Investigation of Reactor Coolant System Depressurization as an Accident Management Strategy.

摘要

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. 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(prime) 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 (PORV), 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(prime) 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 dry out. 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 the level that DCH may be minimized. In the early depressurization, the surge line may also fail, leading to rapid depressurization of the primary. In the late depressurization, surge line failure is not predicted. 23 refs., 11 figs., 3 tabs.