Assessment of criticality and burn up behavior of candu reactors with nuclear waste trans uranium fuel

Abstract

Large quantities of nuclear waste plutonium and minor actinides (MAs) have been accumulated in the civilian light water reactors (LWRs) and CANDU reactors. These trans uranium (TRU) elements are all fissionable, and thus can be considered as fissile fuel materials in form of mixed fuel with thorium or nat-uranium in the latter. CANDU fuel compacts made of tristructural-isotropic (TRISO) type pellets would withstand very high burn ups without fuel change.As carbide fuels allow higher fissile material density than oxide fuels, following fuel compositions have been selected for investigations: ① 90% nat-UC + 10% TRUC, ② 70% nat-UC + 30% TRUC and ③ 50% nat-UC + 50% TRUC. Higher TRUC charge leads to longer power plant operation periods without fuel change. The behavior of the criticality k∞ and the burn up values of the reactor have been pursued by full power operation for > ∼12 years. For these selected fuel compositions, the reactor criticality starts by k∞ = 1.4443, 1.4872 and 1.5238, where corresponding reactor operation times and burn up values have been calculated as 2.8 years, 8 years and 12.5 years, and 62, 430 MW.D/MT, 176,000 and 280,000 MW.D/MT, with fuel consumption rates of ∼16, 5.68 and 3.57 g/MW.D respectively. These high burn ups would reduce the nuclear waste mass per unit energy output drastically. The study has show clearly that TRU in form of TRISO fuel pellets will provide sufficient criticality as well as reasonable burn up for CANDU reactors in order to justify their consideration as alternative fuel.