Uranium vs Thorium for Power Production: IFR vs LFTR—an Overview George S. Stanford September 2013
The possibility of using molten-salt reactors with thorium as the fertile element has captured the imagination of a number of researchers, who see the potential for thorium-based reactors to be superior to today’s uranium-based light-water reactors in several respects. This document is one person’s attempt to express concisely the essence of the thorium/uranium situation. That’s a bit of a challenge, however, since so many variations of uranium- and thorium-based reactors have been proposed, including some hybrids, that a comprehensive discussion would get hopelessly bogged down with esoteric, mind-numbing details. In this paper, therefore, one prominent example of each category of reactor has been selected for discussion: the IFR1 is considered representative of Generation-IV uranium-fueled fast reactors, and the LFTR2 is taken as typical of Gen-IV3 thorium-fueled reactors, even though currently its technology is not nearly as mature as that of the IFR. Thus we will have much-simplified view of the uranium-vs-thorium picture, under the hypothetical assumption that thorium-molten-salt technology has successfully achieved the maturity that has already been reached with uranium-based fast-reactor technology. Let’s be perfectly clear. The issue is not whether to deploy LFTRs (or some other thoriumbased technology)4 instead of IFRs or LWRs—the IFRs are almost ready, the LFTRs are not, and LWRs will probably continue being deployed for several decades at least, regardless. So if this exercise is only hypothetical, why do it at all? The intention is to try to fill the public-information gap regarding the place of the IFR in the energy picture. In the publicity being given to thorium technology’s potential, seldom, if ever is there mention of the moremature fast-reactor technology. This has had the effect of distracting attention from the need to start now to deploy the next-generation reactors that can reduce the consumption of fossil fuel, while dealing with worries about nuclear waste and weapons proliferation. The hope is that this paper will throw light on the plusses and minuses of going with the bird in the hand (the IFR) versus waiting for the one in the bush (the LFTR). The conclusions reached are summarized in tabular form at the end of the document. 1
IFR: Integral Fast Reactor, developed at Argonne National Laboratory, ~1970—1994. The program was aborted, for non-technical reasons, in1994, when it was on the verge of commercial demonstration. 2 LFTR (pronounced “lifter”): Liquid Fluoride Thorium Reactor, an extension (so far on paper only) of early work on thorium reactors (discontinued in the 1970s). 3 The reactors currently being built are considered to belong to the third generation (Gen-III). 4 Besides the LFTR, another thorium-burning concept is attracting interest. Called the DMSR (Denatured Molten Salt Reactor), some U-238 is included to “denature” the U-233. In one of its forms, it is visualized as a oncethrough, 30-year fuel cycle, with a breeding ratio of ~0.8, which makes it sort of a compromise between LWR and LFTR. For simplicity, the DMSR is not discussed further in this overview. Uranium vs Thorium for Power Production
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Definitions and Background Information
A nucleus has an atomic number and a mass number. The atomic number (number of protons) determines the element; e.g., 90, 92, and 94 are the atomic numbers of thorium, uranium, and plutonium, respectively. The mass number is the number of nucleons (protons plus neutrons) in the nucleus. For a given element, the number of neutrons determines the isotope; e.g., the two main isotopes of uranium (U-238 and U-235) have 146 and 143 neutrons, respectively (238 and 235 being the mass numbers). Heavy-metal nuclei (“actinides”—thorium and above) are either fissile (the ones with odd mass numbers) or fertile (even mass numbers). A fissile nucleus has a good probability of fissioning (splitting into two lighter nuclei) when struck by a neutron of any energy; but sometimes it just absorbs the neutron and is transmuted into a fertile nucleus. A fertile nucleus is transmuted into a fissile nucleus upon absorbing one neutron. However, fertile nuclei are said to be fissionable, since they sometimes fission if struck by a fast neutron. The most important fissile nuclei are U-233, U-235, Pu-239, and Pu-241. The most important fertile nuclei are Th-232 (becomes U-233 after absorbing a neutron), U-238 (becomes Pu-239), and Pu-240 (becomes Pu-241). A thermal reactor is one whose neutrons are moderated (slowed down) to improve their chances of causing fissions in the fissile isotopes. Most of the thermal reactors use water (heavy or light) as the moderator, but some use carbon. In a fast reactor, the neutrons remain fast—i.e., the coolant is a non-moderating fluid. Each type of reactor needs fuel with a certain minimum fissile concentration in order to function (achieve and maintain criticality5). Each type of reactor also has a minimum fissile loading (amount of fissile per GWe of capacity). In some cases, as the reactor size gets small (less than ~500 MWe), the needed fissile percentage goes up. Today’s reactors are almost exclusively Gen-II (Generation 2) or Gen-III thermal reactors. Most of them are fueled with low-enrichment uranium (~4% U-235) and cooled and moderated by light water, but some (CANDUs) use heavy water and natural uranium (which is 0.7% U-235). Today’s thermal reactors use less than 1% of the energy in the mined uranium, and cannot exceed 1% even when some of the plutonium is recycled, as is done in France. To get almost all the energy out of the mined uranium, a fast reactor (whose neutrons are not moderated) must be used, not a thermal or epithermal reactor (in the latter, the energy of the neutrons is reduced considerably, but not all the way down to thermal levels). With fast reactors, enough uranium has already been mined to power the world for several centuries.
