mechanical power. The difference between the heat of steam per unit weight at the inlet to the turbine and the heat of steam per unit weight at the outlet to the turbine represents the heat which is converted to mechanical power. Therefore, the more the conversion of heat per pound or kilogram of steam to mechanical power in the turbine, the better is its efficiency. By condensing the exhaust steam of a turbine at a pressure below atmospheric pressure, the steam pressure drop between the inlet and exhaust of the turbine is increased, which increases the amount of heat available for conversion to mechanical power. Most of the heat liberated due to condensation of the exhaust steam is carried away by the cooling medium (water or air) used by the surface condenser.

[edit] Diagram of water-cooled surface condenser

Diagram of a typical water-cooled surface condenser

The adjacent diagram depicts a typical water-cooled surface condenser as used in power stations to condense the exhaust steam from a steam turbine driving an electrical generator as well in other applications.[2][3][4][5] There are many fabrication design variations depending on the manufacturer, the size of the steam turbine, and other site-specific conditions.

[edit] Shell

The shell is the condenser's outermost body and contains the heat exchanger tubes. The shell is fabricated from carbon steel plates and is stiffened as needed to provide rigidity for the shell. When required by the selected design, intermediate plates are installed to serve as baffle plates that provide the desired flow path of the condensing steam. The plates also provide support that help prevent sagging of long tube lengths.

At the bottom of the shell, where the condensate collects, an outlet is installed. In some designs, a sump (often referred to as the hotwell) is provided. Condensate is pumped from the outlet or the hotwell for reuse as boiler feedwater.

For most water-cooled surface condensers, the shell is under vacuum during normal operating conditions.

[edit] Vacuum system

Diagram of a typical modern injector or ejector. For a steam ejector, the motive fluid is steam.

For water-cooled surface condensers, the shell's internal vacuum is most commonly supplied by and maintained by an external steam jet ejector system. Such an ejector system uses steam as the motive fluid to remove any non-condensible gases that may be present in the surface condenser. The Venturi effect, which is a particular case of Bernoulli's principle, applies to the operation of steam jet ejectors.

Motor driven mechanical vacuum pumps, such as liquid ring type vacuum pumps, are also popular for this service.

[edit] Tube sheets

At each end of the shell, a sheet of sufficient thickness usually made of stainless steel is provided, with holes for the tubes to be inserted and rolled. The inlet end of each tube is also bellmouthed for streamlined entry of water. This is to avoid eddies at the inlet of each tube giving rise to erosion, and to reduce flow friction. Some makers also recommend plastic inserts at the entry of tubes to avoid eddies eroding the inlet end. In smaller units some manufacturers use ferrules to seal the tube ends instead of rolling. To take care of length wise expansion of tubes some designs have expansion joint between the shell and the tube sheet allowing the latter to move longitudinally. In smaller units some sag is given to the tubes to take care of tube expansion with both end water boxes fixed rigidly to the shell.

[edit] Tubes

Generally the tubes are made of stainless steel, copper alloys such as brass or bronze, cupro nickel, or titanium depending on several selection criteria. The use of copper bearing alloys such as brass or cupro nickel is rare in new plants, due to environmental concerns of toxic copper alloys. Also depending on the steam cycle water treatment for the boiler, it may be desirable to avoid tube materials containing copper. Titanium condenser tubes are usually the best technical choice, however the use of titanium condenser tubes has been virtually eliminated by the sharp increases in the costs for this material. The tube lengths range to about 55 ft (17 m) for modern power plants, depending on the size of the condenser. The size chosen is based on transportability from the manufacturers’ site and ease of erection at the installation site. The outer diameter of condenser tubes typically ranges from 3/4 inch to 1-1/4 inch, based on condenser cooling water friction considerations and overall condenser size.

[edit] Waterboxes

The tube sheet at each end with tube ends rolled, for each end of the condenser is closed by a fabricated box cover known as a waterbox, with flanged connection to the tube sheet or condenser shell. The waterbox is usually provided with man holes on hinged covers to allow inspection and cleaning.

These waterboxes on inlet side will also have flanged connections for cooling water inlet butterfly valves, small vent pipe with hand valve for air venting at higher level, and hand operated drain valve at bottom to drain the waterbox for maintenance. Similarly on the outlet waterbox the cooling water connection will have large flanges, butterfly valves, vent connection also at higher level and drain connections at lower level. Similarly thermometer pockets are located at inlet and outlet pipes for local measurements of cooling water temperature.

In smaller units, some manufacturers make the condenser shell as well as waterboxes of cast iron.

