Thorium as a Nuclear Fuel
Thorium is not a fissile material but classified as a fertile material. To create a thorium fuel capable of producing energy, a driver component or material is required. During irradiation, thorium transmutes to uranium-233 which is an excellent fissile material that can then yield energy. Plutonium or uranium is used as fissile drivers which are readily found in all spent nuclear inventories. Thorium will absorb neutrons in a thermal reactor and produce 233U.
Thorium oxide (ThO2 ) has excellent material properties for serving as a nuclear fuel. ThO2 has a higher thermal conductivity, a higher melting point than uranium oxide (UO2 ) and it retains fission products better within its crystalline lattice. Thorium oxide fuels can therefore operate at lower temperatures and exhibit less fission gas release than uranium fuels (including MOX). It is therefore recognized that thorium oxide fuels can operate safely to high burn-ups.
The burning of thorium fuel generates smaller amounts of plutonium and minor actinides compared to uranium fuel. Thus thorium based fuels will achieve much greater net plutonium consumption than conventional uranium based fuels, which produces plutonium as it burns.
ThO2 has excellent properties from a waste point of view, even after irradiation. It is highly insoluble, it is non-oxidizable and it retains both fission products and actinides extremely well within its lattice. Thorium oxide would therefore serve as a good matrix for once-through fuel designed specifically to burn plutonium.
Thorium fuel cycles are more resistant to nuclear proliferative actions. Both open and closed thorium fuelling options exhibit higher proliferation resistance than a corresponding uranium based cycle.