Fuel Foaming and Collapse During Light Water Reactor Core Meltdown Accidents

Fuel melting in severe core damage accidents will lead to the rapid release of fission gas from the fuel matrix and the volatilization of low boiling point metallic inclusions, which can be expected to significantly influence molten fuel dynamics. A quantitative analysis of UO₂ foaming potential is based on an assessment of the time characteristics for bubble growth, surface escape, film thinning, and bubble coalescence. Analysis indicates that although the potential exists for early molten UO₂ foaming, such foams are basically unstable and tend to collapse, thereby releasing volatilized fission products from the molten fuel debris. Release of such fission products will impact radiological source term evaluation and can result in up to a 40% reduction in the residual decay heat within the core debris. This reduction in core debris heat level will influence the timing and meltdown sequence for such accidents and can impact the heat load requirements of residual heat removal systems or other engineered melt mitigation devices.