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Supplement 3: Potential for Gas Binding of High-Pressure Safety Injection Pumps During a Loss-Of-Coolant Accident
UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF NUCLEAR REACTOR REGULATION WASHINGTON, D.C. 20555 December 10, 1990 Information Notice No. 88-23, SUPPLEMENT 3: POTENTIAL FOR GAS BINDING OF HIGH-PRESSURE SAFETY INJECTION PUMPS DURING A LOSS-OF-COOLANT ACCIDENT Addressees: All holders of operating licenses or construction permits for pressurized-water reactors (PWRs). Purpose: This information notice supplement is intended to alert addressees to the potential for common-mode failure caused by hydrogen gas binding of the high-head safety injection pumps (charging pumps) during a loss-of-coolant accident (LOCA). 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 supplement do not constitute NRC requirements; therefore, no specific action or written response is required. Description of Circumstances: On August 22, 1990, Unit 2 of the Sequoyah Nuclear Power Plant was at 70-percent power (in coastdown). The licensee was attempting to switch operation of the charging pumps from the "A" to "B" pump in order to perform surveillance (see Attachment 1). Upon start of the "B" charging pump, the licensee observed fluctuation of the pump's motor amperage and rate of flow. The licensee suspected that gas was accumulating on the suction-side of the "B" pump and secured the pump. Further investigation and analysis by the licensee revealed that hydrogen gas was accumulating in the suction piping of the "B" pump and in the RHR crossover piping to the charging header. The licensee was able to vent approximately 5.3 cubic feet of gas. An additional 4.75 cubic feet of gas could not be vented from the RHR crossover piping. On September 6, 1990, with Unit 1 at 100-percent power, the licensee identified the presence of a hydrogen gas bubble on the suction-side of the charging pumps in Unit 1. The gas was collecting in the piping between the "A" residual heat removal (RHR) pump and the charging pumps. The licensee calculated that hydrogen was accumulating at a rate of 0.5 cubic feet per hour. The gas came out of solution (in part) due to localized reductions in pressure because of piping elevation differences and eccentric pipe reducers (see Attachment 1). Immediate corrective action taken by the licensee for both units included venting the suction piping of the idle charging train every 8 hours. 9012040239 . IN 88-23, Supplement 3 December 10, 1990 Page 2 of 3 Discussion: These events at Sequoyah are significant because hydrogen gas accumulation in the suction piping to the charging pumps has the potential to affect multiple trains of pumps in the emergency core cooling system (ECCS). Loss of all high-pressure recirculation capability at Sequoyah during a small-break LOCA is the dominant risk contributor to the core damage frequency as identified in Section 5, Sequoyah Plant Results, NUREG-1150, Volume 1, "Severe Accident Risks: An Assessment For Five U.S. Nuclear Power Plants." During a LOCA, suction of the ECCS pumps must be switched from the refueling water storage tank (RWST) to the containment sump before the RWST is depleted. If the reactor coolant system (RCS) has not yet depressurized to the point that the low-pressure injection pumps (i.e., RHR pumps) can inject into the vessel, then the discharge of the RHR pumps must be directed to the suction of the centrifugal charging pumps (CCPs) and the safety injection (SI) pumps. Successful recirculation of water from the containment sump (with the RCS at high pressure) requires operation of one RHR pump and one of the high head pumps. At Sequoyah, the "A" RHR pump supplies the suction of both CCPs and the "A" SI pump. The "B" RHR pump supplies the suction to the "B" SI pump. Noncondensible gases accumulating in the piping between the "A" RHR pump and the charging pump suction header creates the potential for gas binding of both charging pumps during the switchover from high-pressure injection to high-pressure recirculation. In addition, because the valves isolating the "A" RHR and "A" SI pumps from the charging pump suction header are periodically stroke-time tested, gas may also enter sections of piping normally isolated from this header. Thus, the gas accumulation in the charging pump suction header potentially affects three of the four high-pressure pumps. In recent NRC information notices, the staff addressed gas binding of ECCS pumps. Information Notice (IN) 88-23, "Potential For Gas Binding of High- Pressure Safety Injection Pumps During A Loss-Of-Coolant-Accident (LOCA)," addressed gas-binding problems in the high-pressure safety injection system at the Farley Nuclear Power Plant. The staff issued two supplements to that information notice to address gas accumulation affecting ECCS pumps because of various root causes. IN 90-64, "Potential For Common-mode Failure Of High Pressure Safety Injection Pumps Or Release Of Reactor Coolant Outside Containment During A Loss-Of-Coolant Accident," discusses another mechanism that could lead to gas binding of both CCPs. The two gas-binding events at Sequoyah had root causes that were attributed by the licensee, in part, to inadequate review of IN 88-23. Although most gas accumulation in ECCS systems has been hydrogen, in at least one instance, a mixture of air and hydrogen was found. It is important to consider all potential sources of gas intrusion to the ECCS suction piping, such as leaking bladders on the pulsation dampeners for positive displace- ment charging pumps, ineffective check valves in highpoint venting systems that lead back to the air space in the volume control tank (VCT), any flow restrictions (e.g., orifices) . IN 88-23, Supplement 3 December 10, 1990 Page 3 of 3 which may cause gases to come out of solution, and improper venting and filling operations following maintenance of ECCS flowpaths. Since most plants have no technical specification surveillance requirement for periodic venting of ECCS suction piping (only pump casings and discharge piping), gas may accumulate and remain undetected for extended periods of time, subjecting the plant to a possible common mode failure of the ECCS pumps. 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 NRR project manager. Charles E. Rossi, Director Division of Operational Events Assessment Office of Nuclear Reactor Regulation Technical Contact: John Thompson, NRR (301) 492-1171 Attachments: 1. Charging Pumps and RHR Crossover for SQN Units 1 and 2 2. List of Recently Issued NRC Information Notices .
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