Aluminum/uranium fuel foaming/recriticality considerations for production reactor core-melt accidents

This paper reports on severe accident studies for the Savannah River production reactors which indicate that if coherent fuel melting and relocation occur in the absence of target melting, in-vessel recriticality may be achieved. In this paper, fuel-melt/target interaction potential is assessed where fission gas-induced fuel foaming and melt attack on target material are evaluated and compared with available data. Models are developed to characterize foams for irradiated aluminum-based fuel. Predictions indicate transient foaming, the extent of which is governed by fission gas inventory, heating transient conditions, and bubble coalescence behavior. The model also indicates that metallic foams are basically unstable and will collapse, which largely depends on film tenacity and melt viscosity considerations. For high-burnup fuel, extensive foaming lasting tens of seconds in predicted, allowing molten fuel to contact and cause melt ablation of concentric targets. For low-burnup fuel, contact can not be assured