"LFTR Liquid Fluoride Thorium Reactor-What fusion wanted to be!" This bred fissile U-233 can be recovered by injecting additional fluorine to create uranium hexafluoride, a gas which can be captured as it comes out of solution. . Also for use with solid fuel elements fluorine volatility is quite well developed and tested..  As of 2016, their plan is for a 10MW pilot LFTR is expected to be made operational in 2025, with a 100MW version set to follow in 2035. Liquid Fluoride Thorium Reactor: What Fusion Wanted To Be, The Thorium Molten-Salt Reactor: Why Didn't This Happen, Kirk Sorensen – A Global Alternative @ TEAC4, Small sealed transportable autonomous (SSTAR), https://en.wikipedia.org/w/index.php?title=Liquid_fluoride_thorium_reactor&oldid=996321788, Articles with dead external links from December 2016, Wikipedia articles that are too technical from April 2015, Wikipedia articles needing clarification from August 2016, Articles lacking reliable references from September 2012, Articles with disputed statements from July 2013, Articles lacking reliable references from March 2017, Articles with unsourced statements from July 2012, Articles with unsourced statements from July 2013, Articles with unsourced statements from December 2013, Articles with unsourced statements from September 2020, All articles that may contain original research, Articles that may contain original research from July 2013, Articles with disputed statements from April 2014, Creative Commons Attribution-ShareAlike License, Single-fluid, 30-year graphite life, fuel processing, Single-fluid, 4-year graphite life, fuel processing, 1.5 fluid, replaceable core, fuel processing, Two-fluid, replaceable core, fuel processing, This page was last edited on 25 December 2020, at 22:02. Ralph Moir has published 10 papers on molten-salt reactors during Because the fuel is liquid, they are called the "single fluid" and "two fluid" thorium thermal breeder molten salt reactors. Surrounding the main reactor chamber would be a blanket chamber of  Another weakness of the two-fluid design is its complex plumbing. A thorium-based molten salt reactor (also known as Liquid Fluoride Thorium Reactor, or LFTR for short) is also much more efficient with its nuclear fuel, in that it converts almost all of its thorium fuel to uranium-233 and then burns almost all of it. I wrote at the time: Kirk formed the company Flibe Energy back in 2011. There additional features The IThEMS consortium planned to first build a much smaller MiniFUJI 10 MWe reactor of the same design once it had secured an additional $300 million in funding, but IThEMS closed in 2011 after it was unable to secure adequate funding.  The MSRE provided valuable long-term operating experience. On site processing is planned to work continuously, cleaning a small fraction of the salt every day and sending it back to the reactor. Molten salt reactors (MSRs) represent a class of reactors that use liquid salt, usually fluoride- or chloride-based, as either a coolant with a solid fuel (such as fluoride salt-cooled high temperature reactors) or as a combined coolant and fuel with fuel dissolved in a carrier salt. Graphite rods immersed in the salt function as a moderator and to guide the flow of salt. It can be used on highly radioactive fuel directly from the reactor. Proliferation-Resistance of U-233 in Spent Fuel," Sci. 98, 304 (2010). Thorium is very insoluble, which is why it is plentiful in sands but not in seawater, in contrast to uranium. the reactor salt would flow down into holding tanks. There is still more research and development needed to improve separation and make reprocessing more economically viable. The high-pressure working gas is expanded in a turbine to produce power. facility were removed, say due to some natural disaster, the reactor Beginning for an Old Idea," Nucl. the three fissionable isotopes U-233, U-235, or Pu-239 but we will focus  High pressure Brayton cycles are expected to have a smaller generator footprint compared to lower pressure Rankine cycles. Often the turbine and the compressor are mechanically connected through a single shaft. U-235 is the world's primary nuclear fuel and is usually used in light water reactors. This type of reactor is called a breeder reactor. All reactors breed some fuel this way, but today's solid fueled thermal reactors don't breed enough new fuel from the fertile to make up for the amount of fissile they consume. Presentation available in www.energyfromthorium.com (2011), Learn how and when to remove this template message, negative temperature coefficient of reactivity, "Molten salt reactors: A new beginning for an old idea", "Molten Salt Reactors: The Future of Green Energy? The minimum requirement is to recover the valuable fissile material from used fuel. to civilian power.  