Liquid-Fluoride Thorium Reactors

Advantages, Disadvantages, and Feasibility

By: Dylan Kissinger

Dept. of Civil and Environmental Engineering

UW-Madison

1

Refrences

[1] Breeder. In (2010). U.S NRC Glossary. Retrieved from http://www.nrc.gov/reading-rm/basic-ref/glossary/fissile-material.html

[2] Cooper, N., Minakata, D., Begovic, M., & Crittenden, J. (2011). Should we consider using liquid fluoride thorium reactors for power generation?.Environmental Science and Technology, 45(15), 6237–6238. Retrieved February 3, 2013 from http://pubs.acs.org.ezproxy.library.wisc.edu/doi/pdfplus/10.1021/es2021318

[3] DEPARTMENT OF HEALTH AND HUMAN SERVICES, COMMITTEE TO COORDINATE ENVIRONMENTAL HEALTH AND RELATED PROGRAMS. (n.d.). Assessment of the health risks of fluoride. Retrieved from website: http://www.health.gov/ENVIRONMENT/REVIEWOFFLUORIDE/MAJfind.htm

[4] EBR-I. (n.d.). Idaho National Laboratory. Retrieved February 23, 2013, from www.inl.gov/ebr/

[5] Environmental impact and health effects of the alkali metals. (10, December 2010). Retrieved from http://www.environmental-protections.info/environmental-impact-report/environmental-impact-and-health-effects-of-the-alkali-metals/

[6] Federal council on science and Technology R&D goals, (1972). An evaluation of the molten salt breeder reactor. Retrieved from U.S government printing office website: http://moltensalt.org/references/static/downloads/pdf/WASH-1222.pdf

[7] Fertile material. In (2010). U.S NRC Glossary. Retrieved from http://www.nrc.gov/reading-rm/basic-ref/glossary/fissile-material.html

[8] Fissile material. In (2010). U.S NRC Glossary. Retrieved from http://www.nrc.gov/reading-rm/basic-ref/glossary/fissile-material.html

[9] Folkers, C. (n.d.). Tritium:health consequences. Retrieved from http://www.nirs.org/factsheets/tritiumbasicinfo.pdf

[10] Forsberg, C. W. (2006, June). Molten-salt-reactor technology gaps. 6 international congress on the advances in nuclear power plants, Reno, NV. Retrieved February 22, 2013 from http://www.torium.se/res/Documents/124670.pdf

[11] Juhasz, A. J., Rarick, R. A., & Rangarajan, R. (2009, August). High efficiency nuclear power plants usingliquid fluoride thorium reactor technology. Seventh international energy conversion engineering conference (iecec), Denver, CO. Retrieved February 3, 2013 from http://webcache.googleusercontent.com/search?q=cache:http://ntrs.nasa.gov/archive/nasa/casi.ntrs.n

[12] Leblanc, D. (2010). Molten salt reactors: A new beginning for an old idea. Nuclear Engineering and Design,240(6), 1644-1656. Retrieved from http://www.sciencedirect.com.ezproxy.library.wisc.edu/science/article/pii/S0029549310000191

2

[13] Marder, J. (n.d.). Mechanics of a Nuclear Meltdown Explained | PBS NewsHour .PBS: Public Broadcasting Service. Retrieved February 23, 2013, from http://www.pbs.org/newshour/rundown/2011/03/mechanics-of-a-meltdown-explained.html

[14] Matson, J. (2011, March 15). What happens during a nuclear meltdown. Scientific American, Retrieved 23 Feb. 2013 from http://www.scientificamerican.com/article.cfm?id=nuclear-energy-primer

[15] N.J. Department of Health and Senior Services, (2001).Hazardous substance fact sheet: Uranium hexafluoride . Retrieved from website: http://nj.gov/health/eoh/rtkweb/documents/fs/1970.pdf

[16] Nuclear Power Plants World Wide. (n.d.).European Nuclear Society. Retrieved February 23, 2313, from www.euronuclear.org/info/encyclopedia/n/nuclear-power-plant-world-wide.htm

[17] Nuclear power reactors. In (2012). World Nuclear Association Information Library. Retrieved from http://www.world-nuclear.org/info/Nuclear-Fuel-Cycle/Power-Reactors/Nuclear-Power-Reactors/

[18] Quattrochi, D. (08, June 2006). 3.7 brayton cycle. Retrieved from http://web.mit.edu/16.unified/www/SPRING/propulsion/notes/node27.html

[19] Rosenthal, M. W., Briggs, R. B., & Haubenreich, P. N. U.S Atomic Energy Commission , Oak Ridge National Laboratory. (1972). Molten-salt reactor program semiannual progress report. Retrieved February 2, 2013 from National Technical Information service-U.S Department of Commerce website: http://www.energyfromthorium.com/pdf/ORNL-4728.pdf

[20] U.S Department of Labor, Occupational Safety and Health Administration. (1976). Occupational health guideline for fluorine. Retrieved from website: http://www.cdc.gov/niosh/docs/81-123/pdfs/0289.pdf

[21] Zheng, G., Sellers, R. S., Cheng, W., Kelleher, B., Sridharan, K., Anderson, M., & Allen, T. R. (2012). Corrosion behavior of structural alloys in molten fluoride salts for next generation nuclear reactors. Transactions of the American Nuclear Society, 106, 1312-1313.