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When the chain reaction (or power output) in a reactor is steady—neither increasing nor decreasing—the reactor is said to be critical.
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Some Facts about Uranium and Thorium
There’s enough uranium or thorium to power civilization for as long as it lasts. There are designs for passively safe reactors using either thorium fuel or uranium fuel. Most of the reactors built in the next decade or two will be today’s types of thermal reactors (Gen-III), which are water-moderated and uranium-fueled. However, Gen-IV reactors are being actively developed in several countries—notably India, China, South Korea, Japan, and France (but no longer in the United States, which abdicated its leadership in that area in 1994, with the aborting of the IFR development program when it was on the verge of a commercial-scale demonstration). Various types of thorium-based reactors are being discussed currently, with no consensus yet in the thorium community regarding which types would be the best ones to pursue. Thorium advocates usually cite the advantages that their reactor concept offers over today’s thermal reactors. The problem there is that theirs is really a Gen-IV (or Gen-V) reactor, whose competition will not be today’s variety of thermal reactors. Very similar improvements are offered by at least one uranium-based Gen-IV concept—the IFR, whose existence the thorium community rarely acknowledges voluntarily. The uranium-based IFR is the Gen-IV concept that is closest to commercialization. It is embodied in General Electric Hitachi’s PRISM concept, and that company is ready to build a commercial demonstration plant when given the go-ahead. The LFTR is the thorium-based reactor concept that seems to get the most attention these days. In 2002, a 242-person DOE task force ranked the Argonne-developed IFR (referred to as “Na Metal Pyro”) the most promising, overall, of 19 next-generation (Gen-IV) reactor concepts. Of the other concept categories, the closest to the LFTR was the “molten-salt cooled” reactor (MSR), which ranked sixth. The LFTR’s state of maturity is about where the IFR’s was in the 1970s—a promising concept with significant technological questions to be answered before commercialization can be considered. So the hypothetical question addressed in this essay is this: What would be the pros and cons of IFR vs LFTR technology, if the two technologies were both ready for commercialization—i.e., the technical challenges still to be encountered in commercializing the LFTR are assumed to have been satisfactorily resolved, so that LFTRs perform as predicted by their supporters. LFTR Claims, the IFR, and Related Facts Thorium-fueled reactors are viable energy producers, destined, it seems likely, to be part of the next-generation mix of reactor types in some countries (India for one). In comparing thorium and uranium as reactor fuels, however, the advocates of thorium-based reactors almost always emphasize their potential advantages with respect to today’s Generation-II and -III (uraniumfueled) thermal reactors. But that is not a relevant comparison, because, by the time a thorium cycle has been developed to commercial readiness, the main competition will not be from GenIII, but from Gen-IV—probably uranium-fueled fast reactors. Uranium vs Thorium for Power Production
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Although seldom mentioned by the more vocal thorium supporters, the Gen-IV IFR is now ready for commercial demonstration,6 and its closed fuel cycle has virtually all the advantages over Gen-III that are claimed for the thorium cycle: passive safety waste minimization resource utilization sustainability proliferation resistance thermal efficiency cost of fuel overall cost The various claims made for the LFTR will now be considered, in the context of proven characteristics of the IFR. Passive Safety Claim: LFTRs have a number of design features and physical properties that make them exceptionally immune to accidents that could cause dangerous radioactivity to be dispersed off-site. One of those features is the fact that the LFTR’s coolant is not pressurized, so there can be no “blowdown”7 of pressurized coolant if a pipe breaks. Fact: That’s true. And the same can be said of the IFR (its coolant too runs at atmospheric pressure). Fact: In terms of public health, the safety record of civilian nuclear power is already better than that of any other heavy industry, and further improvement in accident resistance is icing on the cake. Fact: However, passive safety is of value to plant owners, to prevent the kind of expensive power-plant damage that happened at TMI, Chernobyl and Fukushima.
