The Role of Fission Gas and Fuel Melting in Fuel Response during Simulated Hypothetical Loss-of-Flow Transients

Direct electrical heating (DEH) experiments have been performed to study fuel and fission gas behavior during transients with thermal conditions similar to those predicted for flow-coastdown and sodium voiding phases of a reference reactor hypothetical loss-of-flow accident case. Macroscopic fuel response, such as gross fuel swelling and fuel dispersal in DEH fuel pellet stacks, was monitored during the transients. It was noted that in the presence of a mild restraint (e.g., due to quartz “cladding”), fuel melting always occurred prior to any detectable gross fuel motion in the stack. The fuel response at failure was strongly dependent on the thermal history of the simulated flow-coastdown phase and the heating rates during the subsequent phase of the transient experiments. In the presence of a mild restraint, the thermal history before fuel melting occurred in the stack strongly influenced the fuel behavior. The thermal history before melting determines the nature and morphology of fission gas bubbles at the time of melting. These, in turn, strongly influence the fuel behavior after molten fuel appears. Micro-structural analysis of the fuel before and after transients provided additional data that indicate that the interaction between fission gas and molten fuel that may lead to frothing of molten fuel due to expansion of fission gas can play a major role in swelling of the fuel stacks and in fuel behavior at failure.