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Information Notice No. 87-10: Potential for Water Hammer during Restart of Residual Heat Removal
SSINS No.: 6835 IN 87-10 UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF INSPECTION AND ENFORCEMENT WASHINGTON, D.C. 20555 February 11, 1987 Information Notice No. 87-10: POTENTIAL FOR WATER HAMMER DURING RESTART OF RESIDUAL HEAT REMOVAL PUMPS Addressees: All boiling water reactor (BWR) facilities holding an operating license or a construction permit. Purpose: This information notice is to alert addressees of the potential for water hammer in the residual heat removal (RHR) system of BWRs during a design basis loss of coolant accident (LOCA) coincident with a loss of offsite power (LOOP) if the RHR system is aligned to suppression pool cooling. Recipients are expected to review the information for applicability to their facilities and consider actions, if appropriate, to preclude similar problems occurring at their facilities. However, suggestions contained in this information notice do not constitute NRC requirements; therefore, no specific action or written response is required. Description of Circumstances: On December 11, 1986, the Susquehanna nuclear power plant reported that based on results of an ongoing review of the potential effects of water hammer events, the RHR system could be susceptible to water hammer loads that would exceed the allowable stresses in the RHR system and piping. The specific condition of concern involves a design-basis LOCA coincident with a LOOP, while one or one RHR loops are in the suppression pool cooling mode. During the power loss and subsequent valve realignment, portions of the RHR system will void because of the drain down to the suppression pool as a result of elevation differences. A water hammer may occur in those RHR loops that were in the suppression pool cooling mode when the RHR pumps restart after the diesel generators reenergize the buses. The core spray system also may be subject to such a water hammer if it is lined up in the suppression pool mixing mode full flow test. The Susquehanna design basis for LOCA/LOOP assumes that the suppression pool cooling flow path valves are initially closed in the standby lineup. The potential duration factor used in the consideration of the coincident LOCA/LOOP with the RHR in suppression pool cooling mode was one percent, or roughly 90 hours per year. 8702100126 . IN 87-10 February 11, 1987 Page 2 of 2 Contrary to the design basis assumption, a licensee review of operating history found that the worst case RHR system usage factor approached 25% during cycles in which significant safety relief valve weeping was experienced. For interim corrective action, the licensee has modified operating procedures to allow, at a time, only one loop of RHR to operate in suppression pool cooling. In addition, the licensee will revise plant procedures to address the restart of an RHR pump if it trips while operating in the suppression pool cooling mode. The core spray system is currently prohibited from being operated in the suppression pool mixing mode, except for required surveillance testing. Discussion: The NRC discussed the potential for this general type of event in Engineering Evaluation No. AEOD/E309, "The Potential for Water Hammer During the Restart of RHR Pumps at BWR Nuclear Power Plants," dated April 1983. In the type of scenario discussed in AEOD/E309, the line most likely to drain and experience a water hammer is the drywell spray line because it has the largest elevation difference between it and the suppression pool. RHR system pipes less than 33 feet above the suppression pool will not usually drain because atmospheric pressure will support a column of water that high. A water hammer in the drywell spray line could endanger RHR system integrity, and thus jeopardize all modes of RHR including low-pressure coolant injection. The analysis performed by the licensee of the Susquehanna nuclear power plant goes beyond AEOD/E309 in that detailed site-specific computer modeling was performed which shows that piping system integrity could be challenged. Besides Susquehanna, other plants may have high usage factors for suppression pool cooling mode and large elevations differences in the RHR system, making those plants potentially subject to water hammer in the RHR system. No specific action or written response is required by this information notice. If you have questions about this matter, please contact the Regional Administrator of the appropriate NRC regional office or this office. Edward L. Jordan Director Division of Emergency Preparedness and Engineering Response Office of Inspection and Enforcement Technical Contact: Eric Weiss, IE (301) 492-9005 George Lanik, IE (301) 492-9007 Attachment: List of Recently Issued IE Information Notices .
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