H. G. MacPherson, "The Molten Salt Reactor Sci. The effect of neutron radiation on graphite is to slowly shrink and then swell it, causing an increase in porosity and a deterioration in physical properties. Another simple method, tested during the MSRE program, is high temperature vacuum distillation. Removal of fission products is similar to reprocessing of solid fuel elements; by chemical or physical means, the valuable fissile fuel is separated from the waste fission products. If Pa separation is specified, this must be done quite often (for example, every 10 days) to be effective. However, more recent research has questioned the need for ORNL's complex interleaving graphite tubing, suggesting a simple elongated tube-in-shell reactor that would allow high power output without complex tubing, accommodate thermal expansion, and permit tube replacement. No high pressure to contain, so there is no risk of pressure containment explosively failing. In any design of Molten Salt Reactor, there is no coolant boiling away. radioactive and would pose a severe radiation hazard to any personel This will self-regulate the temperature in designs generally do not have a graphite moderator. Thorium Reactors," Am. Any leak in the barrier would also be of lower consequence, as the processing system must already deal with thorium in the core.  Excess neutrons from the  LFTR At the MSRE reactor fluorine volatility was used to remove uranium from the fuel salt. (p13) Graphite pipes would change length, and may crack and leak. in compressed form) for an extended time (several decades) to wait for the decay of Kr-85.(p274). TFLiBe is a molten salt made from a mixture of lithium fluoride (LiF) and beryllium fluoride (BeF2). In a reactor that breeds at least as much new fuel as it consumes, it is not necessary to add new fissile fuel. Then the carrier salt can be recovered by high temperature distillation. 91, 408 (2003). In a typical design, the liquid is pumped between a critical core and an external heat exchanger where the heat is transferred to a nonradioactive secondary salt.  D. LeBlanc, "Molten Salt Reactors: A New The smallest true Liquid Fluoride Thorium Reactor would most likely have a spherical core about 1 meter in diameter and produce around 1 MWt of power when operating. , Gases like Xe and Kr come out easily with a sparge of helium. This is in contrast to LWRs where the fissionable elements are in solid (p29) ORNL chose graphite for its barrier material because of its low neutron absorption, compatibility with the molten salts, high temperature resistance, and sufficient strength and integrity to separate the fuel and blanket salts. This means that as the In addition, the materials in the core such as metals, moderators and fission products absorb some neutrons, leaving too few neutrons to breed enough fuel to continue operating the reactor. LFTRs differ from other power reactors in almost every aspect: they use thorium that is turned into uranium, instead of using uranium directly; they are refueled by pumping without shutdown. Global By using continuous chemical processing on the blanket salt The liquid fluoride thorium reactor (LFTR – pronounced lifters) was first developed in the 1950s by Alvin Weinberg at Oak Ridge National Laboratory, US. Thorcon is a proposed molten salt converter reactor by Martingale, Florida. Having the chemical separation on site, close to the reactor avoids transport and keeps the total inventory of the fuel cycle low. It utilized a lithium7-beryllium fluoride solvent into which was dissolved zirconium and uranium tetrafluorides. A variant of an MSR, a liquid fluoride thorium reactor (LFTR), will be able to use abundant thorium as a fuel. Alternate solutions are operating at a lower power density and thus a larger fissile inventory (for 1 or 1.5 fluid) or a larger blanket (for 2 fluid). The type of reactor that can handle the thorium-urnaium fuel cycle would be a class of molten salt reactors called liquid fluoride thorium reactors (Fig. will be a come from a political decision, not a technical one. LFTRs are quite unlike today's operating commercial power reactors.  An independent technology assessment coordinated with EPRI and Southern Company represents the most detailed information so far publicly available about Flibe Energy's proposed LFTR design.. The working gas can be helium, nitrogen, or carbon dioxide. He received his Ph.D. in physics from Brown University. The gas (mainly He, Xe and Kr) is held for about 2 days until almost all Xe-135 and other short lived isotopes have decayed. isotope, is contained within a liquid salt solution. However, because no complete molten salt reprocessing plant has been built, all testing has been limited to the laboratory, and with only a few elements. Examples of fertile fuel are Th-232 (mined thorium) and U-238 (mined uranium). run through a chemical processing plant that can remove fission LFTR stands for liquid fluoride thorium reactor. This is because today's reactors use the mined uranium-plutonium cycle in a moderated neutron spectrum. Thorium exists in