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Information Notice No. 92-71: Partial Plugging of Suppression Pool Strainers at a Foreign BWR
UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF NUCLEAR REACTOR REGULATION WASHINGTON, D.C. 20555 September 30, 1992 NRC INFORMATION NOTICE 92-71: PARTIAL PLUGGING OF SUPPRESSION POOL STRAINERS AT A FOREIGN BWR Addressees All holders of operating licenses or construction permits for nuclear power reactors. Purpose The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice to alert addressees to the potential for loss of emergency core cooling systems (ECCS) that was identified as a result of an event at a Swedish boiling water reactor (BWR). It is expected that recipients will review the information for applicability to their facilities and consider actions, as appropriate, to avoid similar problems. However, suggestions contained in this information notice are not NRC requirements; therefore, no specific action or written response is required. Description of Circumstances: On July 28, 1992, Sydkraft AB, the utility licensed by the Swedish regulatory agency for the Barseb�ck nuclear power plant, was restarting Unit 2 after the annual outage. While the reactor was at 1 or 2 percent power and at 435 psi, a leaking pilot valve caused a safety valve for the reactor vessel to open. This valve discharges directly to the drywell. Reactor scram, high pressure safety injection, core spray, and containment spray systems initiated automatically in response to the event. Steam from the open safety valve, impinging on thermally-insulated equipment, dislodged 440 pounds of metal- jacketed, mineral wool. An estimated 220 pounds of insulating material was washed into the suppression pool. Two of five strainers on the suction side of the ECCS pumps were in service and became partially plugged with mineral wool. Plugging caused pressure to decrease significantly across the strainers and caused indications of cavitation in one pump in about an hour after the event began. The operators successfully back flushed the strainers and shut down the reactor without additional problems. Discussion In the safety analysis for Unit 2 of the Barseb�ck plant, the licensee concluded that the strainers would not require back flushing during the first 10 hours after a loss-of-coolant accident. Because of the event of July 28, the Swedish regulatory agency required Barseb�ck Unit 2 and four other units of similar design to remain shut down until this problem is resolved. 9209290014. IN 92-71 September 30, 1992 Page 2 of 2 This event at Barseb�ck is directly applicable to an earlier study conducted by the NRC. That study evaluated the potential for displaced thermal insulation to form debris and to be drawn to the sump screen. The results of the study are documented in "Containment Sump Performance," NUREG-0897, Revision 1, dated March 1984. The NRC concluded that the risk associated with the sump clogging concern did not warrant backfitting of domestic plants. However, based on the Barseb�ck event, this conclusion is being reevaluated. This information notice requires no specific action or written response. If you have any questions about the information in this notice, please contact the technical contact listed below or the appropriate Office of Nuclear Reactor Regulation (NRR) project manager. ORIGINAL SIGNED BY Charles E. Rossi, Director Division of Operational Events Assessment Office of Nuclear Reactor Regulation Technical contact: Robert L. Dennig, NRR (301) 504-1156 Attachment: List of Recently Issued NRC Information Notices .
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