Nuclear Power Plant

NUCLEAR POWER PLANT

A nuclear power plant is a thermal power station in which

the heat source is a nuclear reactor. As in a conventional

thermal power station the heat is used to generate steam

which drives a steam turbine connected to a generator which

produces electricity. As of February 2, 2012, there were 439

nuclear power plants in operation.

In Nuclear power plant nuclear fuel (U234 (0.0050), U235,

(0.7198) U238 ((99.27) Pu239 ) are used.

HISTORY Electricity was generated by a nuclear reactor for the first time ever on December 20, 1951 at the experimental station near Arco, Idaho in the United States. On June 27, 1954, the world's first nuclear power plant to generate electricity for a power grid started operations at Obninsk, USSR. The world's first commercial scale power station, Calder Hall in England opened on October 17, 1956.

A process in which bombardment of a nucleus by fast moving neutrons results in its break up in two smallest nuclei of almost equal mass with the liberation of large amount of energy is called Nuclear Fission reaction.

This fission reaction may be of two types(i) Controlled fission reaction and(ii) Uncontrolled fission reaction

NUCLEAR FISSION

Energy can be produced by following two ways:

1. By combining light nuclei, the process known as fusion.

2. By breaking up heavy nuclei into nuclei of intermediate size, process is known as fission.

ISOTOPES OF URANIUM

ISOPOTES Protons Neutrons Atomic MassU-234 92 142 234U-235 92 143 135U-238 92 146 238

Natural uranium consists of three isotopes 99.3% is

U-238 and 0.7 % U-235 with traces of U-234

92U238 + 0n1 92U239 + γ

92U239 has a half life of 23.5 minutes only, it is unstable

hence

92U239 + 1e 98Np239

Neptunium -239 has a half life of few days and it emits β particles

98Np239 + 1e 94Pu239

Types of Radiation

This is a beta-gamma probe, which can measure beta and gamma radiation in millirems per unit of time.

• Radiation Units– Curie (Ci)• Measurement of the activity of a radioactive source.• Measured as the number of nuclear disintegrations per

unit of time• A curie is 3.70 X 1010 nuclear disintegrations per

second.

– Rad unit of dose equivalent to 100 ergs absorbs per gram of irradiated material.• Measures the amount of energy released by radiation striking

living tissue• Short for radiation absorbed dose• One rad releases 1 X 10-2 J/kg

– Rem • Short for Rontgen equivalent man• This takes into account the possible biological damage to humans

of certain types of radiation.• Rem = RBE * RAD where RBE = relative biological effectivenessMinimum dosage 0.5 rem/year for public and 5 rem for working

personals inside plant.D = 5 (N-18) where N= age of worker

• Radiation Exposure– Background radiation is constantly present in our

environment.• Most people are exposed to between 100 to 500

millirems per year.• This background radiation comes from many natural

source.– The harm that radiation does to living organisms is due to

the fact that it produces ionization which can:• Disrupt chemical bonds in biological macromolecules

such as DNA• Produce molecular fragments which can interfere with

enzyme action and essential cell functions.

– The linear model of exposure proposes that any exposure above zero is damaging and can produce cancer and genetic damage, mostly through its effect on DNA

– The threshold model proposes that there is a threshold limit of exposure up to which the human body can repair damage caused by the exposure• It is not until we reach and exceed this threshold that

we begin to see irreversible damage.

CHAIN REACTIONA nuclear chain reaction occurs when one nuclear reaction causes an average of one or more nuclear reactions, thus leading to a self-propagating series of these reactions. The specific nuclear reaction may be the fission of heavy isotopes (e.g., 235U) or the fusion of light isotopes (e.g., 2H and 3H). The nuclear chain reaction releases several million times more energy per reaction than any chemical reaction.

How it Works

The main bit to remember :

Nuclear power stations work in pretty much the same way as fossil fuel-burning stations, except that a "chain reaction" inside a nuclear reactor makes the heat instead.

• Nuclear Power Plants– The nuclear reactor contains the material and is the

vessel for the controlled chain reaction of fissionable materials that will release the energy.

– Usually there is a fissionable enriched material made of 3% U235 and 97% U238 the is fabricated into small beads.

– The beads are enclosed in a fuel rod.– The fuel rods are locked in a fuel rod assembly by locking

collars and arranged so that pressurized water can flow around the rods.