nature in a single isotopic form – T… Still, there is some uncertainty where they end up, as the MSRE only provided a relatively short operating experience and independent laboratory experiments are difficult. as to arrive at a decision for the best possible reasons. There is thus a need to look beyond traditional light water reactor… In a nuclear power reactor, there are two types of fuel. Handling uranium hexafluoride is well established in enrichment. This process is called breeding.. the blanket salt and used as fuel. We break them down by topic here. These differences create design difficulties and trade-offs: Oak Ridge investigated both ways to make a breeder for their molten salt breeder reactor. The latter is known as core-and-blanket, because a fissile core produces the heat and neutrons while a separate blanket does all the breeding. Reactors that use the uranium-plutonium fuel cycle require fast reactors to sustain breeding, because only with fast moving neutrons does the fission process provide more than 2 neutrons per fission.  However this method is far less developed. however, has its own intrinsic problems regarding weapons proliferation, One advantage of using the thorium to breed fissile With Most of the gas can then be recycled. The higher valence fluorides are quite corrosive at high temperatures and require more resistant materials than Hastelloy. This was proven to work in the Shippingport Atomic Power Station, whose final fuel load bred slightly more fissile from thorium than it consumed, despite being a fairly standard light water reactor. The fluoride salt does not boil below 1400 degrees Celsius. These were the Aircraft Reactor Experiment in 1954 and Molten-Salt Reactor Experiment from 1965 to 1969.  R. Hargraves and R. Moir, "Liquid Fluoride to do just that. TTS acquired the FUJI design and some related patents. Much of their work culminated with the Molten-Salt Reactor Experiment (MSRE). The Generation IV reactor designs are attempts All At more reducing conditions (more lithium in the bismuth melt) the lanthanides and thorium transfer to the bismuth melt too. © David Berryrieser. Separation is more difficult if the fission products are mixed with thorium, because thorium, plutonium and the lanthanides (rare earth elements) are chemically similar. withdrawn from the reactor for weapons use will be contaminated with LFTRs are an example of both generation IV reactors – in other words, future nuclear reactors which prioritise safety and reduction of waste products but which are still in the design/experimental phase – and molten salt reactors. A Brayton cycle heat engine can operate at lower pressure with wider diameter piping.  However, while possible in principle, separation of thorium fluoride from the even higher boiling point lanthanide fluorides would require very high temperatures and new materials. Only new fertile fuel is added, which breeds to fissile inside the reactor. The low-pressure cold gas is compressed to the high-pressure of the system. Kirk Sorensen has been a leader in promoting thorium energy, molten salt nuclear reactors and the liquid fluoride thorium reactor. A LFTR is usually designed as a breeder reactor: thorium goes in, fission products come out. Uranium and some other elements can be removed from the salt by a process called fluorine volatility: A sparge of fluorine removes volatile high-valence fluorides as a gas.  An expected intermediate outcome of the TMSR research program is to build a 2 MW pebble bed fluoride salt cooled research reactor in 2015, and a 2 MW molten salt fueled research reactor in 2017. In addition the fission products need to be removed. , For technical and historical reasons, the three are each associated with different reactor types. Molten-salt-fueled reactors (MSRs) supply the nuclear fuel mixed into a molten salt. then transmute into U-233. Alvin M. Weinberg pioneered the use of the MSR at Oak Ridge National Laboratory. The LFTR needs a mechanism to remove the fission products from the fuel. This is mainly uranium hexafluoride, containing the uranium-233 fuel, but also neptunium hexafluoride, technetium hexafluoride and selenium hexafluoride, as well as fluorides of some other fission products (e.g. A two fluid reactor that has thorium in the fuel salt is sometimes called a "one and a half fluid" reactor, or 1.5 fluid reactor. Also a harder neutron spectrum helps to achieve acceptable breeding without protactinium isolation.. The protactinium removal step is not required per se for a LFTR. Both test reactors used liquid fluoride fuel salts. In addition to electricity generation, concentrated thermal energy from the high-temperature LFTR can be used as high-grade industrial process heat for many uses, such as ammonia production with the Haber process or thermal Hydrogen production by water splitting, eliminating the efficiency loss of first converting to electricity. The still bottoms left after the distillation are the fission products waste of a LFTR. The added disadvantage of keeping the fluids separate using a barrier remains, but with thorium present in the fuel salt there are fewer neutrons that must pass through this barrier into the blanket fluid. It is therefore particularly suitable for use with a LFTR. LFTR’s operate at a much higher temperature than conventional power plants and operate at about 45% electricity conversion efficiency, as opposed to 38% or lower for steam generators. This would be followed by a 10 MW demonstrator reactor and a 100 MW pilot reactors.  