Thorium ‒ a better fuel for nuclear technology
Excerpt from: Dr. Rainer Moormann, ‘Thorium ‒ a better fuel for nuclear technology?’, Nuclear Monitor #858, 1 March 2018.
Claim 3: Thorium use has hardly any proliferation risk
The proliferation problem of Th / U-233 needs a differentiated analysis ‒ general answers are easily misleading. First of all, one has to assess the weapon capability of U-233. Criteria for good suitability are a low critical mass and a low rate of spontaneous fission. The critical mass of U-233 is only 40% of that of U-235, the critical mass of plutonium-239 is around 15% smaller than for U-233. A relatively easy to construct nuclear explosive needs around 20 to 25 kg U-233. The spontaneous fission rate is important, because the neutrons from spontaneous fission act as a starter of the chain reaction; for an efficient nuclear explosion, the fissile material needs to have a super-criticality of at least 2.5 (criticality is the amount of new fissions produced by the neutrons of each fission.)
When, because of spontaneous fissions, a noticeable chain reaction already starts during the initial conventional explosion trigger mechanism in the criticality phase between 1 and 2.5, undesired weak nuclear explosions would end the super-criticality before a significant part of the fissile material has reacted. This largely depends on how fast the criticality phase of 1 to 2.5 is passed. Weapon plutonium (largely Pu-239) and moreover reactor plutonium have – different from the mentioned uranium fission materials U-235 and U-233 – a high spontaneous fission rate, which excludes their use in easy to build bombs.
More specifically, plutonium cannot be caused to explode in a so-called gun-type fission weapon, but both uranium isotopes can. Plutonium needs the far more complex implosion bomb design, which we will not go into further here. A gun-type fission weapon was used in Hiroshima – a cannon barrel set-up, in which a fission projectile is shot into a fission block of a suitable form so that they together form a highly super-critical arrangement (see the picture in sheet 7 in reference #1). Here, the criticality phase from 1 to 2.5 is in the order of magnitude of milliseconds – a relatively long time, in which a plutonium explosive would destroy itself with weak nuclear explosions caused by spontaneous fission. One cannot find such uranium gun-type fission weapons in modern weapon arsenals any longer (South Africa’s apartheid regime built 7 gun-type fission weapons using uranium-235): their efficiency (at most a few percent) is rather low, they are bulky (the Hiroshima bomb: 3.6 metric tons, 3.2 meters long), inflexible, and not really suitable for carriers like intercontinental rockets.
On the other hand, gun-type designs are highly reliable and relatively easy to build. Also, the International Atomic Energy Agency (IAEA) reckons that larger terror groups would be capable of constructing a nuclear explosive on the basis of the gun-type fission design provided they got hold of a sufficient amount of suitable fissile material.1 Bombs with a force of at most 2 to 2.5 times that of the Hiroshima bomb (13 kt TNT) are conceivable. For that reason, the USA and Russia have tried intensively for decades to repatriate their world-wide delivered highly enriched uranium (HEU).
A draw-back of U-233 in weapon technology is that – when it is produced only for energy generation purposes – it is contaminated with maximally 250 parts per million (ppm) U-232 (half-life 70 years).2 That does not impair the nuclear explosion capability, but the uranium-232 turns in the thorium decay chain, which means ‒ as mentioned above ‒ emission of the highly penetrating radiation of Tl-208. A strongly radiating bomb is undesirable in a military environment – from the point of view of handling, and because the radiation intervenes with the bomb’s electronics. In the USA, there exists a limit of 50 ppm U-232 above which U-233 is no longer considered suitable for weapons.
Nevertheless, U-232 does not really diminish all proliferation problems around U-233. First of all, simple gun-type designs do not need any electronics; furthermore, radiation safety arguments during bomb construction will hardly play a role for terrorist organisations that use suicide bombers. Besides that, Tl-208 only appears in the end of the decay chain of U-232: freshly produced or purified U-233/U-232 will radiate little for weeks and is easier to handle.2 It is also possible to suppress the build-up of uranium-232 to a large extent, when during the breeding process of U-233 fast neutrons with energies larger than 0.5 MeV are filtered out (for instance by arranging the thorium in the reactor behind a moderating layer) and thorium is used from ore that contains as little uranium as possible.
