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Information Notice No. 93-84: Determination of Westinghouse Reactor Coolant Pump Seal Failure
UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF NUCLEAR REACTOR REGULATION WASHINGTON, D.C. 20555 October 20, 1993 NRC INFORMATION NOTICE 93-84: DETERMINATION OF WESTINGHOUSE REACTOR COOLANT PUMP SEAL FAILURE Addressees All holders of operating licenses or construction permits for pressurized water reactors (PWRs). Purpose The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice to alert addressees to a potential problem that could result from the use of Westinghouse-designed reactor coolant pump (RCP) shutdown procedures for pumps whose No. 1 seals have failed. 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. Background Westinghouse supplied the Braidwood Station RCPs, incorporating a three-stage seal series arrangement to limit coolant flow up the pump shaft. The No. 1 seal, the main seal of the pump, is a controlled-leakage, film riding face seal. The No. 2 and No. 3 seals are rubbing face seals. During normal operation, an injection flow of nominally 30.3 liters [8 gallons] per minute enters the pump below the No. 1 seal (see Attachment 1). Here the flow splits. A portion (approximately 18.9 liters [5 gallons] per minute) flows down through the thermal barrier heat exchanger and enters the reactor coolant system. In this manner, the primary coolant is prevented from entering the radial bearing and seal section of the pump unit. The remainder (11.4 liters [3 gallons] per minute) (controlled leakage) passes through the pump radial bearing and enters the No. 1 seal. Above the No. 1 seal, most of the flow leaves the pump through the No. 1 seal leakoff line and returns to the chemical and volume control system. Minor flow passes through the No. 2 seal and its leakoff line to the reactor coolant drain tank. A back flush injection of 0.8 liters [0.21 gallons] per hour from a head tank flows into the No. 3 seal between its "double dam" seal area. At this point, half of the flow passes through one side of the seal and out the No. 2 seal leakoff line while the remaining flow passes through the other side and out the No. 3 seal leakoff line. This arrangement ensures essentially zero leakage of reactor coolant from the pump. 9310140192. IN 93-84 October 20, 1993 Page 2 of 3 During normal operation, the No. 1 seal is subject to full reactor coolant system pressure. When the injection flow passes through the No. 1 seal, it produces a pressure drop of approximately 15.41 MPa [2235 psi]. The No. 2 seal has the capability to withstand full operating pressure, and its major function is to act as a backup in case the No. 1 seal fails. If the No. 1 seal fails, leakage through the No. 2 seal increases. The No. 2 seal high leak flow alarm actuates at 3.8 liters [1 gallon] per minute. Description of Circumstances In the middle of 1992, while developing shutdown procedures for the RCPs at the Braidwood Station, Commonwealth Edison Company (the licensee) found that its abnormal operating procedure contained inadequate instructions for responding to a failure of the RCP No. 1 seal. On a high RCP No. 1 seal leakoff condition, the abnormal operating procedure directed the operating personnel to check the condition of the No. 2 seal. If the No. 2 seal was acting as the primary pressure boundary on the basis of leakage of at least 3.8 liters [1 gallon] per minute from the No. 2 seal, the procedure required that the RCP be shut down. If the condition of the No. 2 seal was normal, the procedure required no further action and gave no further guidance. Discussions with Westinghouse indicated that the condition of the No. 2 seal might not be a sufficient indication of the operability of the No. 1 seal. Discussion The inadequacies in the operating procedure stem from the limitations of the instrumentation used to monitor the leakage. The relevant No. 1 seal instrumentation limits maximum readings to 22.7 liters [6 gallons] per minute. Although the No. 2 seal high leak flow alarm was set at 3.8 liters [1 gallon] per minute, very high No. 1 seal leak rates, more than 113.6 liters [30 gallons] per minute, are needed for the No. 2 seal to reach this flow rate. Thus, the No. 1 seal could have substantial leakage (e.g., 22.7-113.6 liters [6-30 gallons] per minute) even though the No. 2 seal high leak flow alarm was not actuated. In the mean time, the RCP seal system might be severely damaged if loss of injection were to occur or the No. 1 seal leak rates were more than 30.3 liters [8 gallons] per minute. Therefore, monitoring for high No. 2 seal leakage is not a reliable method of determining whether the No. 1 seal has failed. After finding that the condition of the No. 2 seal might not indicate the operability of the No. 1 seal, Westinghouse provided guidance for the Braidwood Station that significantly enhances operator ability to determine if immediate shutdown of an RCP is required or if a more orderly shutdown can be conducted. Westinghouse completed a safety evaluation and issued Technical Bulletin, NSD-TB-93-01-R0, "Revised Procedures for RCP Shutdown with No. 1 Seal Leakage Outside Operating Limits," dated March 30, 1993, to other PWRs that might be affected. An excerpt from this document is attached to this information notice (see Attachment 2). . IN 93-84 October 14, 1993 Page 3 of 3 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 Office of Nuclear Reactor Regulation (NRR) project manager. /s/'d by BKGrimes Brian K. Grimes, Director Division of Operating Reactor Support Office of Nuclear Reactor Regulation Technical contacts: Steve DuPont, RIII (815) 458-2852 Martin Farber, RIII (708) 790-5774 Chu-Yu Liang, NRR (301) 504-2878 Attachments: 1. Simplified Braidwood Seal Water Injection and Leakoff Flow Diagram 2. Excerpt from Westinghouse Technical Bulletin, NSD-TB-93-01-R0, "Revised Procedures for RCP Shutdown with No. 1 Seal Leakage Outside Operating Limits" 3. List of Recently Issued NRC Information Notices .
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