Reactor pressure vessel lower head structural behaviour during severe accident

The lower head of the reactor pressure vessel (RPV) can be subjected to significant thermal and pressure loads in the event of a core meltdown accident in a typical BWR. The mechanical behavior of the reactor vessel lower head is of importance both in severe accident assessment and the assessment of accident mitigation strategies (SAMG provisions). For severe accident assessment the failure of the lower head defines the initial conditions for all ex-vessel events, e.g., core concrete interactions, fuel coolant interactions, direct containment heating etc. In this paper, these models are used to estimate the failure timing and the failure mechanism of a typical BWR bottom head using ABAQUS and RELAP5/SCDAP. The analysis is carried out for unmitigated station blackout (high pressure loading) scenario where manual depressurization of RPV is not possible as it happened in Fukushima BWR-1. The results show that the maximum vertical displacement of bottom point just before the failure of head will be 22.3cm. Considering the time of lower head failure from start of core melt and delay due to phase transition, the total time to fail the vessel lower head is 1hour 35min