A very elegant way to harvest highly pure U-233 is offered by the proposed molten salt reactors with integrated reprocessing (MSR): During the breeding of U-233 from thorium, the intermediate protactinium-233 (Pa-233) is produced, which has a half-life of around one month. When this intermediate is isolated – as is intended in some molten salt reactors – and let decay outside the reactor, pure U-233 is obtained that is optimally suited for nuclear weapons.
An advantage of U-233 in comparison with Pu-239 in military use is that under neutron irradiation during the production in the reactor, it tends to turn a lot less into nuclides that negatively influence the explosion capability. U-233 can (like U-235) be made unsuitable for use in weapons by adding U-238: When depleted uranium is already mixed with thorium during the feed-in into the reactor, the resulting mix of nuclides is virtually unusable for weapons. However, for MSRs with integrated reprocessing this is not a sufficient remedy. One would have to prevent separation of protactinium-233.9
The conclusion has to be that the use of thorium contains severe proliferation risks. These are less in the risk that highly developed states would find it easier to lay their hands on high-tech weapons, than that the bar for the construction of simple but highly effective nuclear explosives for terror organisations or unstable states will be a lot lower.
In my opinion, the proliferation aspect is a vital issue. Here we would see a severe deterioration of the current situation, because the barriers to the construction of feasible nuclear explosives by, for instance, terror groups would be seriously lowered. This aspect deserves more attention. We can hope that the IAEA, the USA and Russia would oppose uncontrolled propagation of thorium technology, when they would see its introduction thwarting their decades-long efforts to reduce the proliferation risk by repatriation of HEU.
On the other hand, the current thorium hype, partially carried by a fanaticism based on limited knowledge, could lead in a populist environment to incalculable developments. For that reason, I think it important that the environment and peace movements should insist that thorium technology without sufficient proliferation control should be outlawed in the same way as currently is the case with efforts to phase out the use of HEU. As a minimum requirement, thorium technology without U-233 denaturation with U-238 should be banned, and online reprocessing in molten salt reactors should be banned.
Thor-bores and uro-sceptics: thorium’s friendly fire
Excerpt from: Jim Green, 9 April 2015, Nuclear Monitor #801, www.wiseinternational.org/nuclear-monitor/801/thor-bores-and-uro-sceptics-thoriums-friendly-fire
Many Nuclear Monitor readers will be familiar with the tiresome rhetoric of thorium enthusiasts − let’s call them thor-bores. Their arguments have little merit but they refuse to go away. Here’s a thor-bore in full flight − a science journalist who should know better:
“Thorium is a superior nuclear fuel to uranium in almost every conceivable way … If there is such a thing as green nuclear power, thorium is it. … For one, a thorium-powered nuclear reactor can never undergo a meltdown. It just can’t. … Thorium is also thoroughly useless for making nuclear weapons. … But wait, there’s more. Thorium doesn’t only produce less waste, it can be used to consume existing waste.”1
Claims that thorium reactors would be proliferation-resistant or proliferation-proof do not stand up to scrutiny.11 Irradiation of thorium-232 produces uranium-233, which can be and has been used in nuclear weapons.
The World Nuclear Association states:
“The USA produced about 2 tonnes of U-233 from thorium during the ‘Cold War’, at various levels of chemical and isotopic purity, in plutonium production reactors. It is possible to use U-233 in a nuclear weapon, and in 1955 the USA detonated a device with a plutonium-U-233 composite pit, in Operation Teapot. The explosive yield was less than anticipated, at 22 kilotons. In 1998 India detonated a very small device based on U-233 called Shakti V.”2
According to Assoc. Prof. Nigel Marks, both the US and the USSR tested uranium-233 bombs in 1955.6
Uranium-233 is contaminated with uranium-232 but there are ways around that problem. Kang and von Hippel note:
“[J]ust as it is possible to produce weapon-grade plutonium in low-burnup fuel, it is also practical to use heavy-water reactors to produce U-233 containing only a few ppm of U-232 if the thorium is segregated in “target” channels and discharged a few times more frequently than the natural-uranium “driver” fuel.”12
John Carlson discusses the proliferation risks associated with thorium:
“The thorium fuel cycle has similarities to the fast neutron fuel cycle – it depends on breeding fissile material (U-233) in the reactor, and reprocessing to recover this fissile material for recycle. …
“Proponents argue that the thorium fuel cycle is proliferation resistant because it does not produce plutonium. Proponents claim that it is not practicable to use U-233 for nuclear weapons.
