Temperature Limits In The RHR System Prior To ECCS Operation

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 (Lim, 2018).

In 1993, Westinghouse issued the Nuclear Safety Advisory Letter (NSAL), NSAL-93-004, to identify a potential concern associated with steam flashing of hot water in the isolated hot leg suction piping when the RHR system is aligned to the Reactor Coolant System (RCS).

In 2009, Westinghouse issued an additional letter, NSAL-09-8, to clarify the previous guidance and to ensure the consideration of the reduced hydrostatic head when the RHR system water source is transferred from the RWST to the containment sump during the recirculation mode.

The schematic diagram of the Westinghouse three-loop Pressurized Water Reactor (PWR) RHR system. The RHR pumps are normally fed from the RWST until the ECCS suction switchover to the containment sump is initiated based on the RWST water level. The other normal supply path for the RHR pump is from the containment sump.

Phenomena of Concern and Screening Evaluation

Phenomena of Concern:

The fluid can flash and a steam-water mixture will preferentially feed the RHR pump suction as long as the saturation pressure remains above the source pressures from the RWST or the containment sump.
If voiding occurs and the pressure drops below to initiate the injection from the RWST or the containment sump, conditions favorable to condensation-induced water hammer may be created that can challenge the piping and its supports.
If drainage in the RHR pump discharge line occurs during the recirculation from the containment sump, a liquid column separation and rejoining of the water hammer may be expected in the RHR pump discharge line, including the RHR Heat Exchanger (HX) tubes when the RHR pump restarts.

Screening Evaluation:

In this analysis, the RHR HXs and the highpoints of the pump discharge line are horizontally located below the containment sump water surface. Therefore, there should be no drainage in the pump discharge line during the manual switchover to the containment sump.

Based on the static pressures and their corresponding saturation temperatures, the temperature at the highpoint of the isolated hot leg suction piping is limited to 236 °F (113 °C) during the RWST injection mode, while the temperature limit is reduced to 183 °F (84 °C) to preclude any void formation after the switchover to the containment sump during the recirculation mode. However, the maximum temperature based limit of 183 °F (84 °C) for zero voiding would be too restrictive for plant startup operations. As a result, the RELAP5 transient analysis was initiated to provide dynamically based thermal limits.

RELAP5 Analysis

RELAP5 (Reactor Excursion and Leakage Analysis Program, version 5) is a light water reactor transient analysis code developed for the United States Nuclear Regulatory Commission (U.S. NRC) for simulation of a wide variety of hydraulic and thermal transients in both nuclear and non-nuclear systems involving mixtures of steam, water, non-condensable gases, and solutes under single-phase and two-phase conditions.

Description of Cases Analyzed

Injection from RWST:

When the RHR system is isolated by closing the hot leg and the cold leg isolation valves, the hot water at a temperature of 350 °F (177 °C), which corresponds to the saturation pressure of 134.6 psia (928.0 kPa), can be trapped in the isolated hot leg suction piping.

According to the RHR system design, the first water source for the RHR pump is the RWST to provide the RCS cooling. In this case, however, the hot water in the isolated hot leg suction piping can flash to the steam and thus, the steam-water mixture would be pulled into the pump suction header. Simultaneously, the higher pressure in the isolated hot leg suction piping would also close the RWST check valve, thereby preventing water flow from the RWST to the suction header.

Switchover to Containment Sump:

This transition occurs due to the depletion of the RWST. Upon the switchover of the water source from the RWST to the containment sump during the recirculation mode, which is accompanied by the operator action of closing the RWST isolation, the pressure in the suction header would be reduced. There would be additional flashing of the water in the isolated hot leg suction piping and the expansion of any existing steam volume. This results in more steam intrusion into the RHR pump that is sufficient to degrade its performance.

Acceptance Criteria

The objective of the analysis was to determine the temperature limit to preclude the steam ingestion into the RHR pump as well as the conditions favorable to a condensation-induced water hammer in the isolated hot leg suction piping. Therefore, the acceptance criteria for this analysis are:

No void fraction exceeding 2% reaches the inlet of the pump suction header.
Water hammer loads need to be minimal. This is considered to be the case when the piping segment loads are comparable to the flooded weight of the pipe segment, which is the summation of the pipe weight and the weight of water within the pipe.

Result and Conclusion

During the RWST injection mode, the maximum temperature was calculated as 265 °F (129 °C) to limit the void fraction to less than 2% at the inlet of the RHR pump suction header.
During the recirculation from the containment sump, however, the maximum temperature should be reduced to 232 °F (111 °C).
The condensation-induced water hammer was not of concern due to the physical system design not creating conditions favorable to rapid condensation that could lead to a water hammer.

These findings are described in the article entitled Temperature limit in the RHR system when aligned to the ECCS, recently published in the journal Annals of Nuclear Energy. This work was conducted by Jaehyok Lim from Fauske and Associates, LLC.