3

Table of Contents

References………………………………………………………………………………………………………………………………… 1

List of Figures……………………………………………………………………………………………………………………………. 3

Executive Summary…………………………………………………………………………………………………………………… 4

Introduction……………………………………………………………………………………………………………………………… 6

Technical Background: Important Terms………………………………………………………………………………..... 6

Technical Background: Historical Background…………………………………………………………………………… 6

Safety Issues……………………………………………………………………………………………………………………………… 7

a. Nuclear Meltdown in Conventional Nuclear Reactors……………………………………………………….... 7b. Meltdown Prevention in the LFTR……………………………………………………………………………………….. 8

Environmental Issues……………………………………………………………………………………………………………… 10

Environmental and Safety Concerns of LFTRs………………………………………………………………………... 10

Technical Issues……………………………………………………………………………………………………………………… 12

a. Problems with the MSRE…………………………………………………………………………………………………. 13b. Contemporary Solutions in the LFTR………………………………………………………………………………… 14

Conclusion………………………………………………………………………………………………………………………………. 16

List of Figures

Basic diagram of a LFTR Design…………………………………………………………………………………………………. 9

Table of MSR Fission products…………………………………………………………………………………………..… 11

Conceptual Design for a MSBR……………………………………………………………………………………………..... 12

4

Executive Summary

Nuclear power is one of the most promising alternative energy sources available today. It is carbon free, and relatively low-cost and high-efficiency alternative to fossil fuels. As global society makes the transition from fossil fuels to more sustainable options, nuclear power has the potential to be a leading energy sources. However, Problems like the risk of catastrophic nuclear meltdown and environmental detriment due to nuclear waste raise concerns that nuclear power may be too dangerous to be considered a leading alternative energy source. So long as nuclear power carries the risk of meltdown and keeps producing massive amounts of nuclear waste, it will never be trusted to take the place of fossil fuels as the premier energy source of the future.

Fortunately, the liquid-fluoride thorium reactor, or LFTR, may be the solution to the problems plaguing nuclear power. The LFTR is a type of nuclear reactor that uses a molten thorium fluoride fuel salt mixture as a fuel source, as opposed to the solid uranium fuel rods used in conventional nuclear reactors. This type of reactor technology was originally developed in the 1950’s and later dismissed in the 70’s due to technical problems. However, new development in technology have caused a resurgence of interest in reactors using liquid fluoride molten salts. This is largely because LFTRs hold several major advantages over conventional nuclear reactors which either greatly reduce, or effectively solve the major safety and environmental problems associated with nuclear power. In this report I endeavor to examine the advantages and disadvantages of LFTR technology, as well as investigate whether it can be widely implemented in the foreseeable future.

LFTRs offer a safer, more fool proof, reactor design. LFTRs use an innovative freeze plug safety system, which makes meltdown virtually impossible [2]. The system is completely passive, meaning it will activate without need for a power supply or human interaction. In fact, the freeze plug safety system makes it impossible for the reactor to function unless it is at a stable heat and has a steady supply of power [11]. Additionally the LFTR operates in low pressure conditions and is inherently capable of withstanding the extremely high temperatures associated with molten fuel salts. By utilizing a low pressure system specifically designed to generate power from fuel in a molten state, combined with a completely passive safety system, the LFTR has effectively solved the problem of nuclear meltdown.

The LFTRs innovative design not only prevents nuclear meltdown, it greatly increases the efficiency of the reactor [11]. With this increased efficiency comes a substantial reduction in nuclear waste products. LFTRs should be capable of producing 300 times less waste than conventional reactors [2]. Additionally, the radiation levels in LFTR waste takes less than 300 years to drop down to background levels, as opposed to the waste from uranium fueled power plants which takes over 10,000 year to do the same. This drastic reduction in waste combined with a shorter period of irradiation should drastically reduce the LFTRs environmental impact. However, LFTRs are capable of producing dangerous byproducts of its own.

