Modeling of the P2M Past Fuel Melting Experiments with the FEMAXI-8 Code

In the Power to Melt and Maneuverability (P2M) project, a simulation exercise on two past power ramp experiments, xM3 on a medium-burnup rod and HBC4 on a high-burnup rod, was performed with the fuel performance code FEMAXI-8 to investigate fuel behavior under high-power and high-temperature conditions toward centerline fuel melting. In order to treat fuel melting, empirical melting temperature models have been incorporated into the FEMAXI-8 code. The present analysis gives reasonable predictions not only on cladding deformation but also on the fuel melting behavior of the HBC4 rod in which the UO₂ liquidus temperature was reached during the transient. On the other hand, model improvement appears to be needed for a more accurate treatment of the fuel melting behavior of the xM3 rod in which the fuel center temperature reached the solidus line, whereas it may have not reached the liquidus line. A reasonable agreement of estimated fission gas release (FGR) with the measurement suggested that the high-temperature FGR at the given conditions is essentially a temperature-dependent phenomenon rate limited primarily by thermally activated elementary processes, such as fission gas diffusion.