Thorium is not fissile – but fertile. In order to create a reactive thorium fuel capable of producing energy, some form of fresh or recycled fissile material is needed as a ‘driver component’. As the fuel operates, thorium is transmuted to uranium-233 which is an excellent fissile material that then yields energy in the fuel. Reactor grade plutonium is a very good fissile driver since it is available from today’s spent nuclear fuel inventories.

Thorium will absorb neutrons in a thermal reactor and its reactivity will increase as its 233U content grows. It is possible to achieve net ‘breeding’ of 233U in thorium fuels in faster-spectrum variants of light water reactors (LWRs) and in heavy water reactors. Ceramic thorium oxide (ThO 2 ) has excellent material properties for serving as a nuclear fuel. ThO 2 has a higher thermal conductivity and a higher melting point than uranium oxide and it is better able to retain fission products within its crystal lattice. Thorium oxide fuels can therefore operate with lower internal pellet temperatures and exhibit less fission gas release than uranium fuels (including MOX). This implies that thorium oxide fuels should be able to operate safely to high burn-ups. It is possible to design viable thorium-plutonium fuels for LWRs balancing intersecting issues of fuel reactivity, safety margins and reactor operability. To this end, the design and licensing of uranium-plutonium (MOX) fuels has “paved the way” for thorium-plutonium fuels since it is only the added plutonium component that makes MOX fuels different from simple enriched uranium fuels.