The reports from the original research done on this technology indicates that the three technical issues of greatest concern were noble metal plate-out, tritium control, and corrosion [19], [6]. After researching subsequent advancements in technology I have determined that the issue of tritium control has been solved, while noble metal plate-out and corrosion have been largely mitigated, and may be completely solved with subsequent technological advancements [10], [21].

5

In conclusion, the research I have done has led me to reason that the benefits of LFTRs far outweigh the disadvantages. Furthermore, given current state of the technical problems associated with LFTRs, as well as the potential for further improvement I have found it reasonable to infer that the widespread use of LFTRs should be possible in the foreseeable future an appropriate investment in the technology is made.

6

I. Introduction

On December 20, 1951 at the Idaho national laboratory, EBR-1 became the first nuclear reactor to produce electricity [4]. Its operation marked the birth of nuclear power, and a new age in power production. In the decades following three major incidents occurred. Two of these incidents, the Chernobyl meltdown in 1989, and Fukushima meltdown in 2011, resulted in the explosion of nuclear reactors, and the release of massive amounts of dangerous radiation into the environment. Events like these have caused many to have a troubled relationship with nuclear power.

Since the dawn of the nuclear age in the mid-20th century, nuclear power has been both praised and demonized. On one hand, nuclear power is a carbon free, relatively low-cost and efficient alternative to fossil fuels. On the other hand, nuclear power produces significant amounts of dangerous nuclear waste and carries the danger of catastrophic nuclear meltdown. For over 50 years society at large has struggled to trust a power source that can provide a realistic long-term alternative to fossil fuels, yet at the same time is capable of detrimental environmental damage and extremely destructive catastrophic meltdown. So long as nuclear power carries the risk of meltdown and produces large amounts of nuclear waste, it will not be fully trusted to take the place of fossil fuels as the leading energy source of the future. However, the liquid-fluoride thorium reactor, a type of nuclear reactor which uses a molten thorium fuel salt mixture as a fuel source, may offer a solution to many of the problems facing nuclear power today. This report will endeavor to outline the advantages and disadvantages of LFTR technology in the fields of safety and environmental impact, as well as investigate whether the technology can be widely implemented in the foreseeable future.

I. Technical Background

a. Important Terms 1. Fissile Material - As defined by the United States Nuclear Regulatory Commission

“A nuclide that is capable of undergoing fission after capturing low-energy thermal (slow) neutrons” [8]

2. Fertile Material - As defined by the United States Nuclear Regulatory Commission “A material, which is not itself fissile (fissionable by thermal neutrons), that can be converted into a fissile material by irradiation in a reactor” [7]

3. Molten Salt Reactor ( MSR ) - A type of nuclear reactor that uses a molten fuel salt mixture as its primary fuel source. As defined by David Leblanc of Carleton University “A molten salt reactor (MSR) is one in which fluorides of fissile and/or fertile elements such as UF4, PuF3 and/or ThF4 are combined with carrier salts to form fluids” [12]

4. Breeder Reactor - As defined by The United States Nuclear Regulatory Commission “A reactor that produces more nuclear fuel than it consumes. A fertile material, such as uranium-238, when bombarded by neutrons, is transformed into a fissile material, such as plutonium-239, which can be used as fuel” [1]

5. Molten Salt Breeder Reactor ( MSBR ) - A MSR that is also a breeder reactor6. Liquid-Fluoride Thorium Reactor ( LFTR ) - A type of MSBR that uses a thorium based fertile

material. [11]

b. Historical Background Since the early 1950’s nuclear power has been used to generate electricity for mass

consumption. In the decades following the dawn of the nuclear age hundreds of nuclear power plants have been build and are in use around the world [16]. Since that time the design of nuclear power plants, or NPPs, has not significantly changed [17]. Current nuclear power plants,

7

or NPPs, are almost exclusively solid fuel reactors, using uranium fuel rods. The fuel rods generate heat which is used to boil water into steam which will power a turbine [17]. Control rods made of a neutron absorbing material such as cadmium are used to control the reaction and water is most often used to cool the reactor [17].

LFTR technology is the modern day descendant of the MSR technology developed in the 1950’s-1970’s in the U.S. The only comprehensive research done on this technology was done during this period at the Oak Ridge National Laboratory, or ORNL, in Idaho. According to Charles Forsberg, of the ORNL

Between 1950 and 19761, 2 a large Molten Salt Reactor (MSR) development program was conducted in the United States, two test reactors were successfully operated, a design of a 1000-MW(e) reactor was completed, and plans were developed to construct a demonstration reactor. Since then, little research and development (R&D) has been done on MSRs. [10]

The program was discontinued due to technical design problems which, at the time, could not be overcome [6]. However, there has been a renewed interest in MSR technology in recent years, specifically in the form of LFTRs. This resurgence of interest is the result of new technological advancements that may be able to overcome the technical issues that plagued the molten salt reactor experiment, or MSRE, program at ORNL.

