The use of thorium as a nuclear fuel doesn't solve the WMD proliferation problem. Irradiation of thorium (indirectly) produces uranium-233, a fissile material which can be used in nuclear weapons.

The US has successfully tested weapons using uranium-233 (and France may have too).

India's thorium program must have a WMD component − as evidenced by India's refusal to allow IAEA safeguards to apply to its thorium program.

Thorium fuelled reactors could also be used to irradiate uranium to produce weapon grade plutonium. 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 further 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.

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Excerpt from:

Thorium Fuel: No Panacea for Nuclear Power

By Michele Boyd and Arjun Makhijani

http://www.ieer.org/fctsheet/thorium2009factsheet.pdf

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.

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Excerpt from:

ICNND Research Paper No. 8, Revised

Introduction to the Concept of Proliferation Resistance

John Carlson, Director General, Australian Safeguards and Non-Proliferation Office

3 June 2009

http://www.icnnd.org/research/index.html

or direct download http://www.icnnd.org/research/Proliferation_Resistance.doc

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.

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Further reading:

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>.

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Friedman, John S., 1997, "More power to thorium?", Bulletin of the Atomic Scientists, Vol. 53, No.5, September/October.

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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>