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If the use of nuclear energy is to expand and become a sustainable source of energy, the major challenges of improving the utilisation of mineral resources while reducing ultimate waste streams will need to be addressed. In a post-Fukushima context, where deployment of fast neutron reactors is uncertain and the realisation of geological repositories has been delayed in some countries, continuing socio-political concerns are focused on the accumulation of spent fuel and its final disposal.
In the absence of fast neutron reactors, the issue of plutonium management will have to be dealt with in the medium to long term for current and future separated plutonium.
From the early days in the development of nuclear energy, thorium was considered a potential fuel that could supplement or even replace natural uranium, which was considered to be scarce. Thorium is more abundant in the earth’s crust than uranium and thus could be used for longer without being depleted. However, it behaves sufficiently differently to uranium to need a different reactor design, which comes with some technical considerations.
For this OECD report, the use of thorium in the nuclear fuel cycle as a complement to the uranium/plutonium cycle shows potential for improving the medium-term flexibility of nuclear energy and its long-term sustainability. It provides a scientific assessment of thorium’s potential role in nuclear energy both in the short and longer term, addressing diverse options, potential drivers and current impediments to be considered if thorium fuel cycles are to be pursued.
Nuclear power plants currently in operation all use some variation of the uranium fuel cycle, where enriched 235uranium is used to sustain a nuclear disintegration chain reaction where heavy atomic nuclei are split under the impact of neutrons that are emitted from the splitting of other atom nuclei. It is the process that produces energy but also various by-products, amongst which is plutonium, one of the elements, which pose problem for nuclear waste handling and for nuclear weapons proliferation.
In a nuclear fission reactor, a heavy radioactive element, be it uranium, thorium of plutonium, is put in presence of a neutron source. The neutrons are absorbed by the atomic nucleus, which becomes unstable and breaks up, emitting energy and also more neutrons that can break up other nuclei, and so on. The fissile material, or fuel, used in this reaction, has to be of a specific kind, or isotope. In the case of uranium, it is uranium-235 that is used, which has to be concentrated from uranium ore, which contains mostly uranium-238, a much more stable isotope.
1 A short summary from various generic sources
The main initial motivation for the development of thorium fuel is to provide a fuel cycle that could, by replacing uranium fuel, avoid the potential of natural uranium shortage in the event of a rapid growth of nuclear power. The thorium fuel options that may be considered in light water reactors vary according to the purpose envisaged, i.e. reduction in natural uranium consumption, optimising 233U breeding for recycling or its later use in more dedicated reactor types.
The physical and neutronic characteristics of 232Th and 233U could indeed provide the means to achieve longer fuel cycle lengths, higher burn-ups and/or to reduce the burning of 235U or fissile plutonium.
Thorium use could be considered in various reactor systems in a homogeneous as well as in a heterogeneous manner, i.e.:
One of the often-claimed advantages of the thorium cycle is that it produces less plutonium and other actinides and significantly reduces the radiotoxicity of resulting waste. While a pure Th/233U cycle will indeed produce less plutonium and minor actinides, the long-term radiotoxicity of thorium-based spent nuclear fuels is more accurately described as being comparable to that of uranium-based spent nuclear fuels2.
Furthermore, whether or not a reduction in radiotoxicity is translated into an actual advantage will depend in practice on if this reduction leads to a significant change in the likelihood of making a safety case for the disposal and translates into a reduction in disposal cost.
Meanwhile, thorium technologies require significant further development. Thorium fuel R&D is currently funded by countries, which are concerned with long-term nuclear energy sustainability (as it is the case of Canada). However, given their cost and the lack of clear economic incentives for nuclear power plant operators to pursue this route, the OECD report considered that the industrial development activities for thorium remain somewhat limited at present.
2 see more in the Chapter 8 of the OECD report
The thorium fuel cycle is a complex subject even for those familiar with nuclear technology. Any novel fuel cycle proposal must be assessed not only from a multidisciplinary scientific perspective, but also from economic and industrial point of view, each within the broader context of well-established nuclear energy strategies.
