D. GRENECHE, NuclearConsulting, and M. CHHOR, AREVANP, France

Abstract: This chapter compares the use of thorium as a nuclear material with conventional fissile materials. Rather than being a real alternative, thorium is a complement to the current uranium/plutonium fuel. Despite the benefits of thorium, its use presents technical challenges, which are described in this chapter. This review shows that significant experience has been gained on thorium-based fuel in both test reactors and power reactors, but not on an industrial scale.

Key words: thorium, uranium, nuclear power.

8.1 Reasons for considering the thorium cycle

8.1.1 Thorium fuel as an alternative or complement to uranium and plutonium fuels

Almost all of the world’s nuclear reactors in operation today use U-235 to sustain the neutron chain reaction because this isotope is the only naturally occurring isotope, which is fissile by slow neutrons (the fuel is then either natural uranium or, in most cases, uranium enriched in U-235). In this fuel, neutron captures by the fertile material U-238 produce plutonium and, in particular its two thermally fissile isotopes Pu-239 and Pu-241, which are burned partly in situ (typically, about half is consumed in a standard light water reactor). The plutonium that remains in the discharged fuel may be separated by reprocessing and then recycled in reactors in the so-called MOX fuel cycle, which leads to a saving in the use of natural uranium of about 12% for a single plutonium recycle.

As with uranium, thorium is also a naturally occurring material but it contains only one isotope, Th-232, and this is not thermally fissile, although it is a fertile isotope. Therefore, thorium is only useful as a resource for breeding new fissile materials, in this case U-233, which, for reasons explained later, is the best fissile isotope in the thermal neutron spectrum. Furthermore, a neutron chain reaction can only be sustained with thorium if sufficient quantities of fissile materials are available (U-235, U-233, plutonium) and mixed with the thorium. Then, it becomes possible to operate a nuclear reactor with thorium in which U-233 is produced, and, by doing so, the so-called ‘thorium cycle’ would be initiated. As with plutonium, U-233 is partly burnt in reactors and what remains in the discharged fuel can be recycled. However, the potential role and attractiveness of
thorium-based fuels within the nuclear enterprise will depend on the implementation and deployment scenarios, which must take into account economic factors and strategic choices.

Rather than being a real alternative to natural uranium and plutonium fuel, thorium is more accurately considered as a complement to the current uranium/ plutonium fuel. Indeed, it may, for example, be used to increase the available quantity of fissile material by the production of U-233 during irradiation, provided that enough fissile material is initially available to supply the neutrons needed for this breeding process. It can also provide possible avenues towards longer fuel cycles and higher burn-ups as well as multiple recycling of uranium/plutonium fuels in thermal spectrum reactors. Introducing thorium and thus, via irradiation, U-233, in such fuels may also allow the amount of natural uranium needed to be reduced.