Evaluation of Max Allowed Temp for RHR Suction Piping System

Abstract

A series of calculations have been performed to investigate the thermal hydraulic behavior of the Residual Heat Removal (RHR) suction piping system should a Loss-of-Coolant Accident (LOCA) would occur shortly after transition from the shutdown cooling mode to the standby Emergency Core Cooling System (ECCS) injection mode. This condition can occur during startup and shutdown operations and has the potential of hot fluid being trapped in the Residual Heat Removal (RHR) hot leg suction line for the period of time required for the trapped fluid to cool down to ambient conditions.

Depending on the piping configuration, this cooldown could require in excess of 24 hours, leaving a window of time in which the RHR system could be considered vulnerable to behaviors associated with elevated fluid temperatures in this location should the system be needed in response to an accident. These behaviors include the potential for steam intrusion into the RHR pump and to create conditions conducive to condensation induced water hammer on the initiation of Refueling Water Storage Tank (RWST) injection as well as during the switchover to the sump recirculation mode. This issue has been raised in Nuclear Safety Advisory Letter (NSAL), NSAL-93-004 (Ref. [1]) and NSAL-09-8 (Ref. [2]). Therefore, the analysis was intended to address this issue for the Westinghouse three-loop Pressurized Water Reactor (PWR). A RELAP5 model of the RHR system was developed to follow this two-phase, steam-water transient behavior.

The RELAP5 results show that the largest concern for the RHR suction piping is the potential to experience steam ingestion into the RHR pump due to flashing of the trapped hot fluid in the hot leg suction line. This calculation has conservatively generated temperature limits for the isolation of the RHR shutdown cooling to ensure no steam ingestion into the pump will occur. As long as the maximum water temperature is less than 232 °F, there will be no steam intrusion. The considerations for a potential water hammer were not of concern. However, results can be changed for other plants with different geometries and conditions.

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References

1. Westinghouse, 1993, RHRS Operation as Part of the ECCS during Plant Startup, NSAL-93-004: Nuclear Safety Advisory Letter.

2. Westinghouse, 2009, Presence of Vapor in Emergency Core Cooling System/Residual Heat Removal System in Modes 3/4 Loss-of-Coolant Accident Conditions, NSAL-09-8: Nuclear Safety Advisory Letter. 

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