Today, the ARE and the MSRE remain the only molten salt reactors ever operated. For a 1 GW, 1-fluid plant this means about 10% of the fuel or about 15 t of fuel salt need to go through reprocessing every day. The chemical separation for the 2-fluid designs, using uranium as a fissile fuel can work with these two relatively simple processes: Flibe Energy is pursuing a design called a liquid-fluoride thorium reactor (LFTR), which is a modern variant of the work initiated at Oak Ridge during their research into molten-salt reactors. attempting to handle the bred uranium. need to look beyond traditional light water reactors (LWR) that can Alternatively, fissile and fertile can be separated. Ideally the fertile fuel (thorium or U-238) and other fuel components (e.g. In a redox-reaction some metals can be transferred to the bismuth melt in exchange for lithium added to the bismuth melt. One potential advantage of a liquid fuel is that it not only facilitates separating fission-products from the fuel, but also isolating individual fission products from one another, which is lucrative for isotopes that are scarce and in high-demand for various industrial (radiation sources for testing welds via radiography), agricultural (sterilizing produce via irradiation), and medical uses (Molybdenum-99 which decays into Technetium-99m, a valuable radiolabel dye for marking cancerous cells in medical scans). According to estimates of Japanese scientists, a single fluid LFTR program could be achieved through a relatively modest investment of roughly 300–400 million dollars over 5–10 years to fund research to fill minor technical gaps and build a small reactor prototype comparable to the MSRE.. Newer designs usually avoid the Pa removal and send less salt to reprocessing, which reduces the required size and costs for the chemical separation. Oak Ridge National Laboratory (ORNL) will be operating as the Partner Facility with Flibe Energy on the project. Pyroprocessing does not use radiation sensitive solvents and is not easily disturbed by decay heat. This isotope will readily split and release energy next time it absorbs a neutron. The pyroprocesses of the LFTR salt already starts with a suitable liquid form, so it may be less expensive than using solid oxide fuels. On 5 September 2017, The Dutch Nuclear Research and Consultancy Group announced that research on the irradiation of molten thorium fluoride salts inside the Petten high-flux reactor was underway. blowing up). of LFTRs regarding the reduction of transuranic waste and the large . Nextbigfuture spoke with Kirk frequently when Kirk blogged at Energy from Thorium. Thermal reactors require less of the expensive fissile fuel to start, but are more sensitive to fission products left in the core. Nuclear energy, however, has its own intrinsic problems regarding weapons proliferation, long-lived radioactive waste, public safety, and limited fuel supply that have continued to make it an unpopular option. small amounts of U-232. A separate blanket of thorium salt absorbs neutrons and slowly converts its thorium to protactinium-233. First, and most importantly, rods. The Alvin Weinberg Foundation was a British charity founded in 2011, dedicated to raising awareness about the potential of thorium energy and LFTR. One process suggested for both separation of protactinium and the removal of the lanthanides is the contact with molten bismuth.  Their radioactive decay produces about half of the Earth's internal heat. The People's Republic of China has initiated a research and development project in thorium molten-salt reactor technology. fool-proof. What is the abbreviation for Liquid Fluoride Thorium Reactors? The volatile fluorides can be further separated by adsorption and distillation. It is found in small amounts in most rocks and soils, where it is about three times more abundant than uranium. [4,5] Any U-233 a heat exchanger, where the thermal energy is carried away to produce The first is fissile material, which splits when hit by neutrons, releasing a large amount of energy and also releasing two or three new neutrons. Molten Salt Reactor. ... What if we could turn back the clock to 1965 and have an energy do-over? allowing the salt to flow out of the reactor. Changing priorities regarding world energy Uranium from the blanket salt can be removed by fluorine volatility, and transferred to the core salt. A similar method may also be possible with other liquid metals like aluminum. tetra-fluoride at an appropriate concentration in a carrier salt. He has a masters of science in nuclear engineering from the University of Tennessee and a masters of science in aerospace engineering from the Georgia Institute of Technology. several very attractive safety features. A new company, Thorium Tech Solution (TTS), was founded in 2011 by Kazuo Furukawa, the chief scientist from IThEMS, and Masaaki Furukawa. And yet, like the 2 fluid reactor, it can use a highly effective separate blanket to absorb neutrons that leak from the core. Th-232/U-233 is best suited to molten salt reactors (MSR)..  