Claim: The LFTR would be built with a “freeze plug” that melts without operator intervention if the reactor overheats, dumping the liquid fuel into a reservoir where it would be passively cooled and not be able to go critical. Fact: The IFR also is designed to shut itself down if the core overheats, with the low-level heat removed passively by convective cooling. This gives the operators plenty of time to shut it down permanently or restart it, depending on circumstances. Conclusion: In practical terms, the passive-safety features of LFTRs and IFRs are equivalent.
Waste Minimization Claim: Because the LFTR does not use U-238 for its fertile isotope, the amount of longlived transuranics in its used fuel is considerably less than in used LWR fuel. Fact: True. And the waste from an IFR is similar, in that it has only trace amounts of longlived transuranics. 6
This is General Electric’s PRISM reactor, with recycle by pyroprocessing.
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A “blowdown" occurs when the coolant is high-pressure water and a major pipe breaks, permitting the water to flash into steam. There was a blowdown at Chernobyl. Uranium vs Thorium for Power Production
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Fact: While LFTRs could consume the bulk of the plutonium in the used thermal-reactor fuel, they would not use very much of the uranium that’s left (some 94% of the LWR fuel’s original uranium). That left-over uranium, plus about ten times as much depleted uranium, would be treated as radioactive waste (although its activity would be quite low). Conclusion: Overall, there is less waste from an IFR than from a LFTR. Resource Utilization Claim: The LFTR consumes almost 100% of its thorium fuel (an advantage with respect to LWRs). Fact: Perhaps true, depending on the details of the fuel cycle. And the IFR definitely consumes almost 100 percent of its uranium-plus-plutonium fuel. Fact: With IFRs, when all the plutonium from used thermal-reactor fuel has been recycled, the remaining uranium from that fuel plus the depleted uranium left over from the enrichment process could fuel a global fleet of IFRs for the better part of a millennium, and such a fleet could grow exponentially for as long as the energy demand was growing. Conclusion: IFRs utilize resources at least as efficiently as LFTRs. Sustainability Claim: Once a LFTR has been fueled with its initial loading of Th-232 plus the needed fissile (U-235 in uranium enriched to ~20%, or Pu from the weapons inventory or from used thermal-reactor fuel), it will generate its own replacement fissile (U-233). Only a tonne or so of thorium per GWe-year will be needed to keep the reactor running indefinitely. Fact: Probably true. Similarly, once an IFR has been fueled with its initial loading of uranium plus the needed fissile (probably Pu from the weapons inventory or from used thermalreactor fuel), it can generate its own replacement fissile (Pu-239, mainly), needing only a tonne or so of uranium per GWe-year to keep the reactor running indefinitely. Furthermore: Fact: IFRs can be configured to have a breeding ratio8 BR as high as 1.5 or more, leading to self-sustained doubling times of 15 years or less. Fact: The LFTR advocates do not claim a breeding ratio significantly greater than unity, which means that the growth of a LFTR fleet (and of nuclear power in general) would be seriously constrained once all of the easily available fissile had been committed, unless a prolific source of new fissile were to come into play. (Without an external supply of fissile material, a fleet of LFTRs cannot grow.)