In the case of thorium, thorium-232 is used, which is much more common in thorium ore than uranium-235 is in uranium. This is one of the reasons why thorium is considered a more durable potential source of nuclear fuel.
As mentioned in the introduction, the use of thorium in the nuclear fuel cycle as a complement to the uranium/plutonium cycle shows potential for improving the medium-term flexibility of nuclear energy and its long-term sustainability. More specifically, options for thorium’s introduction into the nuclear fuel cycle should be kept open and continue to be investigated. These options include the possibility of reaching higher conversion using thorium-based fuels in the newer types of reactors, with the aim of recycling the fissile material from used fuels. Using thorium would be a means of consuming plutonium, the by-product of the uranium fuel cycle (and possibly other higher actinides) as an option for plutonium management;
Since the thorium cycle needs an initial input of high-energy neutrons from the fission of uranium/plutonium) to generate the first fissile isotope (i.e. uranium 233), any industrial application of thorium as a nuclear fuel would continue to require the input of fissile material from the existing uranium/plutonium cycle until the required amounts of 233U could be produced to ultimately make the thorium cycle self-sustaining.
But, underlines the report, since the development of such new fuels and new reactor concepts is a time- and resource-consuming process likely to span several decades, the required R&D around these options must start in the near term.
Apart from thorium’s abundance in nature and the prospect of 233U breeder systems, the essential reasons for the interest in thorium were the intrinsically good basic physical properties of both 232Th and 233U, including:
Other potential advantages that may explain the current interest in thorium in several academic and R&D institutions as well as among industrial reactor designers and fuel vendors include:
The development of a fully self-sustaining thorium/233U cycle would also require the development of industrial scale reprocessing capabilities to recover 233U from spent fuel, along with fuel fabrication facilities to prepare the material for re-use.
Nevertheless, several factors dimmed the enthusiasm for fuel cycles requiring the recycling of 233U as would be needed in the longer term for so-called pure “thorium fuel cycles”. The full benefits of a closed, self-sustaining thorium/233U fuel cycle may only be realised in dedicated “breeder” reactors (generation IV or beyond), which are still in the design study phase and may not appear before the end of this century.
Thorium is a relatively common element in the earth’s crust. Based on its longer half-life (1.41 1010 years) compared to uranium (4.5 109 years), it is estimated to be roughly three times more abundant than uranium. This figure, however, does not imply that the exploitable reserves of thorium are two or three times higher than those of uranium, as frequently cited in the literature. Because of its very limited use so far, there has never been a comprehensive survey of thorium resources in the world, and current estimates of exploitable world resources of thorium are therefore not very accurate. It should be noted that the overall abundance of thorium is not an issue in whatever nuclear energy system scenario considered for the foreseeable future.
Furthermore, the sustained use of thorium inherently demands the recycling of fuel and its combined use with fissile materials coming from the uranium/plutonium fuel cycle in the near to medium term, which would result in an even lower thorium demand. By-product production of thorium from other industrial mining activities can provide more than ample quantities of thorium for the nuclear industry for this century and beyond.
Anyway, any industrial application of thorium as a nuclear fuel would continue to require the input of fissile material from the existing uranium/plutonium cycle until the required amounts of 233U could be produced and ultimately make the thorium cycle self-sustaining.
Until that point is reached, an important factor governing the rate at which 233U could be produced from the introduction of thorium/plutonium or thorium/uranium/plutonium cycles would be plutonium availability. The limitations imposed by fissile plutonium availability already point to rather long transition periods between thorium/plutonium and Th/233U systems, which are likely to be of the order of many decades.
