Temperature limit in the RHR system when aligned to the ECCS

Highlights: • Westinghouse issued NSALs to identify a potential steam flashing in the RHR system. • The RELAP5 computer code transient analysis was performed to provide dynamically based thermal limits. • The temperature limits were calculated to preclude the steam ingestion into the RHR pump. - Abstract: A thermal hydraulic analysis was performed for the Residual Heat Removal (RHR) system shortly after a transition from the shutdown cooling mode to the standby Emergency Core Cooling System (ECCS) injection mode when a Loss-of-Coolant Accident (LOCA) was postulated to occur. This condition causes the potential of hot fluid to be trapped in the isolated hot leg suction piping when the RHR system is aligned to the Reactor Coolant System (RCS). Subsequently, the RHR system can be realigned to the ECCS, which is at a low pressure, if the system is needed in response to the LOCA. As a result, the hot fluid can flash to steam, so the system may fail to provide the RCS cooling due to the potential of the steam ingestion into the RHR pump. Westinghouse issued two Nuclear Safety Advisory Letters (NSALs), NSAL-93-004 and NSAL-09-8, to ensure that the affected facilities establish the temperature limits to appropriately address the concerns in regard to the RHR pump operability. In this analysis, a detailed calculation of the temperature limits was performed using the RELAP5/MOD3.3 (Patch 03) computer code. The temperature limits were calculated by considering no steam ingestion into the RHR pump.