Thorium based MOX fuel utilization in thermal SCWR

Supercritical Water Reactor (SCWR) is a fourth generation reactor concept. High burnup to be economically beneficial along with proliferation resistance characteristics are two of the major goals set for Gen-IV reactors. Using 4% enriched UO₂ fuel in thermal neutron spectrum SCWR gives burnup values which are even smaller than PWR. Also the risk of proliferation will be there with plutonium-239 production. In order to attain larger burnup values with UO₂ fuel, higher enrichment will be required. Higher initial enrichment will pose the problem of excess reactivity and hence requirement of large quantities of burnable poison. Using thorium and uranium mixed oxide fuel can rectify the problem of small burnup and Plutonium-239 production without having to deal with too much excess reactivity. In this initial study, various fuel compositions i.e. (Th_0.70,U_0.30)O₂, (Th_0.65,U_0.35)O₂ and (Th_0.60,U_0.40)O₂ are studied. Comparisons of these fuels are made with UO2 fuel. Coupled calculations are used to study the neutron physics, safety and thermal hydraulics characteristics for these fuels. Thorium uranium MOX gives a slightly harder spectrum as compared to UO₂ fuel option but still can be used as a full core option. Burnup comparison is made with equivalent enrichment of U-235 in UO₂ fuel. Although equivalent enriched UO₂ fuel shows higher burnup as compared to thorium based MOX fuel, but it has the problem of large excess reactivity at Beginning of Cycle (BOC). On the other hand, thorium based MOX fuel can curb the problem of proliferation as much smaller amounts of plutonium are produced as compared to UO₂ fuel .Comparable values of safety parameters i.e. Doppler, coolant and moderator reactivity coefficients are obtained for the considered fuel options.