Reactor concepts (namely, hybrids of accelerator-driven with molten salt blanket systems, hybrids of fission and fusion reactors, etc.) have been envisaged as potentially making use of thorium. Although these concepts may have interesting theoretical properties, they inevitably reflect the disadvantages, uncertainties and unknowns of the various technologies that enable them. 3
These unknowns are often independent of the fact that these concepts may or may not use thorium and, as such, would first need to be further studied, developed and demonstrated. Consequently, these composite or “hybrid” concepts are very unlikely to provide any credible application for commercial electricity production in this century.
These points would require a thorough explanation of neutron economy and of the various processes, and greatly overshoots the capacity of this summary4.
3 More in the OECD report page 31
4 To understand more on the subject, see the chapter 5 of the OECD report: Thorium fuel cycles in present day reactors.
In 2014, the United States Department of Energy (DOE) published a substantial report entitled Nuclear Fuel Cycle Evaluation and Screening, which highlighted that none of the fuel cycles (promising or potentially promising) are ready to be deployed today and R&D is required to develop the appropriate implementing technologies.
Three main situations and decisions discouraged the implementation of thorium cycles:
However, in a long-term strategy, India has set a programme of reactors entirely fuelled by thorium/233U that is foreseen to be deployed in the third and final stage only beyond 2070 but that could be even much later. In 2011, China also announced the start of an ambitious R&D programme led by the Chinese Academy of Sciences with the creation of the thorium molten salt reactor (TMSR)
Although the thorium fuel cycle has never been fully developed, the opportunities and challenges that might arise from the use of thorium in the nuclear fuel cycle are still being studied in many countries and in the context of diverse international programmes around the world.
If thorium fuel cycles are pursued, it is to be expected that short- to medium-term development of thorium fuels would be carried out in a step-wise fashion and in synergy with the existing uranium/plutonium fuel cycle.
In the longer term, the potential introduction of advanced reactor systems may present an opportunity to realise the full benefits of a closed thorium/233U fuel cycle in dedicated breeder reactors (generation IV or beyond) that are presently in the design study phase.
In particular, molten salt reactors may offer the prospect of using thorium fuels with online recovery and re-use of the 233U while recycling long-lived actinides and ensuring minimal losses to the final waste stream. However, it must be recognised that the development, licensing and construction of such novel systems are long-term undertakings.
In the context of international co-operation, the International Atomic Energy Agency (IAEA) has an existing Co-ordinated Research Project (CRP) on Near-Term and Promising Long-Term Deployment of Thorium Energy Systems5.
Before identifying the different types of general strategies that could be conceived for thorium use in nuclear energy systems, it must be highlighted that the introduction of a new fuel into the industrial fuel cycle is a necessarily progressive process for a variety of reasons.
The development of qualified fuels (even considering evolutionary changes only) is a resource-intensive effort that necessitates a series of different steps, all of which are very time-consuming.
The introduction of thorium into nuclear energy systems, if it occurs, will therefore have to happen progressively, and none of the scenarios envisaging a full transition towards a “100%” thorium/233U fuel cycle in the near term are realistic, both for scientific and for industrial reasons.
Any industrial application of thorium as a nuclear fuel would continue to require the input of fissile material from the existing uranium/plutonium cycle until the required amounts of 233U could be produced and ultimately make the thorium cycle self-sustaining.
Until that point is reached, an important factor governing the rate at which 233U could be produced from the introduction of thorium/plutonium or thorium/uranium/plutonium cycles would be plutonium availability. The limitations imposed by fissile plutonium availability already point to rather long transition periods between thorium/plutonium and Th/233U systems, which are likely to be in the order of many decades.
5 IAEA Coordinated Research Project (CRP) on Near term and Promising Long Term Options for Deployment of Thorium Based Nuclear Energy (T12026)
https://www.iaea.org/OurWork/ST/NE/NEFW/Technical-Areas/NFC/advanced-fuel-cycles-crp-thorium-2011.html
An exercise involving nearly fifty countries and more than 500 experts estimated that the technical obstacles to military use of thorium cycles, with uranium enriched to less than 20%, are similar to those of the uranium/plutonium cycle. However, it should be underlined that the study’s deliberations were based on a vast amount of technical data mixed with some contributions affected by commercial interests, as well as political or diplomatic considerations. Thus, it is not sufficient to refer only to the study’s general conclusions to compare more precisely the non-proliferation relative merits and demerits of the uranium and thorium cycles.
