Proposal of high burnup core concept and optimization of core layout for large LMFBRs with oxide fuel material.

With the aim of reducing the fuel cycle cost of LMFBRs, the high burnup and long operating cycle core concept has been studied and optimized for a large LMFBR.The power shape for the high burnup core is found to be more flattened by dividing the core into different k∞ regions with different fuel volume fraction rather than with conventional different fuel enrichment. An axial heterogeneous core, in which a disk-shaped thin internal blanket is introduced at the core midplane in inner core region with small fuel volume fraction, is furthermore preferable to reduce the power swing during burnup.It becomes clear that the elimination of the core duct material is one of the key components to bring the burnup reactivity swing nearly zero for the core with mixed oxide fuels, in addition to increasing the core height and fuel pin diameter.It is concluded that the core with average burnup of 150GWd/t and continuous operating cycle length of 3 years is feasible by the axial heterogeneous core concept with the radial power distribution flattened by varying the fuel volume fraction.