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Claim: The fissile inventory needed to prime (start up) one IFR could prime five or more LFTRs with the same generating capacity. Fact: That is true. But consider the following: Fact: There will be enough fissile plutonium to prime all the IFRs or LFTRs that will be needed in the next 4 or 5 decades. Fact: But that only holds if the used thermal-reactor fuel is preserved for recycle. If it were to be disposed of irretrievably, that would (a) guarantee the continued operation and expansion of enrichment facilities and uranium mining, and also (b) seriously limit the The breeding ratio is defined to be the number of fissile nuclei created for every heavy-metal atom fissioned.
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expansion rate of nuclear power by restricting the number of advanced reactors—LFTRs or IFRs—that could be deployed. Fact: The larger fissile inventory in an IFR is an advantage when, as currently in the U.K., the desire is to sequester existing plutonium inventory as quickly as possible.
Claim: There is enough accessible thorium in the world that LFTRs would never run out of fertile material. Fact: True, but with the caveat that the sources of readily available fissile material must remain adequate to permit the needed growth in capacity. Fact: The same claim is true of IFRs—but without worry about the long-term supply of fissile material.
Claim: After the available plutonium runs out, new LFTRs could be primed with uranium enriched to approximately 20% U-235 (80% U-238). Fact: True, in principle. However, this approach has a number of problems, such as mentioned in the following three facts: Fact: These starting-up reactors would produce long-lived transuranics (plutonium and up) until all the U-238 in the enriched uranium had been transmuted to plutonium and fissioned (replaced by Th-232). Fact: Therefore startup uranium with a much higher enrichment (i.e., less U-238) would be desirable—but uranium enriched to more than 20% is currently ruled out because it is considered a proliferation hazard. Fact: Possession of enrichment facilities provides a nation with an easy path to fission weapons. A fleet of IFRs could grow indefinitely with never any need for enriched uranium, so that a nation that only used IFRs would have no legitimate need for an enrichment capability.
Claim: With LFTRs there would be no need for enriched uranium—as long as there’s a supply of plutonium from used fuel from thermal reactors. Fact: True. However, once that supply ran out (some decades hence), further expansion of nuclear power—LFTR or IFR—would be stymied without a source of fresh fissile material that is acceptable economically, environmentally, and proliferation-wise. Fact: To expect that source to be uranium is arguably unrealistic, since a lot of uranium would have to be readily available (~200 tons of natural uranium for every ton of U-235). And, importantly, a large enrichment capacity would have to be resuscitated, with its proliferation implications. Fact: As mentioned above, with IFRs no enrichment of uranium would ever again be needed. Conclusion: Both IFRs and LFTRs are sustainable—there is more uranium or thorium than could ever be used. However, since IFRs can breed more fissile than they use, their potential for growth is not limited by the need for fissile material from outside sources.
Proliferation Resistance Claim: LFTRs are proliferation-resistant because the fissile U-233 in them is too contaminated with U-232. Uranium vs Thorium for Power Production
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Fact: That’s very misleading—and it’s not a claim that all LFTR advocates make. Pure U-233 is an excellent material for bombs, and it can be produced by chemically separating from a LFTR’s molten fuel some of the intermediate Pa-233 before it decays (with a 27-day half-life) into pure U-233—and in some of the proposed thorium cycles that would be done routinely.
Claim: But the reactor cannot spare that U-233, because it is needed to keep the fission chain reaction going. Fact: The extracted U-233 can be replaced with poor-quality Pu-239 from used LWR fuel—after all, that’s what was probably used as the fissile material for starting up the LFTR in the first place. Thus an unsafeguarded LFTR could be a good device for converting lousy Pu into good-quality U-233 for bombs.
Claim: The LFTR eliminates the need for aqueous reprocessing (such as PUREX), which is required if plutonium bombs are to be made. Fact: True. And so does the IFR. Fact: To achieve anything near their maximum fuel-use efficiency, both IFR and LFTR have to have their fuel processed regularly to remove the fission products.