“There is no doubt that use of U-233 for nuclear weapons would present significant technical difficulties, due to the high gamma radiation and heat output arising from decay of U-232 which is unavoidably produced with U-233. Heat levels would become excessive within a few weeks, degrading the high explosive and electronic components of a weapon and making use of U‑233 impracticable for stockpiled weapons. However, it would be possible to develop strategies to deal with these drawbacks, e.g. designing weapons where the fissile “pit” (the core of the nuclear weapon) is not inserted until required, and where ongoing production and treatment of U-233 allows for pits to be continually replaced. This might not be practical for a large arsenal, but could certainly be done on a small scale.
“In addition, there are other considerations. A thorium reactor requires initial core fuel – LEU or plutonium – until it reaches the point where it is producing sufficient U-233 for self-sustainability, so the cycle is not entirely free of issues applying to the uranium fuel cycle (i.e. requirement for enrichment or reprocessing). Further, while the thorium cycle can be self-sustaining on produced U‑233, it is much more efficient if the U-233 is supplemented by additional “driver” fuel, such as LEU or plutonium. For example, India, which has spent some decades developing a comprehensive thorium fuel cycle concept, is proposing production of weapons grade plutonium in fast breeder reactors specifically for use as driver fuel for thorium reactors. This approach has obvious problems in terms of proliferation and terrorism risks.
“A concept for a liquid fuel thorium reactor is under consideration (in which the thorium/uranium fuel would be dissolved in molten fluoride salts), which would avoid the need for reprocessing to separate U-233. If it proceeds, this concept would have non-proliferation advantages.
“Finally, it cannot be excluded that a thorium reactor – as in the case of other reactors – could be used for plutonium production through irradiation of uranium targets.
“Arguments that the thorium fuel cycle is inherently proliferation resistant are overstated. In some circumstances the thorium cycle could involve significant proliferation risks.”13
Sometimes thor-bores posit conspiracy theories. Former International Atomic Energy Agency Director-General Hans Blix said “it is almost impossible to make a bomb out of thorium” and thorium is being held back by the “vested interests” of the uranium-based nuclear industry.14
But Julian Kelly from Thor Energy, a Norwegian company developing and testing thorium-plutonium fuels for use in commercial light water reactors, states:
“Conspiracy theories about funding denials for thorium work are for the entertainment sector. A greater risk is that there will be a classic R&D bubble [that] divides R&D effort and investment into fragmented camps and feifdoms.”4
1. Tim Dean, 16 March 2011, ‘The greener nuclear alternative’, www.abc.net.au/unleashed/45178.html
3. UK National Nuclear Laboratory Ltd., 5 March 2012, ‘Comparison of thorium and uranium fuel cycles’, www.decc.gov.uk/assets/decc/11/meeting-energy-demand/nuclear/6300-comparison-fuel-cycles.pdf
4. Stephen Harris, 9 Jan 2014, ‘Your questions answered: thorium-powered nuclear’, www.theengineer.co.uk/energy-and-environment/in-depth/your-questions-answered-thorium-powered-nuclear/1017776.article
5. George Dracoulis, 5 Aug 2011, ‘Thorium is no silver bullet when it comes to nuclear energy, but it could play a role’, http://theconversation.com/thorium-is-no-silver-bullet-when-it-comes-to-nuclear-energy-but-it-could-play-a-role-1842
6. Nigel Marks, 2 March 2015, ‘Should Australia consider thorium nuclear power?’, http://theconversation.com/should-australia-consider-thorium-nuclear-power-37850
