A comparison of the neutronic performance of homogeneous and heterogeneous designs for a 1300 MWe LMFBR

The neutronic performance of the currently preferred homogeneous core design for a 1300 MW(E) CDFR is compared with that of an alternative heterogeneous core. Calculations for the two designs gave been performed with the COSMOS modular code scheme. The heterogeneous design requires fewer fuelled sub-assemblies, has a higher feed enrichment and has a higher breeding gain. Overall the doubling time of the heterogeneous design is lower. With notional fast reactor fuel fabrication and reprocessing costs the fuel cost for the heterogeneous core is marginally greater than that for the homogeneous core. The sodium void coefficient of the heterogeneous core is lower but so too is the Doppler constant which leaves open the question as to whether there is any safety advantage for either design. A major concern with the heterogeneous design is the large increase in the power of the in-core breeder sub-assemblies during their dwell time in the reactor leading to potential ''thermal striping'' problems. (author)