Claim: An IFR can make good-quality Pu-239 by being run with some special U-238 fuel elements. Fact: True. Any type of unsafeguarded reactor can be adapted to do that, including a LFTR.9 Fact: While safeguards are needed for assurance, in reality it is “[unlikely that any] weapons-intent nation would try to pervert either LFTR or IFR for producing weapons material—the proven paths work well, even for impoverished nations such as North Korea, Pakistan, and India.”10 Conclusion: There is no meaningful proliferation-related difference between the LFTR and the IFR. To assure that they are not being diverted for weapons production, both would have to be subject to international inspection.
Thermal Efficiency Claim: Since a LFTR can run at a higher temperature than an IFR, it can produce more electrical energy per unit of thermal energy developed. Fact: True, but the significance is easy to overestimate: At about 45% for the LFTR and 40% for the IFR, the difference is about 10%—which would seem to be significant. But there is a trade-off, because operating at higher temperatures requires more-expensive materials that can stand the heat, so it is not clear whether the higher temperature would be a net advantage. Fact: With very low fuel and operating costs, an efficiency difference of 10% or 15% is a relatively minor consideration.
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A possible exception to this general rule would be a reactor shipped as a small, sealed “nuclear battery.” Robert Hargraves, private communication.
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Fact: In principle, with judicious plant siting, the low-level heat from the power plant need not be entirely wasted, because it could be used for such tasks as desalination, process heat, or district heating.
Claim: The LFTR’s higher thermal efficiency means that less cooling water would be evaporated for a given electrical output. Fact: True, and this might give the LFTR an edge in dry areas where reducing demand for cooling water by 10% or 15% would be important, and where dry cooling towers or constructive uses for the excess heat would be impractical. Conclusion: The LFTR’s higher thermal efficiency might be significant for conserving water in some arid environments.
Cost of Fuel Claim: Thorium is produced in abundance as a waste product from the mining of rare-earth metals, and so the raw thorium would be essentially free for the indefinite future. Fact: True. Similarly, the IFR’s fertile material (uranium from used LWR fuel at first, followed by existing depleted uranium for the next few centuries) will also be essentially free for the better part of a millennium, and after that only one tonne of uranium per year per GWe of capacity would have to be procured. (For an LWR it’s closer to 150 tonnes per GWe-yr.) Fact: Although U-233 is not as attractive as Pu-239 for use in an IFR in breeder mode, there might be special applications where converting IFRs to a thorium/U-233 cycle would become desirable, economically or otherwise. Conclusion: The cost of raw fuel (uranium or thorium) would be a very small part of the cost of energy from either LFTRs or IFRs, and is not a major consideration. Overall Cost Claim: Simplifications stemming from the liquid nature of LFTR fuel make power from the MSR potentially cheaper than from the LWR, coal, or the IFR. Fact: The true cost of electricity is a very slippery topic, due in part to (a) the many external costs of burning fossil fuel that are borne by society, and (b) the fact that there is no commercial experience with either LFTRs or IFRs. Also there are non-technical but very important and hard-to-quantify considerations such as proliferation resistance and the future demand for of fissile material and its availability. Fact: To date there have been no operational, commercial-scale power plants utilizing either LFTR or IFR technology. Fact: However, General Electric Hitachi is confident that power from its PRISM version of the IFR will be economically competitive. Fact: Very high estimates of the cost of IFRs have been made, but they are irrelevant, since they are based on experience with the very complex and expensive PUREX type of processing that is done for oxide fuel. It’s true that on-site pyrometallurgical processing for metallic fuel cannot take advantage of the economy of scale that applies to a large, central
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PUREX plant, but pyro is technically very much simpler, and does not involve transportation of spent fuel. Conclusion: Since there are no empirical cost data, at this point there is no basis for predicting with confidence whether electricity from commercialized IFRs or LFTRs will be more expensive.