II. Safety Issues

The risk of catastrophic nuclear meltdown is quite arguably the most distressing of the major flaws present in current nuclear power plants. Though only three serious meltdown events have ever occurred globally, the risk of a serious meltdown happening is still by far one of the most widely discussed issues concerning nuclear power. The catastrophic potential of a major nuclear meltdown and events like the incidents at the Three Mile Island, Chernobyl, and Fukushima power plants have caused many to harbor understandable doubts as to whether the benefits of nuclear power outweigh these risks. However, LFTRs are designed to have a passive safety system, accounting for some of the major design flaws of solid uranium fuel rod reactors, and making meltdowns almost impossible. Additionally LFTRs do not allow for volatile conditions to develop in the reactor in the event of overheating unlike current nuclear reactors.

a. Nuclear Meltdown in Conventional Nuclear Reactors

Before discussing how LFTRs prevent meltdown and address the safety flaws present in uranium reactors some technical knowledge of the process and effects of the nuclear meltdown process should be understood. Nuclear reactors work by harnessing the massive amount of heat given off by nuclear reactions and using it to generate power. Most nuclear power plants today use solid uranium rods as a fuel source for the reactor [17]. A nuclear meltdown occurs when the reactor becomes overheated and the fuel actually starts to melt and pool at the bottom of the reactor [14]. This overheating would most likely be the result of either design flaw, control system error, or operator error. If the reactor and containment vessel housing the reactor are breached and this molten fuel is released it would be detrimental to the surrounding environment, releasing massive amounts of radiation. The cooling and shut down systems of current nuclear reactors have several design flaws that can allow meltdowns to occur. Most reactors today use a water cooling system and control rods to control the heat of the reactor [13]. Control rods are used to stop the nuclear reaction and shut down the reactor [14]. Control rods made of materials like cadmium or boron for their ability to absorb the neutrons traveling between atoms essential to the nuclear reaction [17]. Water is used to create steam to turn turbines, and to cool the reactor. Generally these systems are more than sufficient to prevent a meltdown form ever

8

happening. However, while multiple safety systems are designed into these cooling and shut down systems, they are far from fool proof and many require the input of a thoroughly trained human operator in order to work properly.

These methods of regulating a nuclear reactor have several major flaws that can result in a loss of reactor control and eventual meltdown. When a reactor starts overheating, the control rods are inserted to stop the nuclear reaction of the uranium fuel [14]. However, this will only stop the reaction of the fuel, not the heating of the reactor. John Matson of Scientific American explains how the reactor continues heating.

Even after the control rods have done their job and arrested the fission reaction the fuel rods retain a great deal of heat. What is more, the uranium atoms that have already split in two produce radioactive by-products that themselves give off a great deal of heat. So the reactor core continues to produce heat in the absence of fissioning [14].

Even after control rods are inserted, in the time it takes for nuclear by-products to stop producing heat and the fuel rods to release their heat, the reactor could undergo a catastrophic meltdown. Water is needed to cool the reactor even after the control rods have fulfilled their role.

This system has some major shortcomings that prevent it from properly functioning, and can even greatly exacerbate the situation. If the pump malfunctions, becomes clogged, or experiences a loss of power, coolant cannot be supplied to the reactor and any water supplied beforehand will quickly boil off. With no water to cool the reactor it will continue to heat and fuel rods will start to melt. At this point any boiled off water vapor will create very high pressure conditions within the reactor, and because of the extreme conditions water can react with materials in the fuel rods while boiling to produce a significant amount of hydrogen gas. [14]. Between the high pressure, the presence of highly flammable gas, and the intense heat, an explosion is very likely to happen and expose the reactor and its radiation to the environment. Cases similar to this resulted in explosions at Chernobyl and Fukushima. Furthermore, these systems usually require electricity, the interaction of a human operator, or both at some point to properly function. This leaves these systems vulnerable to failure due to either total loss of power or operator error.

b. Meltdown Prevention in the LFTR

9

While the controlling systems of contemporary uranium fuel rod reactors can fall short of preventing a meltdown under the right conditions, the design of LFTR reactors makes them far less susceptible to meltdown and features a passive safety system which will automatically stop the reactor in the event of overheating or power failure. Since a liquid thorium fuel is used, the reactor is already in a sort of controlled meltdown. However, the reactor is specifically designed to function at the extremely high temperatures required to keep fluorine thorium salt in a liquid state and allow the nuclear reaction to take place. The molten thorium salt fuel will envelope some fissile material (most likely U233) which will start the nuclear reaction [11]. As the molten thorium salt fuel flows through the reactor nuclear fission occurs under the high temperature conditions and heat is produced. The fuel is then pumped through a cooling salt heat exchanger before being reprocessed to remove waste materials [11]. The cooling salt is then pumped through a second helium heat exchanger which then transfers the energy to turbines to produce power before being cooled by a series of heat sinks [11]. Figure 1 shows a diagram of a basic LFTR design which shows the main components of this process.