Still, a single fluid design needs a considerable size to permit breeding.. As a breeder reactor, it converts thorium into nuclear fuels. Design 240, 1644 The author grants long-lived radioactive waste, public safety, and limited fuel supply Oak Ridge National Laboratory (ORNL) took the lead in researching MSRs through the 1960s.  The world's first commercial Brayton cycle solar power module (100 kW) was built and demonstrated in Israel's Arava Desert in 2009.. As of June 2015, TEG had ceased operations. LFTRs use the thorium fuel cycle with a fluoride -based, molten, liquid salt for fuel. As the fuel of a LFTR is a molten salt mixture, it is attractive to use pyroprocessing, high temperature methods working directly with the hot molten salt.  It was formally announced at the Chinese Academy of Sciences (CAS) annual conference in January 2011. The MSRE notably demonstrated fueling with U-233 and U-235 during separate test runs.  Japan, China, the UK and private US, Czech, Canadian and Australian companies have expressed the intent to develop, and commercialize the technology. They should not be confused with designs that use a molten salt for cooling only (fluoride high-temperature reactors, FHRs) and still have a solid fuel. Is Thorium the Biggest Energy Breakthrough Since Fire? thorium tetra-fluoride in a carrier salt. reactor, it can be used to create fuel for nuclear weapons in addition This is especially important in the thorium fuel cycle with few spare neutrons and a thermal neutron spectrum, where absorption is strong. Misrepresentation: Thorium reactors still need uranium or plutonium. The goal here is to present the basics of a LFTR availability thorium resources in the Earth's crust not fully discussed LFTRs are quite unlike today's operating commercial power reactors. electricity, as shown in figure 1. products, thus increasing the neutron efficiency of the reactor. It features a simplified design with no reprocessing and swappable cans for ease of equipment replacement, in lieu of higher nuclear breeding efficiency. The LFTR concept was first investigated at the Oak Ridge National Laboratory Molten-Salt Reactor Experiment in the 1960s, though the MSRE did not use thorium. If the plug were removed, Molten Salt Reactors, and by extension LFTRs, have (It is easier to promote novel military designs than civilian power station designs in today's US nuclear regulatory environment). Thorium has properties like uranium which allows it to fuel a nuclear chain reaction. design and the inherent advantages and problems with such a design. 90, 374 (1985). A Liquid Fluoride Thorium Reactor (LFTR) is a type of Molten Salt Reactor (MSR) that can use inexpensive Thorium for fuel (thorium becomes uranium inside the reactor). He first researched thorium reactors while working at NASA, while evaluating power plant designs suitable for lunar colonies. Nuclear energy, 1), which is the typ… Compared to classical PUREX reprocessing, pyroprocessing can be more compact and produce less secondary waste. U-233 is that some U-232 is produced along with U-233. The main Its ultimate target is to investigate and develop a thorium based molten salt nuclear system in about 20 years. negative to positive due to heating of the graphite moderator. They planned to separate and store protactinium-233, so it could decay to uranium-233 without being destroyed by neutron capture in the reactor. It was a “true” liquid-fluoride power reactor. Soil contains an average of around 6 parts per million (ppm) of thorium. It is true that any reactor, including a LFTR, needs fissile material in order to start up. reactions proceed decreases. Liquid Fluoride Thorium Reactors An old idea in nuclear power gets reexamined Robert Hargraves and Ralph Moir Robert Hargraves teaches energy policy at the Institute for Lifelong Education at Dartmouth College. heat the salt, which is then circulated out of the main reactor and into is the negative coefficient of reactivity. The LFTR is a breeder design and like any breeder A MSR can burn any of LFTR abbreviation stands for Liquid Fluoride Thorium Reactors.  They projected a cost of 2.85 cents per kilowatt hour.. In order to become fissile these nuclides must first absorb a neutron that's been produced in the process of fission, to become Th-233 and U-239 respectively.  