A core design study to convert a breakeven core into a TRU burner is performed for a 600 MWe rated fast reactor. No change in the core and subassembly layouts is assumed, which only allows geometry variations within the fuel rods. Investigated alternatives are to use variable cladding thicknesses, smearing fraction adjustments and annular fuel rod concepts with a central liner of a variable diameter consisting of void, Zr, B4C, Al, W, etc.The variable cladding thickness concept could not be employed due to too high a clad inner wall temperature. A smearing fraction adjustment below a typical fraction of 75% leads to a moderate TRU burning and reduced sodium void worth, but to a relatively high burnup swing. Placing a central non-fuel rod with the fuel arranged in an annular ring affects the core performance and reactivity coefficients, depending on whether it is a moderator or an absorber. In general, candidate materials of high atomic numbers contribute to large positive sodium void worths, but enhanced negative expansion effects. Among the light elements, vanadium reveals a favourable performance with a comparable TRU burning and a reduced sodium void worth, suggesting this material can be regarded as a solid substitute for sodium.
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