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UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF NUCLEAR REACTOR REGULATION WASHINGTON, D.C. 20555 March 25, 1994 NRC INFORMATION NOTICE 94-25: FAILURE OF CONTAINMENT SPRAY HEADER VALVE TO OPEN DUE TO EXCESSIVE PRESSURE FROM INERTIAL EFFECTS OF WATER Addressees All holders of operating licenses or construction permits for nuclear power reactors. Purpose The U.S. Nuclear Regulatory Commission is issuing this information notice to alert addressees to the potential for valves to fail to open because of unexpectedly high differential pressures caused by the inertial effects of water moving in partially filled piping systems. 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 September 13, 1993, at the Waterford Steam Electric Station, Unit 3, the train A containment spray header isolation valve failed to open when it was actuated following the successful completion of a test on the containment spray system. The containment spray system configuration includes a shutdown cooling heat exchanger and two check valves located in the piping between a containment spray pump and the subject isolation valve (Attachment 1). The isolation valve is a 25-cm [10-inch] WKM solid gate valve, which is closed by air pressure and opened by a spring, and is designed to open with a differential pressure of 2070 kPa [300 psi] across the gate. The design rating of the piping near the valve is 2170 kPa [300 psig] and the design shutoff head of the pump is approximately 2000 kPa [275 psig]. During the test, the operators had successfully cycled the isolation valve open and closed as required by the inservice surveillance testing program. Subsequently, the operators started the containment spray pump. These two functions are tested separately to prevent the actual spraying of water into the containment. After completing this testing, the operators attempted to open the isolation valve to refill the containment spray riser, but the valve 9403210215. IN 94-25 March 25, 1994 Page 2 of 3 would not open. Therefore, the licensee declared train A of the containment spray system inoperable. The header subsequently became depressurized and the operators were able to open the valve. The licensee performed a special test on train A of the containment spray system to determine the pressures that were developed in the piping. With the isolation valve closed, the pump was started, run, and shut down while pressure measurements were made at various locations in the piping. The discharge pressure of the pump, which was run with minimum recirculation flow, was measured at 1900 kPa [260 psig]. The pressure at the inlet of the heat exchanger, downstream of the first check valve, was measured at 2230 kPa [308 psig]. The pressure between the second check valve and the containment isolation gate valve reached 3330 kPa [469 psig] and then stabilized at 3210 kPa [450 psig], 1310 kPa [190 psi] higher than the pump discharge pressure. Data gathered during this testing also indicated that air had been allowed to enter the piping system during maintenance activities. Discussion The licensee concluded that excessive differential pressure across the isolation valve, which had developed during the earlier surveillance test, had prevented the isolation valve from opening on September 13. Apparently the air in the piping had allowed the pump discharge water to accelerate to a higher-than-normal velocity. As the air was forced into the space between the isolation valve and the check valves, it was compacted to a pressure that was higher than the pump discharge pressure by the inertia (the hydraulic ram effect) of the water. This excessive pressure was then trapped against the isolation valve when the second check valve closed. The licensee initially believed that the high pressure would occur only during the surveillance tests because during an emergency actuation the isolation valve was expected to open before the pump started. However, further investigation revealed that, if offsite power was maintained, the containment spray pump might start before the isolation valve opened. This could cause the valve to remain shut under accident conditions. Consequently, as an interim solution, the licensee has requested and received a technical specification amendment allowing the train A isolation valve to remain open during normal operation. In order to prevent the inadvertent spraying of the containment, the licensee is taking compensatory measures such as disabling the pump or closing and disabling the isolation valve before performing maintenance on the system. The licensee performed an evaluation and determined that the system was not degraded due to the overpressure condition. The licensee also performed system evaluations on containment spray train B, and concluded that, due to a shorter piping configuration and a better capability for venting this system, train B should be considered operable until a permanent solution is . IN 94-25 March 25, 1994 Page 3 of 3 implemented. This conclusion was supported by testing which showed that, although somewhat higher-than-expected differential pressures did occur in this train also, the containment isolation valve would consistently open. The licensee is exploring various options for a permanent solution. These include using a different isolation valve or delaying the pump starting time to ensure that the isolation valve will open before pressure build-up can occur. The event illustrates that valve stroke-time inservice testing alone does not ensure the capability of a valve to operate under all postulated design conditions. This event also indicates that the inertia of water (hydraulic ram effects) may not have been accounted for in the design of certain systems. This information notice requires no specific action or written response. If you have any questions regarding this matter, please contact one of the technical contacts listed below or the appropriate Office of Nuclear Reactor Regulation project manager. /s/'d by BKGrimes Brian K. Grimes, Director Division of Operating Reactor Support Office of Nuclear Reactor Regulation Technical contacts: Thomas F. Westerman, RIV Paula A. Goldberg, RIV (817) 860-8145 (817) 860-8168 Linda J. Smith, RIV Patricia L. Campbell, NRR (501) 968-3290 (301) 504-1311 Attachments: (see IN94025.WP1 for figure) 1. Figure: Containment Spray System (Typical of Two Trains)
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