Irradiation Performance of Metallic Driver Fuel in Experimental Breeder Reactor II to High Burnup

The Experimental Breeder Reactor II Mark-II metallic-driver-fuel element has been irradiated to high burnup to assess element lifetime and performance reliability. The elements breached at 10 at.% burnup or greater. This increase in burnup potential compared to its predecessor, the Mark-IA (limited to a burnup of 3 at.%), is due to the reduction of fuel-cladding mechanical interaction (FCMI) resulting from the smaller smear density, a lower fission-gas-induced cladding stress resulting from the increased plenum volume and increased cladding thickness, and a reduction in fuel-cladding chemical interaction (FCCI) due to a change in cladding material. The cladding breach in the solution-annealed Type 316 stainless-steel-clad elements was in the restrainer dimple located above the original fuel column, not in the upper half of the fuel column as in the Type 304L stainless-steel-clad elements. During irradiation, the prime cause of cladding deformation was swelling. Due to the extensive release of fission gas after interconnected porosity developed, the fuel deformation was restricted by the cladding. After fuel-cladding contact, a small amount of FCCI, as predicted by out-of-pile measurements, occurred, but little FCMI is thought to have taken place.