Thorium energy fuel cycle
Although not fissile itself 232Th will absorb slow neutrons to produce
after two beta decays 233U which is fissile. Hence like 238U it is
fertile. Also preparation of thorium fuel does not require isotopic
separation.
The thorium fuel cycle creates 233U which if separated from the reactor’s
fuel can be used for making nuclear weapons. This is why a liquid-fuel
cycle (e.g. MSR or molten salt reactor) is preferred – only a limited
amount of 233U ever exists in the reactor and its heat-transfer systems
preventing any access to weapons material; however the neutrons produced by
the reactor can be absorbed by a thorium or uranium blanket and fissile 233U
or 239Pu produced. Also the 233U could be continuously extracted from the
molten fuel as the reactor is running. Neutrons from the decay of
uranium-233 can be fed back into the fuel cycle to start the cycle
again.
The neutron flux from spontaneous fission of 233U is negligible. 233U can
thus be used easily in a simple gun-type nuclear bomb design. In 1977 a
light-water reactor at the Shippingport Atomic Power Station was used to
establish a 232Th-233U fuel cycle. The reactor worked until its
decommissioning in 1982. Thorium can be and has been used to
power nuclear energy plants using both the modified traditional Generation
III reactor design and prototype Generation IV reactor designs. The use of
thorium as an alternative fuel is one innovation being explored by the
International Project on Innovative Nuclear Reactors and Fuel Cycles
(INPRO) conducted by the International Atomic Energy Agency (IAEA).
Unlike its use in MSRs when using solid thorium in modified light water
reactor (LWR) problems include: the undeveloped technology for fuel
fabrication; in traditional once-through LWR designs potential problems in
recycling thorium due to highly radioactive 228Th; some weapons
proliferation risk due to production of 233U; and the technical problems
(not yet satisfactorily solved) in reprocessing. Much development work is
still required before the thorium fuel cycle can be commercialized for use
in LWR. The effort required has not seemed worth it while abundant uranium
is available but geopolitical forces (e.g. India looking for indigenous
fuel) as well as uranium production issues proliferation concerns and
concerns about the disposal/storage of radioactive waste are starting to
work in its favor.