For such a comparison, there is a need to consider the physical characteristics that must be taken into account in assessing the difficulty of using fissile materials for atomic bombs.
In general, the diversion and processing of spent nuclear power reactor fuel is not the preferred method to obtain material for a nuclear explosive device. It is likely that separated uranium from spent fuel would be used in a single explosive device rather than in creating a stockpiled arsenal of weapons. It is generally assumed that for thorium based fuels, uranium, after being separated, would be used without much delay, such that a substantial amount of 232U will not have had significant time to decay.
A way claimed to reduce the risk of proliferation with the thorium cycle is to design systems in which the 233U produced in the reactor is diluted “at source” by 238U. This can easily be achieved by mixing thorium initially with natural or depleted uranium. However, this option would lead to increased production of plutonium and also raise proliferation concerns. There is also a ban in some countries on the reprocessing of spent fuel, which was put in place to limit the availability of plutonium, and which also prevents the recovery of uranium-233.
The physical and neutronic characteristics of 232Th and 233U could provide the means to achieve longer fuel cycle lengths, higher burn-ups and/or to reduce the burning of 235U or fissile plutonium6. Thorium use could be considered in various reactor systems both in a homogeneous and heterogeneous manner:
The thorium fuel options that may be considered in light water reactor vary according to the purpose envisaged, i.e. reduction in natural uranium consumption, optimising 233U breeding for recycling or its later use in more dedicated reactor types.
A performance study phase is expected post-2025, where the decision to support further development of molten salt reactor (MSR) systems within Generation IV International Forum (GIF) will need to be supported by adequate data. For innovative reactors such as liquid-fuelled MSRs, an approved licensing and regulation basis, yet to be developed, is required to support their technical feasibility and commercial viability.
6 See more in the OECD report page 30
Given that the thorium cycle has not yet been deployed on an industrial scale, there is no accurate data on the costs associated with different stages of this cycle. The only evidence available on this subject came from assessments based on limited experience from the manufacture of thorium fuel for different reactors.
The estimates found in the literature on this subject were quite disparate, as they depend on many parameters, such as the types of fuel cycles considered, calculation methods used, assumptions made on the cost of goods and services, and economic models applied in the calculations, especially in terms of discounting rates. For a more accurate comparison between the thorium/uranium and the uranium/plutonium fuel cycles, it is necessary to know the cost breakdown between different stages of the nuclear fuel cycle.
The use of thorium as nuclear fuel is often associated with advantages in the radiotoxicity of the resulting waste compared to conventional uranium fuels. It must not be overlooked, however, that the implementation of thorium fuels with the view of developing a self-sustainable thorium fuel cycle will require the use of mixed fuel forms (thorium-LEU or thorium-plutonium fuels) during very long transition phases before a full Thorium/233U cycle can be achieved.
One of the often-claimed advantages of the thorium cycle is that it produces less plutonium and other actinides, which significantly reduces the radiotoxicity of waste. While a pure Th/233U cycle will indeed produce less plutonium and minor actinides than conventional uranium oxide (UO2) fuels, this is not the case for thorium-plutonium mixed fuel forms, and is less clear for thorium-LEU fuels.
A self-sustainable thorium fuel cycle will require a very long transition phase during which thorium is going to be accompanied by uranium and/or plutonium before the waste advantage will take a long time to come into play. The benefits of improved chemical stability of ThO2 compared to uranium oxide (UO2) may have advantages for the final disposal of spent fuels in geological sites if this option is chosen.
For a given amount of thermal energy produced in a reactor core, the amount of fission products is similar in thorium or uranium fuels since, on a first approximation, a fission event releases approximately 200 MeV, regardless the nuclide that fissions, and gives birth to two fission products.
