Core melt down and vessel failure: a coupled problem Original Research Article

Analyses of lower head failure have been performed for a variety of core slump scenarios that result from three contrasting reactor accident sequences in a PWR. The cases cover a range of thermalhydraulic conditions in the vessel and core debris characteristics. The results show lower head failure occurs at a time which depends on the internal thermal-hydraulic conditions and debris characteristics. Failure may be local or global and may be due to one or more of the following processes: creep; plasticity (including thermo-plasticity); and melt-through. At low to moderate pressure, creep damage accumulates over a wide area, leading to probable global failure. Local plastic deformation becomes increasingly important at higher pressures or following a pressure spike, with a possibility of local failure. Local melting can occur before failure if there is a large concentrated heat flux. A question of particular interest for future study is raised by the CORVIS experiments, namely that the deformation can cause a gap to open between the structure and debris crust and hence increase the thermal resistance. Modest estimates of the gap resistance show a significant delay in failure. A coupled treatment of the thermal and mechanical response is needed to assess the dynamic gap behaviour effectively.