BWR Reactor Vessel Bottom Head Failure Modes

This chapter describes the approach taken by the Boiling Water Reactor Severe Accident Technology Program at Oak Ridge National Laboratory towards understanding the probable sequence of events for an unmitigated boiling water reactors severe accident. There are many associated uncertainties, and experimental verification of the approach is certainly desirable. The chapter addresses the events occurring after debris relocation past the core plate and to describe the subsequent expected modes of bottom head pressure boundary failure. For an unmitigated BWR severe accident involving the progressive relocation of material from the core region into the lower plenum of the reactor vessel, the control rod guide tube structure and the large amount of water in the lower plenum would be expected to provide for distribution and quenching of the relocating debris. The individual components of the debris bed would be expected to leave the vessel in the order in which they reach their melting points and transform to the liquid state.