The Nuclear Regulatory Commission (NRC) last week instructed staff to address a potential problem at nuclear power plants around the U.S. where the containment sump—an emergency long-term cooling water source—could be clogged by debris accumulating after a high-pressure coolant break.
Reactor building sumps are designed to collect water lost after a pipe break, providing a potential source of coolant for recirculation after a reactor’s other water sources are exhausted. According to the NRC, the original technical issue involved pipe-break scenarios where large amounts of fibrous insulation could be knocked off coolant pipes and collect in the sump, blocking recirculation.
“This issue might impede the long-term operation of the emergency core cooling system or containment spray system,” the NRC says on its website. “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).”
The new directions set a near-term schedule for smaller loss-of-coolant accident (LOCA) scenarios and a longer-term schedule for the less-likely larger LOCAs. The commission also asked staff to develop additional risk-informed implementing guidance for the containment sump issue.
“While much has been done by the staff, and licensees have physically modified their sump screens, resulting in significant safety improvements, the agency needs to finally resolve this issue,” Chairman Gregory B. Jaczko said. “Nuclear power plants have installed larger sump strainers and taken steps to eliminate materials that could end up in the sump. We’re confident these first steps provide for continued safe operation, but we need to move forward in resolving the final technical issues and closing out this long-standing issue.”
Sump strainer performance issues were first identified in 1979, in an NRC report, Unresolved Safety Issue A-43, “Containment Emergency Sump Performance.” Boiling-water reactors resolved the issue by installing much larger strainers, but in the late 1990s the staff identified new concerns for pressurized-water reactors concerning debris generated following a design-basis LOCA.
According to the NRC, 48 of the 69 U.S. pressurized water reactors have already used the integrated resolution process to develop NRC-accepted analysis and testing to show that their sumps will not clog. The commission said it has directed the agency’s staff to continue working with industry representatives to perform additional testing—expected to be completed by the end of 2011—and formulate a path forward by mid-year 2012.
“In the interim, the Commission has directed the staff to defer further plant modification actions (such as fibrous material removal) until the tests and analyses are complete. The staff will report to the Commission in approximately 18 months, identifying proposed policy options for resolving [the containment sump issue].”
The NRC said it expects upcoming sump performance assessments to take into account data including the full range of possible pipe break sizes, plant-specific compensatory measures, and other design features that could reduce clogging risk. “The Commission is also interested in better understanding the radiation dose that workers might receive if plants had them remove additional materials that could contribute to sump clogging,” it said.
Sources: NRC, POWERnews