Effect of Burnup on the Response of Stainless Steel‐Qad Mixed‐Oxide Fuels to Simulated Thermal Transients

Direct electrical heating experiments were performed on irradiated fuel to study the fuel and cladding response as a function of burnup during a slow thermal transient. The results indicated that the nature and extent of the fuel and cladding behavior depended on the quantity of fission gas retained in the fuel. Fission-gas-driven fuel ejection occurred as the molten cladding flowed down the stack exposing bare, radially unrestrained fuel. The fuel dispersion occurred in the absence of molten fuel and the amount of fuel ejected increased with increasing burnup. In tests with medium- and high-burnup fuel, the cladding ballooned prior to melting. In regions where the cladding remained intact, helium-filled cavities were observed on the cladding. The swelling due to the cavities increased with increasing burnup.