– Control rods are made of material that can absorb neutrons and are inserted between the fuel rods.

– The rate of the chain reaction is controlled by raising or lowering the control rod.

– In a pressurized water reactor the energy is carried way from the reactor by water in a closed pipe called a primary loop.• This water acts as a coolant and also as a moderator to

slow neutrons so they can more readily be absorbed by the U235 nuclei.

– The water from the primary loop is circulated to a heat exchanger called a steam generator.• The water in the heat exchanger moves through

hundreds of small loops and heats the feedwater in the steam generator.• The water then returns to the core to be heated again.

• A schematic representation of the basic parts of a nuclear reactor. The largest commercial nuclear power plant reactors are nine- to eleven-inch-thick steel vessels with a stainless steel liner, standing about 40 feet high with a diameter of 16 feet. Such a reactor has four pumps, which move 440,000 gallons of water per minute through the primary loop.

Main Parts of a reactor and reactor control

• Reactor Core• Moderator• Reflector• Shielding• Control of reactor• Cooling system

ModeratorsThe moderator is the one reduces the speed of the neutron. The most moderators are Beryllium, Heavy water, or a Graphite. To allow the chain reaction to proceed, enough of these fast neutrons must be slowed down so that there are just enough slow neutrons to produce more fissions and allow a steady power level to be achieved. It is the function of the moderator to slow the fast neutrons, in fact we talk of thermalizing them, and such reactors are sometimes called thermal reactors.Control RodsTo prevent overheating, control rods made of a material that absorbs neutrons are inserted into the uranium bundle using a mechanism that can raise or lower the control rods. Raising and lowering the control rods allow operators to control the rate of the nuclear reaction.The rods can also be lowered completely into the uranium bundle to shut the reactor down in the case of an accident or to change the fuel.ShieldingThe large mass of concrete absorbs gamma radiation well, and as it contains a lot of water molecules it also stops neutrons from penetrating.

CoolantThe heat is absorbed by the colant that's produced in the reactor. most nuclear used water, liquid sodium or a carbon dioxide.

Nuclear reactor principle

Nuclear Plant

• Spent fuel rod assemblies are removed and new ones are added to a reactor head during refueling. This shows an initial fuel load to a reactor, which has the upper part removed and set aside for the loading.

Categories of Nuclear Reactors

• Light Water Reactors– Use H2O– Most Prevalent

• Heavy Water Reactors– Use D2O– Prevalent in Canada and Russia

• Other Reactor Categories– Liquid Metal Cooled– Gas Cooled

Light Water Reactors

• Boiling Water Reactors (BWRs)– Located throughout U.S., Europe, and Asia– Power: 570 -- 1300 MW; Efficiency: ~30% – Tmax: 570oF; PRxr Coolant: 1000 psi

• Pressurized Water Reactors (PWRs)– Manufactured by U.S., European, and Asian

companies– Tmax: 590oF; PRxr Coolant: 2250 psi

• VVER Reactors (VVERs) -- Russian PWR

Pressurized water reactor

A pressurized heavy water reactor (PHWR) is a nuclear power

reactor, commonly using unenriched natural uranium as its fuel,

that uses heavy water (deuterium oxide D2O) as its coolant and

moderator. The heavy water coolant is kept under pressure,

allowing it to be heated to higher temperatures without boiling,

much as in a PWR. While heavy water is significantly more

expensive than ordinary light water, it yields greatly

enhanced neutron economy, allowing the reactor to operate

without fuel enrichment facilities (mitigating the

additional capital cost of the heavy water) and generally

enhancing the ability of the reactor to efficiently make use of

alternate fuel cycles.