7. Idaho National Laboratory, Sept 2009, ‘AFCI Options Study’, INL/EXT-10-17639, www.inl.gov/technicalpublications/Documents/4480296.pdf
8. John Carlson, 2014, submission to Joint Standing Committee on Treaties, Parliament of Australia, www.aph.gov.au/DocumentStore.ashx?id=79a1a29e-5691-4299-8923-06e633780d4b&subId=301365
9. Oliver Tickell, August/September 2012, ‘Thorium: Not ‘green’, not ‘viable’, and not likely’, www.no2nuclearpower.org.uk/nuclearnews/NuClearNewsNo43.pdf
10. George Dracoulis, 19 Dec 2011, ‘Thoughts from a thorium ‘symposium”, http://theconversation.com/thoughts-from-a-thorium-symposium-4545
12. Jungmin Kang and Frank N. von Hippel, 2001, “U-232 and the Proliferation-Resistance of U-233 in Spent Fuel”, Science & Global Security, Volume 9, pp.1-32, www.princeton.edu/sgs/publications/sgs/pdf/9_1kang.pdf
13. John Carlson, 2009, ‘Introduction to the Concept of Proliferation Resistance’, http://d3n8a8pro7vhmx.cloudfront.net/foe/legacy_url/863/Carlson_20ASNO_20ICNND_20Prolif_20Resistance.doc
14. Herman Trabish, 10 Dec 2013, ‘Thorium Reactors: Nuclear Redemption or Nuclear Hazard?’, http://theenergycollective.com/hermantrabish/314771/thorium-reactors-nuclear-redemption-or-nuclear-hazard
15. Pia Akerman, 7 Oct 2013, ‘Ex-Shell boss issues nuclear call’, The Australian, www.theaustralian.com.au/national-affairs/policy/ex-shell-boss-issues-nuclear-call/story-e6frg6xf-1226733858032
Thorium and nuclear weapons
Thorium fuel cycles are promoted on the grounds that they pose less of a proliferation risk compared to conventional reactors. However, whether there is any significant non-proliferation advantage depends on the design of the various thorium-based systems. No thorium system would negate proliferation risks altogether.
Neutron bombardment of thorium (indirectly) produces uranium-233, a fissile material which can be used in nuclear weapons (1 Significant Quantity of U-233 = 8kg).
The USA has successfully tested weapon/s using uranium-233 cores. India may be interested in the military potential of thorium/uranium-233 in addition to civil applications. India is refusing to allow safeguards to apply to its entire ‘advanced’ thorium/plutonium fuel cycle, stongly suggesting a military dimension.
The possible use of highly enriched uranium (HEU) or plutonium to initiate a thorium-232/uranium-233 reaction, or proposed systems using thorium in conjunction with HEU or plutonium as fuel, present risks of diversion of HEU or plutonium for weapons production as well as providing a rationale for the ongoing operation of dual-use enrichment and reprocessing plants.
Thorium fuelled reactors could also be used to irradiate uranium to produce weapon grade plutonium.
Kang and von Hippel conclude that “the proliferation resistance of thorium fuel cycles depends very much upon how they are implemented”. For example, the co-production of uranium-232 complicates weapons production but, as Kang and von Hippel note, “just as it is possible to produce weapon-grade plutonium in low-burnup fuel, it is also practical to use heavy-water reactors to produce U-233 containing only a few ppm of U-232 if the thorium is segregated in “target” channels and discharged a few times more frequently than the natural-uranium “driver” fuel.” (Kang, Jungmin, and Frank N. von Hippel, 2001, “U-232 and the Proliferation-Resistance of U-233 in Spent Fuel”, Science & Global Security, Volume 9, pp 1-32, <www.princeton.edu/~globsec/publications/pdf/9_1kang.pdf>.)
One proposed system is an Accelerator Driven Systems (ADS) in which an accelerator produces a proton beam which is targeted at target nuclei (e.g. lead, bismuth) to produce neutrons. The neutrons can be directed to a subcritical reactor containing thorium. ADS systems could reduce but not negate the proliferation risks.
