PWR Sump Performance

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The containment sump (also known as emergency or recirculation sump) in a pressurized water reactor (PWR) is part of the emergency core cooling system. Every nuclear power plant is required by regulations (i.e., 10 CFR 50.46) to have an emergency core cooling system to mitigate a design basis accident. The emergency core cooling system (ECCS) is one of several safety systems required by the NRC.

The NRC identified a potential susceptibility of PWR recirculation sump screens and associated flow paths to debris blockage during loss-of-coolant accidents that require recirculation operation. This issue might impede the long-term operation of the emergency core cooling system or containment spray system. The additional head loss due to the accumulation of debris has the potential to exceed the net positive suction head margin required for the successful operation of the emergency core cooling system and containment spray system pumps. Debris can also pass through sump screens and affect equipment downstream (such as valves, pumps, and nuclear fuel assemblies).

As a result, all operating PWR licensees were requested through Generic Letter (GL) 2004-02 to perform a mechanistic evaluation of the recirculation functions and, as appropriate, to take additional actions (e.g., plant modifications) to ensure system functionality. As licensees pursued development of responses to GL 2004-02, testing indicated that licensees needed to address additional issues (e.g., chemical effects and in-vessel downstream effects) whose scope was not fully known at the time GL 2004-02 was issued. These issues have required additional testing and plant modifications for many licensees.

NRC's and licensees' efforts are focused on resolving remaining issues to support closing the GL for all PWRs. In response to GL 2004-02, licensees have made major modifications to their plants to ensure satisfactory sump performance. All licensees have significantly increased the size of their sump screens, and some have replaced fibrous insulation with reflective metal insulation whose debris is considered less likely to reach or impede flow through sump screens. The remaining issues are involved with adequately demonstrating that performance of the modified sumps would be adequate. The plant systems involved cannot be tested in a debris environment, so testing must be done at an offsite facility on a model of the sump. The challenge is for the licensee to show that the test conducted on the model is adequately similar to what would occur in the plant in the unlikely event of a loss-of-coolant accident. Many assumptions must be made to support the evaluation and test, which are highly complex. The staff is in the process of carefully evaluating the licensee tests and evaluations. In some cases, licensees have responded to questions raised by the staff by redoing their tests and evaluations with different assumptions. While these evaluation issues are being resolved, the NRC staff believes that plants may continue to operate safely, based on the modifications made to date, the very low likelihood of a worst-case event that could challenge strainer performance, and the numerous mitigative measures licensees have taken.

The NRC has concluded that the majority of PWR licensees have demonstrated adequate sump performance with the exception of in-vessel downstream effects. The NRC staff is currently evaluating a topical report submitted by the PWR Owners Group that is intended to address this remaining issue. The NRC has asked for additional information from licensees that have not fully addressed sump performance issues. The results of licensee evaluations, which contain numerous conservatisms, may indicate the need for some licensees to make additional plant modifications to fully address the sump performance issue. The NRC staff will expect licensees to make such modifications as soon as practicable and will track all such modifications to completion.

The NRC and the BWR Owners Group are evaluating whether additional work is needed to address BWR ECCS strainer performance in light of information obtained for PWRs. BWR licensees had previously addressed BWR strainer performance during the 1990s. The NRC has not yet made a determination regarding whether additional BWR licensee actions are needed. Additional information on this subject can be found via a link below under "NRC Activities."

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Reports and Correspondence

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Page Last Reviewed/Updated Wednesday, March 11, 2020