BWR

PWR

VVER

Heavy Water Reactors

• Canadian Deuterium-Uranium (CANDU) Reactors– Use natural (non-isotopically concentrated) uranium – Tmax: 590oF; PRxr Coolant: 1500 psi– Can run 2 ½ years without refueling/downtime

• RBMK Reactors– Chernobyl-style reactor– Graphite and D2O in reactor core– Much improved designs today

CANDU Reactor

RBMK Reactor (High Power channel)

Other Reactor Categories

• Liquid Metal Fast Breeder Reactors (LMFBRs)– Use liquid sodium as coolant– 3-loop cycle– None operating commercially

• Advanced Gas-cooled Reactors (AGRs)– Use natural uranium– He or CO2 gas coolant at 1430oF and 600 psi– Primarily operated in United Kingdom

TYPES OF POWER REACTOR

Main types of nuclear reactors are1. Advanced Gas Cooled-Reactor2. Boiling Water reactor3. Pressurized Water reactor4. CANDU Type reactor5. Sodium Graphite reactor6. Fast Breeder Reactor

Boiling Water Reactor (BWR)

Pressurized Water Reactor (PWR)

PWR in Operation

A Primary circuit B. Secondary circuit 

C. Tertiary circuit 1. Reactor 2. Fuel assemblies 3. Control rods 4. Pressurizer

5. Steam generator  6. Primary pump 7. Feedwater primary circuit 8. Feedwater secondary circuit 9. Steam secondary circuit 10. High pressure turbine 11. Low pressure turbine

12. Condenser 13. Feedwater pump 14. Generator 15. Exciter 16. Transformer 17. High voltage line 18. River    

19. Intake cooling water 

20. Cold cooling water 21. Warm cooling water 22. Cooling water 23. Upward airflow 24. Steam 25. Outlet cooling water

CLASSIFICATIONS BY TYPE

Main parameters

Magnox AGR BWR PWR CANDU Fast

Fuel U UO2 UO2 UO2 UO2 PUO2, UO2

Can material

Mg SS Zr Zr Zr SS

Fuel enrichment

Natural 2-2,5% 2,5% 3% Natural 15%

Coolant CO2 CO2 H2O H2O D2O Na

Pressure (kg/cm2)

22 30 70 150 95 Zero

Inlet/out let temperature

245/400 300/700 275/285 275/300 250/290 400/600

Moderator Graphite Graphite H2O H2O D2O None

Annual Fuel Requirements for 1000 MW Plant

• 2,000,000 mTons of Coal• 1,960,000,000 Gallons of Oil• 30 mTons of Uranium• 0.6 mTons of Tritium and Hydrogen

(Fusion)

Advantages

• Nuclear power costs about the same as coal, so it's not expensive to make.

• Does not produce smoke or carbon dioxide, so it does not contribute to the greenhouse effect.

• Produces huge amounts of energy from small amounts of fuel…so cheaper logistics

• Produces small amounts of waste.• Nuclear power is reliable.

Disadvantages •Although not much waste is produced, it is very dangerous.

It must be sealed up and buried for many thousands of years to allow

the radioactivity to die away.

For all that time it must be kept safe from earthquakes, flooding,

terrorists and everything else. This is difficult.

•Nuclear power is reliable, but a lot of money has to be spent on

safety - if it does go wrong, a nuclear accident can be a major

disaster.

People are increasingly concerned about this - in the 1990's nuclear

power was the fastest-growing source of power in much of the world.

In 2005 it was the second slowest-growing.

Nuclear Power Plant in Pakistan• In Pakistan, nuclear power makes a small contribution to total energy

production and requirements, supplying only 2.34% of the country's

electricity.  Total generating capacity is 20 GWe and in 2006, 98 billion

kWh gross was produced, 37% of it from gas, 29% from oil.

• The Pakistan Atomic Energy Commission (PAEC) is responsible for all

nuclear energy and research applications in the country.

• Its first nuclear power reactor is a small (125 MWe) Canadian

pressurized heavy water reactor (PHWR) which started up in 1971 and

which is under international safeguards - KANUPP near Karachi, which

is operated at reduced power.

Nuclear Power Plant in Pakistan

• The second unit is Chashma-1 in Punjab, a 325 MW

pressurized water reactor (PWR) supplied by China. The

main part of the plant was designed by Shanghai Nuclear

Engineering Research and Design Institute (SNERDI).  It

started up in May 2000 and is also known as CHASNUPP-1.

• Construction of its twin, Chashma-2, started in December

2005. It is reported to cost PKR 51.46 billion (US$ 860

million, with $350 million of this financed by China).

Pakistan Nuclear Power Reactors

Reactor Type MW net

Construction start

Commercial operation

Karachi

 

PHWR 

125 1966 12/72

Chashma 1PWR 300 1993 6/00

Chashma 2PWR 300 2005 expected 2011

Total 

 425 operating