United States Nuclear Regulatory Commission - Protecting People and the Environment

Frequently Asked Questions About PWR Sump Performance

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Index to All Frequently Asked Questions Pages

How long has the NRC been studying PWR sump performance?

The NRC first published regulatory guidance on the performance of pressurized-water reactor (PWR) containment sumps and boiling-water reactor (BWR) suction strainers in 1974 with the issuance of revision 0 of Regulatory Guide (RG) 1.82, "Water Sources for Long-Term Recirculation Cooling Following a Loss-of-Coolant Accident." BWR suction strainers perform the same function as PWR containment sump screens. The NRC continually assesses the design and operation of nuclear power plants to determine whether its regulations, its guidance, or nuclear power plant design or operations need to be modified. The NRC began its first reassessment of PWRs sump and BWRs suction strainer performance in 1979. The first reassessment (including all technical and regulatory actions) was completed in 1985. In 1992, the NRC started a second reassessment of BWR suction strainer performance and completed the reassessment (including all technical and regulatory actions) in 2000. In 1996, the NRC started a second reassessment of PWR sump performance and that reassessment is ongoing. After the completion of all necessary regulatory actions, the reassessment of PWR sump performance will be completed (i.e., resolved).

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What are the results of the recent NRC technical assessment of PWR sump performance?

The technical assessment of PWR sump performance was to designed to answer
two questions:

  1. Is the emergency core cooling system (ECCS) sump clogging issue a plausible concern for domestic PWRs?
  2. Is there a need for additional regulatory action regarding PWR sumps?

The technical assessment provided a generic, not a plant-specific, basis answer to these two questions. Based on the results from the technical assessment the NRC concluded that excessive head loss caused by debris accumulation on sump screens was a plausible concern that merited additional plant-specific evaluation.

As part of the technical assessment, the NRC commissioned a generic parametric evaluation. This evaluation is documented in NUREG/CR- 6762, Volume 1. The focus of the parametric evaluation was to examine the range of conditions present in operating PWRs without the reliance on bounding or conservative assumptions. The study was conducted using a generic plant piping and containment configuration. Various plant parameters were then overlayed onto the generic plant. The combinations of parameters were reflective of actual licensed plants to determine a reasonable range of the susceptibility of excessive head loss caused by debris accumulation on sump screens. Since each case was calculated using a combination of a generic plant piping and containment configuration, some generalized assumptions, and some actual plant characteristics, none of the combinations (i.e., parametric cases) represent any of the 69 operating PWRs.

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How will the NRC identify which plants do not meet the regulations based upon their PWR sump performance?

Plant-specific evaluations for excessive head loss across the containment sump screen because of the accumulation of debris on the containment sump screen are needed to determine whether a operating pressurized reactor meets the applicable safety regulations. Based on the information available to date, continued operation of PWRs is justified until the plant-specific evaluations are completed. To provide additional assurance until those evaluations are completed, the NRC asked the licensees of PWRs to implement interim compensatory measures. However, if the results of ongoing NRC inspections and reviews indicate that unsafe conditions exist at any operating PWR, the NRC will take immediate plant-specific actions to ensure the continued health and safety of the public. Also, if a licensee discovers that it is not in compliance with the NRC regulations during the implementation of the requested actions in Bulletin 2003-01, it is required to take prompt corrective actions.

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Page Last Reviewed/Updated Tuesday, December 11, 2012