Parametric Studies on Plutonium Transmutation Using Uranium-Free Fuels in Light Water Reactors

To compare the once-through use of U-free fuels for plutonium burnup in light water reactors (LWRs), plutonium transmutation, minor actinide (MA) and long-life fission product (LLFP) buildup and radiotoxicity hazards were compared for PuO₂ + ZrO₂ (rock-like oxide: ROX) and PuO₂ + ThO₂ (thorium oxide: TOX) fuels, loaded in a soft-to-hard neutron spectrum LWR core (a moderator-to-fuel volume ratio V_m/V_f is from 0.5 to 3.0). For better understanding and proper improvement of the reactivity coefficient problem of ROX, the fuel temperature coefficient, the void coefficient, and the delayed neutron fraction were also studied. A mixed-oxide (MOX)-fueled LWR was considered for reference purposes.

From the result of the cell burnup calculation, ROX fuel transmutes 90% of net initially loaded weapons-grade Pu, and 2.5% of initially loaded Pu is converted to MAs when V_m/V_f is 2.0 and discharge burnup in effective full-power days is equivalent to that of 33 GWd/t in MOX fuel. Reactor-grade Pu-based ROX fuel transmutes 80% of net initially loaded Pu, and 6.7% of initially loaded Pu converts to MAs with the same condition as the weapons-grade Pu ROX fuel. TOX fuel also has a good Pu transmutation capability, but the ²³³U production amount is approximately a half of the fissile Pu transmutation amount. The MA production amount in TOX fuel is lower than that in MOX and ROX fuels. The LLFP production amount in ROX fuel is lower than that in MOX and TOX fuels. The radiotoxicity hazard of ROX spent fuel is lower compared to that in TOX and MOX spent fuels.

The thermal neutron energy region is important in ROX fuel for fuel temperature coefficient and void coefficient problems. From these calculations, 15 to 20% ²³²Th-added ROX fuel seems the best to use as a once-through Pu-burning fuel compared to TOX and MOX fuels in conventional LWRs, because of its higher Pu transmutation, lower radiotoxicity hazard.