[edit] Corrosion

On the cooling water side of the condenser:

The tubes, the tube sheets and the water boxes may be made up of materials having different compositions and are always in contact with circulating water. This water, depending on its chemical composition, will act as an electrolyte between the metallic composition of tubes and water boxes. This will give rise to electrolytic corrosion which will start from more anodic materials first.

'Sea water based condensers,' in particular when sea water has added chemical pollutants, have the worst corrosion characteristics. River water with pollutants are also undesirable for condenser cooling water.

The corrosive effect of sea or river water has to be tolerated and remedial methods have to be adopted.

On the steam (shell) side of the condenser:

The concentration of undissolved gases is high over air zone tubes. Therefore these tubes are exposed to higher corrosion rates. Some times these tubes are affected by stress corrosion cracking, if originally stress is not fully relieved during manufacture. To overcome these effects of corrosion some manufacturers provide higher corrosive resistant tubes in this area.

[edit] Effects of corrosion

As the tube ends get corroded there is the possibility of cooling water leakage to the steam side contaminating the condensed steam or condensate, which is harmful to steam generators. The other parts of water boxes may also get affected in the long run requiring repairs or replacements involving long duration shut-downs.

[edit] Protection from corrosion

Cathodic protection is typically employed to overcome this problem. Sacrificial anodes of zinc (being cheapest) plates are mounted at suitable places inside the water boxes. These zinc plates will get corroded first being in the lowest range of anodes. Hence these zinc anodes require periodic inspection and replacements. This involves comparatively less down time. The water boxes made of steel plates are also protected inside by epoxy paint.

Cross-sectional schematic diagram of a power plant condenser for condensing exhaust steam from a steam turbine. This condenser is single-pass on both the tube and shell sides with a large opening at the top for the exhaust steam to enter and a hotwell at the bottom where condensate water drips down to and collects. Circulating water for cooling is shown in light greenish color and condensate is light blue.

[edit] Other applications of surface condensers

Vacuum evaporation Vacuum refrigeration Ocean Thermal Energy (OTEC) Replacing barometric condensers in steam-driven ejector systems Geothermal energy recovery Desalination systems

Surface condenser

From Wikipedia, the free encyclopedia

  (Redirected from Condenser (steam turbine))Jump to: navigation, search

Surface condenser is the commonly used term for a water cooled shell and tube heat exchanger installed on the exhaust steam from a steam turbine in thermal power stations.[1][2][3] These condensers are heat exchangers which convert steam from its gaseous to its liquid state at a pressure below atmospheric pressure. Where cooling water is in short supply, an air-cooled condenser is often used. An air-cooled condenser is however significantly more expensive and cannot achieve as low a steam turbine exhaust pressure as a surface condenser.

Surface condensers are also used in applications and industries other than the condensing of steam turbine exhaust in power plants.

Contents

[hide] 1 Purpose 2 Why is it required? 3 Diagram of water-cooled surface condenser

o 3.1 Shell o 3.2 Vacuum system o 3.3 Tube sheets o 3.4 Tubes o 3.5 Waterboxes

4 Corrosion o 4.1 Effects of corrosion o 4.2 Protection from corrosion

5 Other applications of surface condensers 6 See also 7 References

8 External links

[edit] Purpose

In thermal power plants, the primary purpose of a surface condenser is to condense the exhaust steam from a steam turbine to obtain maximum efficiency and also to convert the turbine exhaust steam into pure water (referred to as steam condensate) so that it may be reused in the steam generator or boiler as boiler feed water.

[edit] Why is it required?

The steam turbine itself is a device to convert the heat in steam to mechanical power. The difference between the heat of steam per unit weight at the inlet to the turbine and the heat of steam per unit weight at the outlet to the turbine represents the heat which is converted to mechanical power. Therefore, the more the conversion of heat per pound or kilogram of steam to mechanical power in the turbine, the better is its efficiency. By condensing the exhaust steam of a turbine at a pressure below atmospheric pressure, the steam pressure drop between the inlet and exhaust of the turbine is increased, which increases the amount of heat available for conversion to mechanical power. Most of the heat liberated due to condensation of the exhaust steam is carried away by the cooling medium (water or air) used by the surface condenser.

[edit] Diagram of water-cooled surface condenser

Diagram of a typical water-cooled surface condenser

The adjacent diagram depicts a typical water-cooled surface condenser as used in power stations to condense the exhaust steam from a steam turbine driving an electrical generator as well in other applications.[2][3][4][5] There are many fabrication design variations depending on the manufacturer, the size of the steam turbine, and other site-specific conditions.