Excerpt from: Thorium Fuel: No Panacea for Nuclear Power
By Michele Boyd and Arjun Makhijani
A Fact Sheet Produced by Physicians for Social Responsibility and the Institute for Energy and Environmental Research
Thorium is not actually a “fuel” because it is not fissile and therefore cannot be used to start or sustain a nuclear chain reaction. A fissile material, such as uranium-235 (U-235) or plutonium-239 (which is made in reactors from uranium-238), is required to kick-start the reaction. The enriched uranium fuel or plutonium fuel also maintains the chain reaction until enough of the thorium target material has been converted into fissile uranium-233 (U-233) to take over much or most of the job.
The use of enriched uranium or plutonium in thorium fuel has proliferation implications. Although U-235 is found in nature, it is only 0.7% of natural uranium, so the proportion of U-235 must be industrially increased to make “enriched uranium” for use in reactors. Highly enriched uranium and separated plutonium are nuclear weapons materials.
In addition, U-233 is as effective as plutonium-239 for making nuclear bombs. In most proposed thorium fuel cycles, reprocessing is required to separate out the U-233 for use in fresh fuel. This means that, like uranium fuel with reprocessing, bomb-making material is separated out, making it vulnerable to theft or diversion. Some proposed thorium fuel cycles even require 20% enriched uranium in order to get the chain reaction started in existing reactors using thorium fuel. It takes
90% enrichment to make weapons-usable uranium, but very little work is needed to move from 20% enrichment to 90% enrichment.
It has been claimed that thorium fuel cycles with reprocessing would be much less of a proliferation risk because the thorium can be mixed with uranium-238. In this case, fissile uranium-233 is also mixed with non-fissile uranium-238. The claim is that if the U-238 content is high enough, the mixture cannot be used to make bombs without a complex uranium enrichment plant. This is misleading. More uranium-238 does dilute the uranium-233, but it also results in the production of more plutonium-239 as the reactor operates. So the proliferation problem remains – either bomb-usable uranium-233 or bomb-usable plutonium is created and can be separated out. Even if the mixture of U-238 and U-233 contains so much U-238 that it cannot be used for making weapons, the U-233 proportion can be increased by enrichment – the same process used to enrich natural uranium in U-235. The enrichment of U-233 is easier than the enrichment of U-235 because U-233 is much lighter than U-235 relative to U-238 (five atomic weight units lighter compared to three).
There is just no way to avoid proliferation problems associated with thorium fuel cycles that involve reprocessing. Thorium fuel cycles without reprocessing would offer the same temptation to reprocess as today’s once-through uranium fuel cycles.
Excerpt from: ICNND Research Paper No. 8, Revised
John Carlson, Director General, Australian Safeguards and Non-Proliferation Office, 3 June 2009, ‘Introduction to the Concept of Proliferation Resistance’, www.icnnd.org/
For reasons unknown the paper appears to have been removed from the International Commission on Nuclear Non-proliferation and Disarmament website but here is a link to the paper (Word file)
In principle, another route for avoiding the need for enrichment is the thorium fuel cycle, but as will be discussed in section 5.C, a thorium reactor requires enriched uranium or plutonium for the initial operating cycles, and current thorium reactor types also require reprocessing. Although reprocessing is for recovery of uranium-233 rather than plutonium, U-233 can also be used in nuclear weapons. A liquid fuel reactor concept is being considered which would avoid the need for U-233 separation.
5.C Thorium fuel cycle
The thorium fuel cycle has similarities to the fast neutron fuel cycle – it depends on breeding fissile material (U-233) in the reactor, and reprocessing to recover this fissile material for recycle.
Thorium is not a fissile material, so cannot be used as reactor fuel. The basis of the thorium fuel cycle is irradiation of the fertile thorium isotope, Th-232, to produce the fissile material U-233 through neutron capture (rather like production of plutonium from U‑238). The thorium fuel cycle requires separation – i.e. reprocessing – of U-233 produced in the fuel, and the recycle of U‑233 as fresh fuel.
Proponents argue that the thorium fuel cycle is proliferation resistant because it does not produce plutonium. Proponents claim that it is not practicable to use U-233 for nuclear weapons.
