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 in this paper. Both primary and secondary feed-and-bleed strategies are examined as a mechanism to depressurize the RCS. For a station blackout, where the ac power is lost, there is no safety injection; and for a TMLB' transient, the failure of the auxiliary feedwater is also 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, 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 were performed using the SCDAP/RELAP5 code for a hypothetical TMLB' sequence. Two cases with different times for the initiation of depressurization are considered. In the first case, the depressurization is initiated at the time of steam generator dryout. This case is called early depressurization. In the second case, the depressurization is initiated at the time of core uncovery. This case is called late depressurization. Both calculations show that the system can be depressurized to a level where DCH may be minimized. Uncertainties in these calculations are discussed, and strategies are subjectively judged. Available strategies, in the order of preference, are: (a) to perform a feed-and-bleed operation if the ac power is recovered within a certain period of time; (b) to depressurize the primary system after core heat up has begun, if an emergency source of feedwater is not available; and (c) to take no action