Behavior of Metallic Fast Reactor Fuels During an Overpower Transient

Abstract A slow-ramp (0.1%/s), extended overpower (∼32%) transient was conducted in the Experimental Breeder Reactor-II (EBR-II) on 19 metallic alloy fuel elements, including both binary (U-Zr) and ternary (U-Pu-Zr) fuel designs. The elements were clad in either Type 316 SS or HT9. Before the transient, the elements were pre-irradiated under steady-state or steady-state plus duty-cycle (periodic 15% overpower transient) conditions in various EBR-II sub-assemblies and reached burnups ranging from ∼5 to ∼12 at%. The fuel pins were then consolidated into the X512 sub-assembly for the overpower transient. Cladding integrity was maintained for all fuel pins throughout the entirety of normal operation and the overpower transient test. Post-irradiation examination (PIE) demonstrated that mild restructuring was evident within the fuel phase. However, these changes were not significant enough to expect changes in irradiation performance under normal operating conditions if the fuel elements were returned to service. These results empirically validate that limits on sodium fast reactor operational transients (e.g. power ramps) could be applied with adequate safety margins to fuel failure or non-reversible damage.