Mitigation of direct containment heating by intentional reactor coolant system depressurization

An investigation of intentional depressurization of the reactor coolant system (RCS) as a means to mitigate direct containment heating (DCH) is presented. A DCH event could occur as the result of the dispersal of molten corium into the containment caused by a high pressure core melt ejection. The resulting containment pressure increase might be sufficient to cause containment failure. The probability of occurrence for a DCH event is a function of the driving pressure for the core melt ejection, the quantity of molten core ejected and the composition of the core melt. Although the latter two parameters are important, the RCS pressure is monitored and can be controlled to some extent by the plant operators during a severe accident. Possible strategies to mitigate DCH include (a) prevent core melting, (b) depressurize the RCS, or (c) take no action. Primary and secondary feed-and-bleed strategies are considered as basic options to prevent core damage. For a station blackout, where the ac power is lost, there is no safety injection. For a TMLB transient, the additional failure of the auxiliary feedwater is assumed. For this transient, a primary or secondary feed-and-bleed strategy using existing systems is not possible because of unavailability of the safety injection and auxiliary feedwater. The operators can take action to depressurize the primary system using the pressurizer power-operated relief valves (PORVs), take no action, or find an emergency source of water independent of ac power to perform a feed-and-bleed operation. The last option may require some minor plant system modifications. Analyses for depressurization of the Surry Nuclear Power Plant are being performed using the SCDAP/RELAP5 code for a hypothetical TMLB sequence. Two cases with different times for the initiation of depressurization are considered. 2 refs. 1 fig