Figure 1: A basic diagram of an LFTR design. Note the series of heat exchangers allowing the thermal energy to transfer from the fuel salts to the turbine for electrical power generation. Also of note is the freeze plug safety system located below the reactor. [11]

Though the reactor is designed to withstand the high temperatures needed for the reaction, if too much heat is produced overheating can still occur. However, because the reactor uses cooling salts instead of water as coolant and medium to transfer heat energy to turbines, high pressure volatile conditions will not develop if overheating occurs. This eliminates the risk of an explosion releasing radiation. Though if the reactor should overheat, it will be quickly shutdown by the freeze plug safety system [2].

10

LFTRs feature a passive safety system using a freeze plug designed to stop the reaction in the event of overheating or power failure. The freeze plug is designed to be built somewhere at the bottom of the reactor, as seen in figure 1, and the plug is made of a material which has a melting point below the temperature of the fuel-salt. A cooling system keeps the fuel from melting the plug. However, if the reactor starts to overheat, the fuel will overcome the cooling system and melt the plug, draining the fuel-salt, separating it from the fissile material, and allowing it to cool in secure tanks away from the reactor. Additionally, since the system requires power to cool the freeze plug it will automatically engage should a loss of power occur. This safety system operates without need for operator interaction or electricity, making it truly passive. The freeze plug design goes beyond simply preventing overheating from occurring by actually preventing the reactor from functioning unless its heat is under control and a steady power supply is available.

III. Environmental issues

For decades, perhaps the most enticing aspects of nuclear power, has been its carbon emission free energy. Nuclear power plants do not emit CO2 or any other greenhouse gasses into the atmosphere during power generation. Though, while nuclear power is a carbon free energy source, the nuclear waste it produces emits harmful radiation and can be detrimental to the environment if not stored and handled properly. The LFTR has the capability to diminish this environmental issue. However, LFTR power generation does not come without its own environmental risks.

The LFTR can greatly reduce the environmental effects of nuclear waste seen in conventional NPPs. The LFTR has significantly improved efficiency compared to the solid fuel rod NPPs in operation today [11]. With this improved efficiency comes a reduction in the amount of waste LFTRs produce compared to contemporary NPPs [2]. According to a peer reviewed article from Environmental Science and Technology “The volume of waste products from a LFTR is approximately 300 times less than that of a uranium reactor” [2]. Implementing LFTR reactors could drastically reduce the amount of nuclear waste produced by nuclear power. The LFTR’s impact on the environment is further reduced by the fact that thorium has a far shorter half-life than uranium. While the waste from a solid uranium fuel NPP takes over 10,000 years for its radiation to drop to background levels [11]. A LFTR takes approximately 300 years to decay, only 3% of the time needed for uranium waste to decay to safe radiation levels [11]. This significant reduction in waste combined with the drastically shorter period in which the waste emits harmful radiation mean that NPPs using LFTRs will have far less potential to harm the environment with radiation emissions than solid fuel rod NPPs. This would be a major improvement in the environmental impact of nuclear power and would make it an even more viable option as the leading energy sources as the world makes the transition from fossil fuels to more sustainable options.

IV. Environmental & Safety Concerns of LFTRs

11

Though LFTRs offer the advantages of reduced waste and half-life, as well as making meltdown

virtually impossible, the technology has its own unique issues with dangerous waste products. As the fission reaction produces heat in the reactor various fission products result from this process. Inevitably all of these products must be removed from the reactor and appropriately stored, recycled, or disposed of. Figure 2 from a 1971 MSRE progress report shows a table of some of these fission products as well as the removal method for each. While some of these fission products like Xe, and Kr are inert and harmless, some products, like certain noble and alkali metals, can be hazardous to the environment and human health if improperly disposed of [5].

