ICONE15-10021 EVALUATION OF LATE INTENTIONAL DEPRESSURIZATION AS A SEVERE A ACCIDENT MANAGEMENT STRATEGY

It has identified that the pressure in the reactor coolant system (RCS) remains high in some severe accident sequences at the time of reactor vessel failure, causing direct containment heating (DCH). Intentional depressurization is an effective accident management strategy to prevent DCH or to mitigate its effects. Two strategies to mitigate DCH by depressurizing the RCS pressure are usually considered. One is called "early depressurization" and the other is called "late depressurization". The late depressurization is preferred as the severe accident management strategy because there are greater opportunities to recover plant functions prior to core damage, e.g. emergency core cooling systems (ECCS). According to the pressurizer PORVs design of the specific PWR plant, three late depressurization strategies are proposed in the analysis. During a station blackout TMLB' sequence, the phenomenological behavior is evaluated to determine how the strategies could depressurize the RCS pressure to a sufficiently low value where the effects of DCH will be mitigated.