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Information Notice No. 88-23, Supplement 2: 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 January 31, 1990 Information Notice No. 88-23, SUPPLEMENT 2: 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 potential problems resulting from the transport of hydrogen from the volume control tank (VCT) to the safety-related high-pressure injection (HPI) pumps during testing of the VCT outlet isolation valves. Both trains of high pressure injection could possibly be rendered inoperable during this operation. 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: On October 30, 1989, during inservice testing of VCT outlet isolation valves at the Trojan Nuclear Plant, plant personnel discovered that both high-pressure injection pumps could be rendered inoperable if a safety injection signal (SIS) occurred while the manual "bypass" valve around the motor-operated VCT outlet isolation valves is opened (as specified by procedure). The test procedure involved timed opening and closing of VCT outlet isolation valves MO-112B and MO-112C (see Figure 1). In order to maintain HPI pump suction from the VCT during testing, the test procedure specified locking open manual valve 8484, bypassing the isolation valves. An SIS causes MO-112D and MO-112E to open to align the HPI pump suction to the refueling water storage tank (RWST). An SIS also causes MO-112B and MO-112C to close after MO-112D and MO-112E are open to isolate the VCT from the HPI pumps' suction. If a safety injection signal is received while valve 8484 is open, the VCT would not be automatically isolated from the suction of the HPI pumps. If VCT hydrogen cover gas pressure were sufficiently high, suction of the pumps could be preferentially provided from the VCT, even though the pump suctions were aligned to the RWST. Hydrogen gas could then be entrained 9001250020 . IN 88-23, Supplement 2 January 31, 1990 Page 2 of 2 into the HPI pumps, rendering them inoperable and possibly causing pump damage. The inservice test of MO-112B and MO-112C is performed quarterly. The total time manual bypass valve 8484 was open was 15 to 30 minutes each quarter. Discussion: The licensee evaluated a number of flow and pressure conditions that might occur following an SIS to determine when the VCT would be drained and the cover gas ingested into the HPI pumps' suction when the VCT isolation valves are bypassed. Conditions during a large-break loss-of-coolant accident (LOCA), a small-break LOCA, and a main steamline break were examined. The licensee determined that following a small-break LOCA, with low initial VCT pressure, the HPI pumps could continue to operate for up to 2 hours before gas entrainment would occur. However, for a large-break LOCA with an initial VCT pressure of 15 to 40 psig, the HPI pumps might be made inoperable by gas binding after as little as a minute of operation. Similar results were determined for a main steamline break. The likelihood of HPI pump failure from VCT cover gas injection following an SIS would be exacerbated by inadvertent failure to close the 8484 valve after testing is completed. If the manual bypass valve were inadvertently left in the open position, both HPI trains could be unknowingly rendered inoperable for an extended period. Test procedures for other valves are being reviewed by the licensee to determine if similar problems exist. At another utility, the VCT isolation valves are tested with the bypass line closed and the isolation valves in the RWST line to the HPI pump suction open to provide suction flow to the HPI pumps when the VCT isolation valves are cycled. 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: L. Mark Padovan, AEOD (301) 492-4445 Walton Jensen, NRR (301) 492-1190 Attachments: 1. Figure 1, Charging System Overview 2. List of Recently Issued NRC Information Notices .
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