Transmutation of minor actinides in Candu reactors

Abstract

Large quantities of nuclear waste plutonium have been accumulated in the civilian LWRs and CANDU reactors in form of minor actinides (MAs). Reactor grade plutonium and other transuranium elements can be used as a booster fissile fuel material in form of mixed ThO2/MAO2 fuel in a CANDU fuel bundle in order to assure reactor criticality. Following fuel compositions have been selected for investigations; Reactor grade plutonium: Circled digit one 96 % thoria (ThO2) + 4 % PuO2 and Circled digit two 91 % ThO2 + 5 % UO2 + 4 % PuO2. The latter is used for the purpose of denaturing the new 233U fuel with 238U. The behavior of the criticality k∞ and the burn-up values of the reactor have been pursued by full power operation for > ∼ 8 years. The reactor starts with k∞ = ∼ 1.39 and the criticality drops down asymptotically to values k∞ > 1.06, still acceptable and useable in a CANDU reactor. Reactor criticality k ∞ remains nearly constant between the 4th year and 7th year of plant operation and then a slight increase is observed thereafter, along with a continuous depletion of thorium fuel. Totality of nuclear waste actinides after the extraction of uranium isotopes: The best fuel compositions with respect to power flattening as well as long term reactivity have been found by mixing thoria with 14 % minor actinides in form of MAO 2 in the central fuel bundle and decreasing the MAO2 content in radial direction at discrete levels down to 2 % at the periphery. The temporal variation of the criticality k∞ and the burn-up values of the reactor have been calculated for a period of 10 years, operated at full power. The criticality starts at time zero near to k∞ = ∼ 1.24 for both fuel compositions. A sharp decrease of the criticality has been observed during the first year as a consequence of rapid plutonium burnout in the actinide fuel. The criticality becomes quasi constant after the 2 nd year after sufficient 233U is accumulated and remains close to k∞,end = ∼1.06 over ∼ 10 years. Quasi-uniform power generation density has been realized in the fuel bundle throughout the reactor operation. In all investigated cases, plutonium burns up rapidly and after the 2nd year, the CANDU reactor begins to operate practically as a thorium burner.