A major challenge associated to thorium reprocessing is related to the unavoidable presence of 232U, which accompanies 233U. Remotely operated and fully shielded recycled fuel fabrication processes will be required, for which there are currently no proven equipment or processes at the industrial scale. A related challenge is the handling and storage of excess thorium, which will contain 228thorium and its highly radioactive daughters for about 20 years.
Thorium fuel is generally reprocessed by a form of the THOREX liquid-liquid extraction process successfully used at pilot scale, notably in the United States and also in France to reprocess about 900 tonnes of thorium fuel. However, extrapolating the THOREX process to industrial scale will require further development of the dissolution, liquid-liquid extraction and conversion steps, and the dissolution of thorium metal or oxide is more complex than for uranium. Currently, no developed alternative to variants of the THOREX exists for reprocessing thorium-based fuels although other extractants have been investigated. A related challenge is the handling and storage of excess thorium, which will contain 228Th and its highly radioactive daughters for about 20 years.
The benefits of improved chemical stability of ThO2 compared to UO2 may have advantages for the final disposal of spent fuels in geological sites (if this option is chosen). The long-term radiotoxicity of thorium-based spent nuclear fuels is therefore more accurately described as being comparable to that of uranium-based spent nuclear fuels. Whether a reduction in radiotoxicity is translated into an actual advantage will depend in practice on whether this reduction leads to a significant change in the likelihood of making a safety case for the disposal. The most direct driver of disposal difficulty is more likely to be the amount of power generated rather than the system it is generated by. Similar arguments can be advanced against waste volume per se being the major driver of disposal economics, but a potential advantage of thorium fuel would result from the possibility of achieving higher burn-up compared to uranium.
A disadvantage of the thorium-uranium fuel cycle arising from reprocessing spent fuel (for recovery and recycle of 233U), is the requirement to avoid off-gas radon transport of the highly radioactive radon daughters through High-Efficiency Particulate Air (HEPA) filters to the environment.
Both the design of future reactors and a safety analysis methodology are currently being developed according to the following steps:
The consideration of the Integrated Safety Analysis Methodology (ISAM) developed by the GIF Risk and Safety Working Group (RSWG) has already identified strengths and limitations of the Molten Salt Fast Reactor (MSFR) concept. Calculation tools presently available do not allow for the same level of analysis for all of the accidents considered. Several limiting factors have been identified in the development of the new reactor concept at the pre-conceptual stage.
The grinding and crushing of ores would release 220Rn, the analogue of the U-decay-chain’s 222Rn. 220Rn has a much shorter half-life than 222Rn (56 seconds vs. 3.8 days), so it would only pose a hazard to those in the immediate vicinity. This radon hazard, however, could be heightened as its concentration in bastnasite or monazite ores used to extract thorium7 is higher than in other, less radioactive ores.
The use of concentrated strong acids and strong bases represents a significant chemical hazard regardless of processing scheme. The digestion process also produces radioactive waste streams. In any scheme, undigested solids with a moderate specific activity must be disposed. The bulk of the radioactivity is carried by 228Ra, which chemically tends to follow the Rare Earth Elements (REEs). This results in a waste stream with a high specific activity during REE processing, but this would not be attributable to the nuclear fuel cycle.
At the refining stage, in addition to the presence of strong acids and bases, extractants for solvent extraction systems are toxic, and exposure would need to be limited. Any raffinate streams leaving the system would represent radioactive waste, albeit of a low specific activity. As in previous stages, radon inhalation would continue to be a hazard.
An important factor when determining safety requirements for handling fuel after it has been discharged and stored for five years is the gamma radiation produced.
7 There are more than 100 thorium-bearing minerals in the crust of the earth, of which about 60 have a thorium concentration larger than 0.1%. Among the most notable of these minerals are thorite, thorianite, bastnasite and monazite.
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