Thorium-Based Fuels in Fast Breeder Reactors

A number of thorium-based fuels for fast breeder reactors using either sodium or helium coolant are considered. Thorium-plutonium mixed-oxide fuels have similar or slightly better material properties than those for mixed urani-um-plutonium oxides. Their thermal performance is also very similar to that of the UO₂-PuO₂ mixed-oxide fuel. Their nuclear performance shows a substantially lower breeding gain, but a much lower positive sodium void coefficient than those for the UO₂-PuO₂ system. The material properties of Th-U-Pu and Th-U metal alloys are more suitable for reactor application than those of the uranium metal alloy. The Th-U-Pu metal alloy system has higher breeding gain, much lower positive sodium void coefficient, and a possibly higher negative Doppler coefficient of reactivity than the magnitude of those parameters for the UO₂-PuO₂ system. The Th-²³³U metal alloy system has a slightly lower breeding gain than the UO₂-PuO₂ system, but it has a negative reactivity coefficient for sodium voiding from the core. Equilibrium fuel cycle calculations reveal that all of the thorium metal alloy systems have a longer cycle length than the UO₂-PuO₂ system for the same burnup constraint. Thermal-hydraulic calculations show that sodium-bonded thorium metal alloy fuel elements may be able to operate up to an ∼82 kW/m (25 kW/ft) peak power rating in sodium coolant.