Discovery of Sump Performance Issue
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.
As part of its responsibility to ensure public health and safety, 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.
Because of internal questions by the NRC staff, the NRC first sponsored research to study the accumulation of debris on PWR containment sump screens and BWR suction strainers in the late 1970s (approximately 1979). With the information and engineering tools available in the late 1970s and early 1980s, the NRC concluded that its regulatory guidance needed to be revised and issued in 1985 revision 1 of RG 1.82. As documented in Generic Letter 85-22, “Potential for Loss of Post-LOCA Recirculation Capability Due to Insulation Debris Blockage,” the NRC concluded that no additional regulatory action was warranted for operating nuclear power plants, but that new nuclear power plants would need to satisfy the guidance in the revised RG 1.82, and that operating nuclear power plants should consider the guidance in the revised RG 1.82 when making plant modifications, namely changing thermal insulation.
In the early 1990s, because of an event at a BWR in Sweden and several events at BWRs in the United States, the NRC sponsored new research to study the accumulation of debris on BWR suction strainers. Based on the information learned from operational experience and the new research using more sophisticated engineering tools than those readily available in the late 1970s, the NRC concluded that its guidance needed additional revision for BWRs. In 1996, the NRC issued revision 2 of RG 1.82. Using new research data and better engineering tools, the NRC concluded that additional regulatory action was needed to ensure safety is maintained at BWR power plants. As documented in Bulletin 96-03, “Potential Plugging of Emergency Core Cooling Suction Strainers by Debris in Boiling-Water Reactors,” the NRC concluded that additional regulatory action was warranted for operating BWRs and asked BWRs to conduct plant-specific evaluations of their suction strainer performance and, if necessary, modify their plant design and/or operation.
Because of the information the NRC learned during the assessment of BWR suction strainers and oversight of BWR plant-specific evaluations and modifications, the NRC sponsored a new research effort to study the accumulation of debris on PWR containment sump screens. Based on the most recent research study, “GSI-191 Technical Assessment: Parametric Evaluations for Pressurized Water Reactor Recirculation Sump Performance,” the NRC concluded that its guidance needed additional revision for PWRs. In November 2003, the NRC issued revision 3 of RG 1.82. The NRC has concluded that additional regulatory action is warranted. Currently, the NRC is implementing its plan to have all PWR licensees perform a plant-specific evaluation for the potential for excessive head loss across the containment sump screen because of the accumulation of debris on the containment sump screen. The NRC also expects licensees to evaluate effects of debris that might pass through the sump screens.
Based on the information available to date, continued operation of PWRs is justified until plant-specific evaluations are completed. To provide additional assurance regarding the continued operation of PWRs, the NRC asked the licensees of PWRs to implement compensatory measures. This was done through the issuance of Bulletin 2003-01, “Potential Impact of Debris Blockage on Emergency Sump Recirculation at Pressurized-Water Reactors.” If the results of ongoing NRC inspections and reviews or ongoing and planned studies indicate that unsafe conditions exist at any operating PWR, the NRC will take immediate 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.