There is no doubt that use of U-233 for nuclear weapons would present significant technical difficulties, due to the high gamma radiation and heat output arising from decay of U-232 which is unavoidably produced with U-233. Heat levels would become excessive within a few weeks, degrading the high explosive and electronic components of a weapon and making use of U‑233 impracticable for stockpiled weapons. However, it would be possible to develop strategies to deal with these drawbacks, e.g. designing weapons where the fissile “pit” (the core of the nuclear nuclear weapon) is not inserted until required, and where ongoing production and treatment of U-233 allows for pits to be continually replaced. This might not be practical for a large arsenal, but could certainly be done on a small scale.
In addition, there are other considerations. A thorium reactor requires initial core fuel – LEU or plutonium – until it reaches the point where it is producing sufficient U-233 for self-sustainability, so the cycle is not entirely free of issues applying to the uranium fuel cycle (i.e. requirement for enrichment or reprocessing). Further, while the thorium cycle can be self-sustaining on produced U‑233, it is much more efficient if the U-233 is supplemented by additional “driver” fuel, such as LEU or plutonium. For example, India, which has spent some decades developing a comprehensive thorium fuel cycle concept, is proposing production of weapons grade plutonium in fast breeder reactors specifically for use as driver fuel for thorium reactors. This approach has obvious problems in terms of proliferation and terrorism risks.
A concept for a liquid fuel thorium reactor is under consideration (in which the thorium/uranium fuel would be dissolved in molten fluoride salts), which would avoid the need for reprocessing to separate U-233. If it proceeds, this concept would have non-proliferation advantages.
Finally, it cannot be excluded that a thorium reactor – as in the case of other reactors – could be used for plutonium production through irradiation of uranium targets.
Summary Arguments that the thorium fuel cycle is inherently proliferation resistant are overstated. In some circumstances the thorium cycle could involve significant proliferation risks.
Comparison of thorium and uranium fuel cycles
UK National Nuclear Laboratory Ltd.
A report prepared for and on behalf of Department of Energy and Climate Change
Issue 5, 5 March 2012
Here is the Exec Summary and an extract about proliferation risks.
The UK National Nuclear Laboratory has been contracted by the Department for Energy and Climate Change (DECC) to review and assess the relevance to the UK of the advanced reactor systems currently being developed internationally. Part of the task specification relates to comparison of the thorium and uranium fuel cycles. Worldwide, there has for a long time been a sustained interest in the thorium fuel cycle and presently there are several major research initiatives which are either focused specifically on the thorium fuel cycle or on systems which use thorium as the fertile seed instead of U-238. Currently in the UK, the thorium fuel cycle is not an option that is being pursued commercially and it is important for DECC to understand why this is the case and whether there is a valid argument for adopting a different position in the future.
NNL has recently published a position paper on thorium  which attempts to take a balanced view of the relative advantages and disadvantages of the thorium fuel cycle. Thorium has theoretical advantages regarding sustainability, reducing radiotoxicity and reducing proliferation risk. NNL’s position paper finds that while there is some justification for these benefits, they are often over stated.
The value of using thorium fuel for plutonium disposition would need to be assessed against high level issues concerning the importance of maintaining high standards of safety, security and protection against proliferation, as well as meeting other essential strategic goals related to maintaining flexibility in the fuel cycle, optimising waste arisings and economic competitiveness. It is important that the UK should be very clear as to what the overall objectives should be and the timescales for achieving these objectives.
Overall, the conclusion is reached that the thorium fuel cycle at best has only limited relevance to the UK as a possible alternative plutonium disposition strategy and as a possible strategic option in the very long term for any follow-up reactor construction programme after LWR new build. Nevertheless, it is important to recognise that world-wide there remains interest in thorium fuel cycles and as this is not likely to diminish in the near future. It may therefore be judicious for the UK to maintain a low level of engagement in thorium fuel cycle R&D by involvement in international collaborative research activities. This will enable the UK to keep up with developments, comment from a position of knowledge and to some extent influence the direction of research. Participation will also ensure that the UK is more ready to respond if changes in technology or market forces bring the thorium fuel cycle more to the fore.
