CANDU reactor as minor actinide/thorium burner with uniform power density in the fuel bundle

Abstract

A CANDU reactor fueled with a mixed fuel made of thoria (ThO 2) and nuclear waste actinides has been investigated. The mixed fuel composition has been varied in radial direction to achieve a uniform power distribution and fuel burn-up in the fuel bundle. 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 with a radially decreasing MAO 2 content at discrete levels down to 2% at the periphery. Furthermore, as an alternative fuel composition, 5% UO 2 has been added to the mixed fuel for the sake of a higher degree of nuclear safeguarding through denaturing the 233U component with 238U. 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 2nd 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. Very high burn-up could be achieved with the same fuel material (up to 200,000 MW D/MT), provided that the fuel rod claddings would be replaced periodically. This would reduce fuel fabrication costs and nuclear waste mass for final disposal per unit energy drastically.