Thorium-based transmuter fuels for use in light water reactors

Advanced fuels for thermal reactors will be needed in the future for several reasons: reduced production of plutonium and minor actinides (MA, i.e., neptunium, americium, curium, and the higher elements), achieving higher burnups, and a more robust fuel-form/waste-form for temporary storage awaiting reprocessing or for holding the fission products and actinides in a permanent repository. Thorium-based fuels reduce the total amount of plutonium produced and produce a mixture of plutonium isotopes high in ²³⁸Pu. Because of the high decay heat and spontaneous neutron generation by ²³⁸Pu, this isotope helps to provide a higher measure of intrinsic proliferation resistance. Thoria as a fuel-form and as a waste-form is difficult to dissolve, thus making the diversion of spent fuel for weapons' production purposes more difficult. This analysis has been conducted to determine if ThO₂-UO₂ fuels can be used to further increase the proliferation resistance of LWR fuels, while reducing the plutonium and MA inventory. Some of the proliferation concern in the world today stems from plutonium that has already been separated from spent fuel. Currently separated plutonium is being incorporated in UO₂-PuO₂ mixed oxide (MOX) fuel. However, because MOX fuel contains 90 wt % ²³⁸U, substantial amounts of ²³⁹Pu are produced in the MOX fuel and net plutonium burnup rates are only 30-50% per cycle. The incorporation of plutonium into a ThO₂ matrix will consume already-separated plutonium without breeding additional ²³⁹Pu. The MAs would be included in the ThO₂ to further reduce the overall long-term radiotoxicity of the fuel cycle. In the work presented here, 11% of the fuel pins in a typical PWR are replaced by the thorium-based fuel pins, where the first cycle shows a small net increase of plutonium. Further optimization should show a net decrease in the total core inventory of plutonium with modest loadings of the thorium fuel.