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Information Notice No. 89-71: Diversion of the Residual Heat Removal Pump Seal Cooling Water Flow During Recirculation Operation Following a Loss-Of-Coolant Accident
UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF NUCLEAR REACTOR REGULATION WASHINGTON, D.C. 20555 October 19, 1989 Information Notice No. 89-71: DIVERSION OF THE RESIDUAL HEAT REMOVAL PUMP SEAL COOLING WATER FLOW DURING RECIRCULATION OPERATION FOLLOWING A LOSS-OF-COOLANT ACCIDENT Addressees: All holders of operating licenses or construction permits for nuclear power reactors. Purpose: This information notice is intended to alert addressees to a potential problem resulting from a design deficiency that may affect the operability of the residual heat removal (RHR) pumps during recirculation operation following a loss-of-coolant accident (LOCA). The problem, which can occur as a result of a single failure can cause a diversion of the cooling water flow from the RHR pump seal coolers, resulting in inadequate seal cooling and the potential for ultimate failure of the RHR pumps. 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 do not constitute NRC requirements; therefore, no specific action or written response is required. Description of Circumstances: During a probabilistic risk assessment (PRA) study of the emergency core cooling system at Haddam Neck, the licensee discovered that a single failure in response to a LOCA could disable both RHR pumps. The single failure would cause the diversion of the cooling water flow from the RHR pump seal coolers resulting in inadequate seal cooling and possible pump failure. As indicated in Attachment 1, cooling water for the RHR pump seal coolers comes from the cooling water inlet lines to each RHR heat exchanger. During normal operation, the component cooling water system provides cooling to the RHR heat exchangers and pump seal coolers. However, during a LOCA condition, component cooling water is isolated, and service water is used to provide cooling to the RHR components. Because the RHR pump seal cooler water supply line is cross-connected to both the component cooling and service water systems, a failure of one of the service water motor-operated valves to open following a LOCA would result in only one branch of service water being available to provide cooling to both RHR heat exchangers and the seal water coolers. 8910130014 . IN 89-71 October 19, 1989 Page 2 of 2 Under these conditions, the RHR seal water coolers would receive inadequate flow, with the potential for ultimately causing the failure of both RHR pumps. The licensee corrected this problem by installing check valves in the seal water cooling piping (shown in Attachment 2) to preclude one branch of service water from feeding two RHR heat exchangers. Discussion of Safety Significance: The design deficiency described above was identified in a plant that switches cooling water systems during a LOCA. However, the problem could occur in any auxiliary cooling water system that provides sizably different cooling needs for different redundant components such as RHR heat exchangers and seal coolers. The important feature in the system is the interconnecting piping between the auxiliary cooling water system branches and the piping to the individual components. In the situation described in this notice, a single failure of a valve to open would cause one branch of auxiliary cooling water to service multiple redundant components, thereby reducing flow below design requirements. Licensees may wish to review their auxiliary cooling water system designs for similar deficiencies. Several other flow design problems that affect operation while in the recirculation mode following a LOCA have been previously identified in IN 87-63, "Inadequate Net Positive Suction Head in Low Pressure Safety Systems," and IN 88-74, "Potentially Inadequate Performance of ECCS in PWRs During Recirculation Operation Following a LOCA." It is important to note that a flow balance analysis would determine the impact of asymmetric flow balances caused by single failures in these systems. A simple review of piping diagrams may be sufficient to identify systems with potential flow problems, but hydraulic analyses are necessary to confirm suspected problems. This information notice requires no specific action or written response. If you have any questions about the information in this notice, please contact one of the technical contacts listed below or the appropriate NRR project manager. Charles E. Rossi, Director Division of Operational Events Assessment Office of Nuclear Reactor Regulation Technical Contacts: S. Israel, AEOD (301) 492-4437 D. Prochnow, NRR (301) 492-1166 Attachments: 1. Diagram of Haddam Neck's Original RHR Cooling Water System 2. Diagram of Haddam Neck's Modified RHR Cooling Water System 3. List of Recently Issued NRC Information Notices .
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