Summary Provided the LFTR works as hoped, in almost every case the technical comparison of LFTR with IFR comes out a wash or close to it. Relevant to energy policy, therefore, the important question is whether commercial demonstration of a technology that is ready now (the IFR) should be delayed for two decades or more while waiting for R&D to bring to maturity a technology (such as the LFTR) that might have some minor technical advantages—especially when those technical advantages cannot be more than marginal in any event. One should keep in mind that R&D can proceed regardless, and new technology can always be deployed when commercially ready. The most important differences between the two systems discussed here are three: (a) the degree of readiness for commercialization, (b) the fissile material needed, now and in the future, and (c) the potential breeding rate. In category (a), currently the advantage is clearly with the IFR, since construction of a commercial demonstration could start tomorrow. Meanwhile, R&D on advanced thorium systems could go forward, on the way to seeing whether there are applications where thorium has an economic advantage over uranium. Assuming equal readiness for commercialization, considerations in categories (b) and (c) favor one system or the other, depending on circumstances. A large fissile inventory in the plant is an advantage for segregating a lot of plutonium in a hurry (advantage IFR), but it’s a disadvantage if extremely rapid early growth of nuclear energy is wanted (advantage LFTR). However, if the lion’s share of global energy by the end of the century is to be supplied by nuclear power, the IFR can grow into that in an orderly manner, whereas deployment of a large number of LFTRs could preempt the readily available fissile material, putting serious constraints on continued growth of nuclear energy after mid-century. The first table on the next page summarizes the conclusions reached above, and the second one mentions more quantitatively some important additional considerations pertinent to comparing the demonstrated properties of IFRs with the expected properties of LFTRs. See below for ther conclusions in tabular form.
ACKNOWLEDGEMENTS Thanks to Robert Hargraves, Ralph Moir, Per Peterson, Ken Kok, and Dan Meneley for constructive and informative comments on earlier versions of this paper. Uranium vs Thorium for Power Production
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SUMMARIZED CONCLUSIONS REGARDING THE EXPECTED RELATIVE PERFORMANCE OF IFRS AND LFTRS Note: With the exception of growth potential, breeding ratio, and readiness for commercialization, all technical differences are minor or relatively unimportant. PROPERTY Passive safety Waste minimization Resource utilization Sustainability
Proliferation resistance
Thermal efficiency
Cost of fuel
Overall cost
COMPARISON, LFTR vs IFR In practical terms, the passive-safety features of LFTRs and IFRs are equivalent. Overall, there is less waste from an IFR than from a LFTR. IFRs utilize resources at least as efficiently as LFTRs. Both IFRs and LFTRs are sustainable—there is more uranium or thorium than could ever be used. However, since IFRs can breed more fissile than they use, their potential for growth is not limited by the amount of fissile material from outside sources. There is no meaningful proliferation-related difference between the LFTR and the IFR. To assure that they are not being diverted for weapons production, both would have to be subject to international inspection. The LFTR’s higher thermal efficiency might turn out to be important where water is in short supply and where dry cooling towers or constructive uses for the excess heat would be impractical. The cost of fuel (uranium or thorium) would be a very small part of the cost of energy from either LFTRs or IFRs, and is not a major consideration. Since there is no empirical cost data yet, there is no basis for predicting with confidence whether electricity from commercialized IFRs or LFTRs will be the more expensive.
Advantage --IFR -----
---
LFTR
---
Unknown
Additional Important Considerations PROPERTY Fissile loading Pu sequestration Growth potential, near term Breeding ratio Self-sustained growth rate Growth potential, long term (>90 yr) Readiness for commercialization
IFR 7-10 tonnes per GWe 7-10 tonnes Pu per GWe Current fissile inventory (U.S.) sufficient for 40-50 GWe ~0.5-1.5 (designer’s choice)
LFTR 1 tonne per GWe 1 tonne Pu per GWe Current fissile inventory (U.S.) sufficient for ~400 GWe Enough >1 to be self-sustaining
Advantage --IFR
0-6% per year
Expected to be zero or negative
IFR
Can grow indefinitely with doubling time down to 15 yr or less
Potentially limited to ~500 GWe (U.S.) by available fissile* Commercial demo only after ~2 decades of r&d
Ready now for commercial demo
LFTR IFR
IFR IFR
* Without using enriched mined uranium or using accelerator or fusion neutrons to convert thorium to U-233.
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