It should be noted that this paper is not intended to provide an exhaustive review and assessment of potential advanced reactor technologies in order for DECC or other UK interested parties to immediately down select reactor options. The study and the approach developed was deliberately limited in its assessment of reactor options primarily due to time and in particular budget constraints. As such, only a limited cross section of reactor technologies were assessed and no design variants were assessed either e.g. prismatic or pebble VHTR options.
The UK NNL would like to also recognise and thank all of the external reviewers for their time taken to review the study and for their comments on the paper. As with any such review process, not all of the comments were able to be included in the final version of the report either due to opposing views not simply between the authors and the reviewers, but also between the reviewers themselves. Nevertheless, every comment was considered and included where appropriate.
Measures to increase the inherent proliferation resistance of the reprocessing fuel, such as avoiding the separation of pure plutonium oxide are considered desirable in designing new reactors and associated fuel cycle facilities. However, reducing proliferation risk is not a factor in strategic decision making for utilities and is unlikely to become so in the foreseeable future. Therefore, there currently is no incentive for utilities to seek alternatives to U-Pu fuel.
Section 4.5. Proliferation risk
The absence of plutonium is in the thorium fuel cycle is claimed to reduce the risk of nuclear weapons proliferation, though Reference  questions whether is this is completely valid, given that there were a number of U-233 nuclear tests (the “Teapot tests”) in the US in the 1950s. U-233 is in many respects very well suited for weapons use, because it has a low critical mass, a low spontaneous neutron source and low heat output. It has been stated [eg Wikipedia entry on U-233] that because U-233 has a higher spontaneous neutron source than Pu-239, then this makes it more of a technical challenge. However, this is erroneous, because even in weapons grade plutonium the main neutron source is from Pu-240. A further consideration is that the U-233 produced in thorium fuel is isotopically very pure, with only trace quantities of U-232 and U-234 produced. Although the U-232 presents problems with radiological protection during fuel fabrication, the fissile quality does not degrade with irradiation. Therefore, if it is accepted that U-233 is weapons useable, this remains the case at all burnups and there is no degradation in weapons attractiveness with burnup, unlike the U-Pu cycle.
The presence of trace amounts of U-232 is beneficial in that it provides a significant gamma dose field that would complicate weapons fabrication and this has been claimed to make U-233 proliferation resistant. However, there are mitigating strategies can be conceived and the U-232 dose rate cannot be regarded as a completely effective barrier to proliferation. As such, U-233 should be considered weapons usable in the same way as HEU and plutonium. This is also the position taken by the IAEA, which under the Convention on the Physical Protection of Nuclear Materials  categorises U-233 in the same way as plutonium. Under the IAEA classification, 2 kg or more of U-233 or plutonium are designated as Category I Nuclear Material and as such are subject to appropriate controls. By way of comparison, the mass of U-235 for Category I material is 5 kg. Attempts to lower the fissile content of uranium by adding U-238 are considered to offer only weak protection, as the U-233 could be separated relatively easily in a centrifuge cascade in the same way that U-235 is separated from U-238 in the standard uranium fuel cycle.
The overall conclusion is that while there may be some justification for the thorium fuel cycle posing a reduced proliferation risk, the justification is not very strong and, as noted in Section 3.5, this is not a major factor for utilities. Regardless of the details, those safeguards and security measures in place for the U-Pu cycle will have to remain in place for the thorium fuel cycle and there is no overall benefit.
Further reading on thorium and weapons proliferation
Feiveson, Harold, 2001, “The Search for Proliferation-Resistant Nuclear Power”, The Journal of the Federation of American Scientists, September/October 2001, Volume 54, Number 5, www.fas.org/faspir/2001/v54n5/nuclear.htm
Friedman, John S., 1997, “More power to thorium?”, Bulletin of the Atomic Scientists, Vol. 53, No.5, September/October.
Kang, Jungmin, and Frank N. von Hippel, 2001, “U-232 and the Proliferation-Resistance of U-233 in Spent Fuel”, Science & Global Security, Volume 9, pp 1-32, www.princeton.edu/sgs/publications/sgs/pdf/9_1kang.pdf