One of the most concerning and dangerous fission byproducts of LFTRs is Tritium, a radioactive isotope of hydrogen. Though tritium does not emit as much radiation as nuclear fuels like thorium or uranium, exposure to tritium can result in deadly health problems like cancer and genetic effects [9]. Furthermore, tritium can easily and permanently contaminate the surrounding environment as well as the organisms therein as explained by the Nuclear Information and Resource Service “Tritium will bind anywhere hydrogen does, including in water, and in plant, animal and human tissue. It cannot be removed from the environment once it is released. Tritium can be inhaled, ingested, or absorbed through skin” [9]. Meaning that once it is exposed to the environment it quickly contaminates the water as well as any organisms it comes in contact with and cannot be cleaned. This problem is exacerbated by the fact that LFTRs produce far more tritium than conventional nuclear reactors. According to a report on MSRBs released by ORNL in 1972 “tritium would be-produced at a rate of about 2400 curies/day in a 1000 MWe MSBR. This compares with about - 30 - 40 to 50 curies/dry for light water, gas-cooled, and fast breeder reactors”[6]. All of this tritium is going to have to be somehow contained, and then properly handled and disposed of or stored, adding to the amount of dangerous waste produced by LFTRs.

Figure 2 a table of MSR fission products from a 1971 MSRE progress report showing the products, removal time, and primary removal method [19]

12

Additionally, there are concerns with long term storage of the fuel salts used in LFTRs. When fuel salts are stored over a period of decades, dangerous fluorine gas and uranium hexafluoride can be released. According to Charles Forsberg of ORNL “In 1994, a gas sample taken from the MSRE off-gas system (which remained connected to the fuel and flush-salt drain tanks) showed the generation of fluorine from the fuel salt and the partial transport of uranium (in the form of UF6) from the salt into the off-gas system” [10].Though while at reactor temperatures fluorine gas is kept within the fuel salt mixture, when the fuel is stored at ambient temperatures fluorine gas is released from the fuel salts [10]. Fluorine gas can cause a variety of health problems in humans and other [20], [3] and uranium hexafluoride is a radioactive and extremely dangerous material [15]. Both of these materials must be handled and stored with extreme care because of the various, and very serious, health risks of exposure to humans or other organisms.

V. Technical Issues

13

While LFTRs show great promise as the answer to many of the major problems seen in conventional NPPs, very little applied research has ever been done on this type of technology. The only comprehensive applied research ever done on MSRs was done at ORNL, during the 1950’s-1970’s. The reactor in this research, was the precursor to the modern LFTR. Figure 3 shows a conceptual design for a MSBR, done by ORNL researchers, based on the MSRE.

Figure 3 a conceptual design for a MSBR from a 1972 ORNL report evaluating MSBR potential. LFTRs are a type of MSBR. Note the single heat exchanger and use of a steam powered generator. [6]

Note that the design uses a steam turbine instead of the helium gas turbine seen in the LFTR design in figure 1. However, the project was shut down in the 1970’s due to technical problems that the technology of the day was unable to overcome. The following sections will explore what the major issues preventing the further study and development of MSR, and ultimately LFTR, technologies were during the MSR research, as well as if these issues are capable of being overcome using current technology.

a. Problems with MSRE

During the 1960’s as the ORNL was conducting research on its MSRE, as is true for any new technology, a number of technical problems arose which prevented the reactor from working safely and effectively. The researchers at ORNL were able to solve many of these technical issues. However, some problems were simply beyond the capability of the period technology to overcome. Although several issues still remained at the time of the MSRE shut down, for the sake of brevity this report will only

14

examine the four issues that proved to be most important and most problematic for the MSRE researchers: noble metal plate-out, tritium control, and corrosion.

1. Noble Metal Plate-out

As stated in section V, the fission reactions required to heat LFTRs produce various byproducts, which must be removed from the system and properly disposed of. However, the removal of noble metals in particular proved especially difficult in the MSRE. Noble metal production due to the fission reactions was very problematic because these noble metals tend to plate-out on the metal surfaces of the reactor and build up there [19], [10]. These noble metals also give off a significant amount of heat due to radioactive decay, which can cause damage to components like the heat exchanger if plate-out is excessive and proper cooling of the component is not maintained [10]. This problem not only prevented the removal of noble metal fission products from the system, it posed the danger of damaging essential components. This is just one of several problems caused by fission products.

2. Tritium Control

Tritium, another fission byproduct, is a dangerous radioactive isotope of hydrogen which can be extremely hazardous to the environment and human health as was discussed in section V. ORNL researchers struggled to prevent tritium from escaping the MSRE, posing a major hazard. The problem lied in the ability of tritium to diffuse through metals [19], [10].This issue was most problematic at the heat exchangers which were comprised of very thin metal with high surface area, making diffusion into the coolant salt easier [19], [10].The tritium then has the ability to diffuse into the steam generator and contaminate the water being used. This presents a major safety hazard and environmental concern.