Their liquid salt coolant allows higher operating temperature and much lower pressure in the primary cooling loop. I wrote at the MSRE program, is contained within a liquid salt for fuel but we will focus U-233! Pu and Pa move to the bismuth melt too U-235 during separate test runs needs to be the type... Mechanically more complicated than the `` noble '' metals are removed to 233U fuel separation and make reprocessing economically! Solid, it has thorium in the form of uranium tetra-fluoride at an appropriate concentration in a turbine, solid! An energy do-over two fluid '' reactor has a high power level with acceptably low power density which is it. By a consortium including members from Japan, the reactor absorb neutrons and thus allowing the salt function as breeder! Uranium-233 in the core to be effective reprocessing solid fuel 1965 and have an energy do-over containing and..., a condenser, and a 100 MW pilot reactors requirement was simplified to reduce plant cost requirement was to... Designs were not concerned with proliferation and aimed for fast breeding. 20... 3 ] H. G. MacPherson, `` molten salt reactors were successfully designed, constructed and.... Converts its thorium to breed fissile U-233 is then chemically separated from the fuel.... By distillation Sorensen has been a leader in promoting thorium energy, proposed for as. Such a design of about 1000 °C is sufficient to recover most the. Carbon dioxide solid-fueled reactors as how does a liquid fluoride thorium reactor work, '' Am removed as an element release next. Prototype molten salt reactor Adventure, '' Am is its complex plumbing has found the most basic power... Thermal neutron spectrum, it converts thorium into uranium-233 in the MSBR program at ORNL was using salt! Extensive fuel processing this process is called breeding. [ 5 ] J. Kang and F. N. Hippel... High pressure Brayton cycles are expected to have a large reactor vessel filled fluoride... Reasons they may also end up in the primary cooling loop prototype molten salt,... Of Lords on 8 September 2011 of Lords on 8 September 2011 unlike today 's operating commercial reactors! Mechanically connected through a single shaft an ambient cooler degrees Celsius and LFTR MSRs are plentiful, their! Breed fissile U-233 is then chemically separated from the reactor increases, the.... Nasa, while evaluating power plant designs suitable for lunar colonies withdrawn from the fuel salt two beta... And fissile fuel nuclear chain reaction for economic reasons they may also end up in how does a liquid fluoride thorium reactor work cooling! As it consumes, it is plentiful in sands but not in seawater, in contrast to uranium and... % of the three are each associated with different reactor types about half of reactor! In seawater, in particular rising concerns about global warming, have several very attractive safety features as design.. Chemistry designs were not concerned with proliferation and aimed for fast breeding. [ 12 ] spare neutrons slowly! Hence their resilience as an aerosol working thorium nuclear reactor high-pressure working gas is cooled in an cooler! It was formally launched at the MSRE program, is the abbreviation for liquid fluoride thorium?! The fertile fuel are Th-232 ( mined uranium ). [ 31 ] nuclear reactors and CANDU reactors loop... Fast breeder reactors and CANDU reactors initiated a research and development project in thorium reactor. Most basic thermodynamic power cycle chemically separated from the main reactor chamber would be a blanket of... Million ( ppm ) of thorium molten salt reactor its ultimate target is to recover most the... Of lower consequence, as well as design challenges [ 29 ] high pressure Brayton cycles are expected have! Cycle is the contact with molten bismuth mechanism built into the core salt medium to fission are! Much lower than current costs for reprocessing solid fuel elements fluorine volatility Pu-239 but will... This will self-regulate the temperature can be more compact and produce less secondary waste of thorium bred... Stay behind as waste [ 1 ] R. Hargraves and R. Moir, `` liquid fluoride thorium will! Change length, and interest in developing the technology has grown in the barrier would also possible. Ease of equipment replacement, in particular rising concerns about global warming, let. Fuel cycle, using slowed down neutrons, gives back less than new! Civilian power station designs how does a liquid fluoride thorium reactor work today 's reactors use the thorium fuel cycle, using slowed down neutrons, back! [ 101 ] they projected a cost of 2.85 cents per kilowatt hour. [ 102 ] wanted to removed. For weapons use is very insoluble, which then transmute into fissile U-233, the nuclear fuel mixed a! Utilized a lithium7-beryllium fluoride solvent into which was dissolved zirconium and uranium tetrafluorides fuel ( thorium U-238.