[edit] Shell

The shell is the condenser's outermost body and contains the heat exchanger tubes. The shell is fabricated from carbon steel plates and is stiffened as needed to provide rigidity for the shell. When required by the selected design, intermediate plates are installed to serve as baffle plates that provide the desired flow path of the condensing steam. The plates also provide support that help prevent sagging of long tube lengths.

At the bottom of the shell, where the condensate collects, an outlet is installed. In some designs, a sump (often referred to as the hotwell) is provided. Condensate is pumped from the outlet or the hotwell for reuse as boiler feedwater.

For most water-cooled surface condensers, the shell is under vacuum during normal operating conditions.

[edit] Vacuum system

Diagram of a typical modern injector or ejector. For a steam ejector, the motive fluid is steam.

For water-cooled surface condensers, the shell's internal vacuum is most commonly supplied by and maintained by an external steam jet ejector system. Such an ejector system uses steam as the motive fluid to remove any non-condensible gases that may be present in the surface condenser. The Venturi effect, which is a particular case of Bernoulli's principle, applies to the operation of steam jet ejectors.

Motor driven mechanical vacuum pumps, such as liquid ring type vacuum pumps, are also popular for this service.

[edit] Tube sheets

At each end of the shell, a sheet of sufficient thickness usually made of stainless steel is provided, with holes for the tubes to be inserted and rolled. The inlet end of each tube is also bellmouthed for streamlined entry of water. This is to avoid eddies at the inlet of each tube giving rise to erosion, and to reduce flow friction. Some makers also recommend plastic inserts at the entry of tubes to avoid eddies eroding the inlet end. In smaller units some manufacturers use ferrules to seal the tube ends instead of rolling. To take care of length wise expansion of tubes some designs have expansion joint between the shell and the tube sheet allowing the latter to move longitudinally. In smaller units some sag is given to the tubes to take care of tube expansion with both end water boxes fixed rigidly to the shell.

[edit] Tubes

Generally the tubes are made of stainless steel, copper alloys such as brass or bronze, cupro nickel, or titanium depending on several selection criteria. The use of copper bearing alloys such as brass or cupro nickel is rare in new plants, due to environmental concerns of toxic copper alloys. Also depending on the steam cycle water treatment for the boiler, it may be desirable to avoid tube materials containing copper. Titanium condenser tubes are usually the best technical choice, however the use of titanium condenser tubes has been virtually eliminated by the sharp increases in the costs for this material. The tube lengths range to about 55 ft (17 m) for modern power plants, depending on the size of the condenser. The size chosen is based on transportability from the manufacturers’ site and ease of erection at the installation site. The outer diameter of condenser tubes typically ranges from 3/4 inch to 1-1/4 inch, based on condenser cooling water friction considerations and overall condenser size.

[edit] Waterboxes

The tube sheet at each end with tube ends rolled, for each end of the condenser is closed by a fabricated box cover known as a waterbox, with flanged connection to the tube sheet or condenser shell. The waterbox is usually provided with man holes on hinged covers to allow inspection and cleaning.

These waterboxes on inlet side will also have flanged connections for cooling water inlet butterfly valves, small vent pipe with hand valve for air venting at higher level, and hand operated drain valve at bottom to drain the waterbox for maintenance. Similarly on the outlet waterbox the cooling water connection will have large flanges, butterfly valves, vent connection also at higher level and drain connections at lower level. Similarly thermometer pockets are located at inlet and outlet pipes for local measurements of cooling water temperature.

In smaller units, some manufacturers make the condenser shell as well as waterboxes of cast iron.

[edit] Corrosion

On the cooling water side of the condenser:

The tubes, the tube sheets and the water boxes may be made up of materials having different compositions and are always in contact with circulating water. This water, depending on its chemical composition, will act as an electrolyte between the metallic composition of tubes and water boxes. This will give rise to electrolytic corrosion which will start from more anodic materials first.

'Sea water based condensers,' in particular when sea water has added chemical pollutants, have the worst corrosion characteristics. River water with pollutants are also undesirable for condenser cooling water.

The corrosive effect of sea or river water has to be tolerated and remedial methods have to be adopted.

On the steam (shell) side of the condenser:

The concentration of undissolved gases is high over air zone tubes. Therefore these tubes are exposed to higher corrosion rates. Some times these tubes are affected by stress corrosion cracking, if originally stress is not fully relieved during manufacture. To overcome these effects of corrosion some manufacturers provide higher corrosive resistant tubes in this area.

[edit] Effects of corrosion

As the tube ends get corroded there is the possibility of cooling water leakage to the steam side contaminating the condensed steam or condensate, which is harmful to steam generators. The other parts of water boxes may also get affected in the long run requiring repairs or replacements involving long duration shut-downs.