3. Corrosion protection

By far the issue most consistently examined in the MSRE reports from ORNL, and subsequent examinations of its findings was corrosion caused by the fuel salt mixture, due to the radiation, extreme heat, and most importantly the chemical properties of the fuel-salt mixture used in MSRs. This issue was of extreme importance to the success of MSR technology and was, in large part, why the MSRE program was discontinued [12]. An MSRE progress report from 1971 stated that “The success of a molten-salt reactor system is strongly dependent on the compatibility of the materials of construction with the salts comprising the primary and secondary circuits of the reactor” [19]. To put it simply, MSR technology cannot be successful unless a material of suitable corrosion resistance is developed.

Hastelloy-N, a nickel alloy with strong corrosion resistance, was developed for use in the MSRE to combat this problem, and indeed outperformed other materials used [21]. Nevertheless, the MSRE encountered corrosion problems throughout its operation, and a 1971 MSRE progress report states that “The observations are consistent with our general conclusions that all Hastelloy-N surfaces exposed to fuel salt crack.” [19]. The report further examines the common problem of grain boundary cracking in Hastelloy-N, and concludes that heat and radiation were not necessarily requirements for cracking, but exposure to fuel salt was. ORNL researchers continued to test new alloys and materials, but were unable to find a suitable material before termination of the experiment.

These as well as other technical difficulties resulted in the termination of the MSR program at ORNL. In the decades that followed, MSR technology was largely ignored. However, in that time technology and scientific knowledge of the processes involved in MSR fission reactions has greatly advanced prompting a renewed interest in MSR technology, specifically in LFTRs.

b. Contemporary Solutions in the LFTR

15

Since the termination of the MSRE, extraordinary strides in technology and scientific understanding of nuclear physics have caused many to reexamine MSR technology in the last decade. Additionally, as this technology has been reexamined, the LFTR emerged as the most promising variation of this technology. Between these advances and the improved design of the modern LFTR, the four main issues preventing the development and use of MSR technology in the form of the LFTR have all been either eliminated or largely mitigated.

1. Noble Metal Plate-Out

At the time of the MSRE shut down noble metal plate-out was a major problem which not only formed a coating of dangerous fission products on exposed metal, but threatened to cause considerable damage to the heat exchanger. In the decades since that time major advancements have been made in material sciences that will help to greatly reduce this problem. Given the fact that noble metals prefer to plate-out over metals, new carbon-carbon composites could be used for the heat exchanger and significantly reduce plate-out compared to a metallic heat exchanger [10]. Additionally, several new methods have been proposed to remove noble metals from fuel salt [10]. According to Forsberg “The molten salts can be purged with inert gases, with the noble metals preferentially forming aerosols that can be filtered from the gas stream. Newer options include ultra-high-surface-area metal or coated carbon-foam matrixes designed to preferentially encourage plate-out of noble metals in the salt cleanup system” [10]. The use of one or a combination of any of these methods along with a carbon-carbon composite could drastically reduce the amount of noble metal plate-out on the heat exchanger as well as the amount of noble-metal fission products present in the fuel salts. These advancements may be further improved by an increasing understanding of the physical processes causing this problem. During and after the MSRE project, a lack of understanding of noble metal fission products was a major problem hindering the plate-out issue from being solved [19]. However, recent breakthroughs have made research on molten salts containing noble metals far easier, which should lead to a far greater understanding of the plate-out process in the near future [10]. The advancements in materials and removal methods combined with a quickly growing understanding of noble metal plate-out should lead to this problem being drastically reduced with the potential for the issue being completely resolved as understanding of the process continually develops.

2. Tritium Control

The problem of tritium control was a major health and environmental concern at the time of the MSRE project. However, the use of a helium gas turbine without need for water or steam anywhere in the system effectively eliminates this problem in the LFTR design [10]. The LFTR design allows for tritium to be removed from the system for safe disposal as explained by Forsberg “Adoption of a Brayton cycle provides an alternative tritium trapping option where the tritium is removed from the helium in the Brayton power cycle. This is potentially a high-performance low-cost option based on demonstrated inexpensive methods to remove tritium gas or tritiated water from helium” [10]. With the use of the Brayton power cycle, which uses gas instead of steam turbines [18], the LFTR eliminates the risk of producing tritium contaminated water and has a low cost and effective method to remove tritium for proper disposal. Unlike the MSRE, tritium control is not a problem in LFTRs because the design does not use a steam power cycle, instead opting for a design using Brayton cycle gas turbines.