[edit] Protection from corrosion

Cathodic protection is typically employed to overcome this problem. Sacrificial anodes of zinc (being cheapest) plates are mounted at suitable places inside the water boxes. These zinc plates will get corroded first being in the lowest range of anodes. Hence these zinc anodes require periodic inspection and replacements. This involves comparatively less down time. The water boxes made of steel plates are also protected inside by epoxy paint.

Cross-sectional schematic diagram of a power plant condenser for condensing exhaust steam from a steam turbine. This condenser is single-pass on both the tube and shell sides with a large opening at the top for the exhaust steam to enter and a hotwell at the bottom where condensate water drips down to and collects. Circulating water for cooling is shown in light greenish color and condensate is light blue.

[edit] Other applications of surface condensers

Vacuum evaporation Vacuum refrigeration Ocean Thermal Energy (OTEC) Replacing barometric condensers in steam-driven ejector systems Geothermal energy recovery Desalination systems

Spent fuel pool

From Wikipedia, the free encyclopedia

Jump to: navigation, search

Example of a spent fuel pool (though this one isn't holding large amounts of fuel) from the shut down Caorso Nuclear Power Plant.

Spent fuel pool (SFP) are storage pools for spent fuel from nuclear reactors. Typically 40 or more feet deep, with the bottom 14 feet equipped with storage racks designed to hold fuel assemblies removed from the reactor. These fuel pools are specially designed at the reactor in which the fuel was used and situated at the reactor site. In many countries, the fuel assemblies, after being in the reactor for 3 to 6 years, are stored underwater for

10 to 20 years before being sent for reprocessing or dry cask storage. The water cools the fuel and provides shielding from radiation.

While only about 8 feet of water is needed to keep radiation levels below acceptable levels, the extra depth provides a safety margin and allows fuel assemblies to be manipulated without special shielding to protect the operators.

Contents

[hide] 1 Operation 2 Other possible configurations 3 Status 4 See also

5 External links

[edit] Operation

About one-fourth to one-third of the total fuel load of a reactor is removed from the core every 12 to 18 months and replaced with fresh fuel. Spent fuel rods generate intense heat and dangerous radiation that must be contained. Fuel is moved from the reactor and manipulated in the pool generally by automated handling systems, although some manual systems are still in use. The fuel bundles fresh from the core normally are segregated for several months for initial cooling before being sorted in to other parts of the pool to wait for final disposal. Metal racks keep the fuel in safe positions to avoid the possibility of a “criticality”— a nuclear chain reaction occurring. Water quality is tightly controlled to prevent the fuel or its cladding from degrading. Current regulations permit re-arranging of the spent rods so that maximum efficiency of storage can be achieved.

The maximum temperature of the spent fuel bundles decreases significantly between 2 and 4 years, and less from 4 to 6 years. The fuel pool water is continuously cooled to remove the heat produced by the spent fuel assemblies. Pumps circulate water from the spent fuel pool to heat exchangers then back to the spent fuel pool. Radiolysis, the dissociation of molecules by radiation, is of particular concern in wet storage, as water may be split by residual radiation and hydrogen gas may accumulate increasing the risk of explosions. For this reason the air in the room of the pools, as well as the water must permanently be monitored and treated.

[edit] Other possible configurations

Rather than manage the pool’s inventory to minimise the possibility of continued fission activity, the Chinese are building a 200 MWt nuclear reactor to run on used fuel from nuclear power stations to generate process heat for district heating and desalination.

Essentially an SFP operated as a deep pool-type reactor; it will operate at atmospheric pressure, which will reduce the engineering requirements for safety. [1]

Other research envisions a similar low-power reactor using spent fuel where instead of limiting the production of hydrogen by radiolysis, it is encouraged by the addition of catalysts and ion scavengers to the cooling water. This hydrogen would then be removed to use as fuel. [2]

[edit] Status

Without cooling, the fuel pool water will heat up and boil. If the water boils or drains away, the spent fuel assemblies will overheat and either melt or catch on fire. Fear has been expressed that sabotage, an accident, or an attack which partially or completely drains a plant's spent fuel pool or disables its cooling, might be capable of causing a high-temperature fire that could release large quantities of radioactive material into the environment. Since there is no standard design, most SFPs are housed in far less robust structures than reactor containment vessels and moreover, an SFP often contains much more radioactive material than the reactor core, so this is not a misplaced concern.

It is estimated that by 2014, all of the nuclear power plants in the United States will be out of room in their spent fuel pools, most likely requiring the use of temporary storage of some kind. Yucca Mountain is expected to open in 2017 at the earliest.

[edit] See also

Radioactive waste Nuclear fuel cycle Dry cask storage Spent nuclear fuel shipping cask Deep geological repository