3. CorrosionCorrosion caused by the fuel salt mixture is still arguably the greatest concern with MSR

technology including LFTRs. As stated earlier, MSR technology cannot succeed without a material of suitable corrosion resistance. Towards the end of the MSRE program ORNL researchers had made significant progress in improving the corrosion resistance of the Hastelloy-N used in the MSRE [12], but

16

the program was discontinued before an alloy of suitable resistance could be developed. However, much like the problem of noble metal plate-out, major advancements in material science may offer a solution to corrosion problems. Research done at the University of Wisconsin-Madison found a nickel based alloy with strong corrosion resistance properties in molten salts1. The report on the research states that “Results from the static corrosion tests show that Fe-Ni-Cr alloys are typically corroded by chromium depletion along the grain boundaries… Consequently, Ni-201 alloy, a nearly pure nickel alloy, showed the best corrosion resistance due to low thermodynamic stability of nickel-based corrosion products” [21]. This Ni-201 alloy helps to combat corrosion at its source along the grain boundaries. The research also studied corrosion resistance techniques involving using additives on the molten salt which yielded positive results. In addition to new nickel based alloys, silicone impregnated carbon-carbon composite has shown great promise as a fuel salt corrosion resistant material [12]. While both of these materials, as well as the molten salt additives, must be put through further testing to determine corrosion resistance to the molten salt mixtures proposed for LFTR use, as well as longer term corrosion resistance, both show great promise for use in LFTRs. While a material shown to be capable of resisting corrosion over the lifetime of an LFTR has not been found as of yet, the potential shown by materials like these demonstrate that a solution may be close at hand. As understanding of material sciences continues to advance, it is not unreasonable to surmise that a material of suitable corrosion resistance may be found in the near future if proper focus is put into this area of research.

In the decades since the shutdown of the MSRE project, general advancements in science and technology have solved the problem of tritium control. Additionally, subsequent research on molten salts and advancements in material sciences have greatly reduced the problem of noble metal plate-out, and corrosion. While these problems have not been fully remediated, experts have suggested that if proper funding and interest is given to LFTR research these problems could be easily solved [12], [2]. According to Cooper, Minakata, Miroslav, and Crittenden “A LFTR program could be achieved through a relatively modest investment of roughly 1 billion dollars over 5-10 years to fund research to fill minor technical gaps, then construction of a reactor prototype, and finally a full-scale reactor” [2]. It would appear that given proper attention, the remaining problems hindering LFTR implementation can be solved with modern technology and scientific knowledge. While the MSRE had several technical issues preventing MSR technology from being safety and effectively implemented on a large scale, many of these issues have been either solved or greatly reduced through scientific and technological advancement in the decades since the MSRE shutdown. If proper funding and interest is invested into finding a solution to noble metal plate-out and corrosion problems it is reasonable to predict that these issues will be solved in the foreseeable future.

VI. Conclusion

As global reserves of fossil fuels continue to deplete alternative energy sources will need to continually take on the burden of providing for humanity’s energy needs. Nuclear power offers the enticing prospects of being a relatively efficient, widely available, carbon free energy source. However, risk of catastrophic nuclear meltdown and the large amount of dangerous nuclear waste produced by NPPs, make nuclear power a less than desirable option for the successor to fossil fuels. Before nuclear power can be trusted to assume the role of a leading energy production method in the post fossil fuel era these problems with safety and environmental protection must be either substantially reduced or solved outright.

The LFTR is capable of solving or greatly reducing the major problems associated with nuclear energy. Its innovative freeze plug safety system makes meltdown virtually impossible, eliminating the

1 The molten salt used in this research was a “FLiNaK (LiF-NaF-KF: 46.5-11.5-42 mol%) molten salt” [21]

17

problem of greatest public concern. Additionally, the LFTR waste is dangerously radioactive for only 3% of the time that conventional nuclear waste is, and the LFTR produces 300 time less waste overall. This is a huge reduction in the amount and lifespan of nuclear waste produced. However, some of the fission products of LFTRs can be hazardous to human health and the environment at large, so the must be handled and stored with extreme care. Though dangerous fission products may still be an issue, the major issues originally hindering development of MSR and therefore LFTR technology have all been solve or largely mitigated.

In the decades since the shutdown of the MSRE project at ORNL, technological and scientific advancements have offered solutions or effective reduction methods to the major problems of MSR operation. Through development of gas turbine systems, tritium control is no longer an issue. Likewise the problems of noble metal plate-out and corrosion have been greatly reduced with the development of new materials, processing techniques, and fuel salt additives. Furthermore, if proper funding and attention is given to developing LFTRs for common use, these problems could be further reduced or even solved completely.

After reviewing the advantages and disadvantages of LFTR use, it has become apparent that the benefits of using LFTRs as opposed to conventional solid fuel nuclear reactors far outweigh the disadvantages. Additionally, examination of the progress made in overcoming the technological barriers of the MSRE program that preceded LFTR technology, it is reasonable to infer that widespread implementation of LFTRs should be possible if the technology receives increased interest and investment.