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Information Notice No. 83-77: Air/Gas Entrainment Events Resulting in System Failures
SSINS No.: 6835 IN 83-77 UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF INSPECTION AND ENFORCEMENT WASHINGTON, D.C. 20555 November 14, 1983 Information Notice No. 83-77: AIR/GAS ENTRAINMENT EVENTS RESULTING IN SYSTEM FAILURES Addressees: All holders of a nuclear power reactor operating license (OL) or construction permit (CP). Purpose: This information notice is provided as notification of events that rendered redundant safety systems inoperable because air or gas entrainment caused pump cavitation. It is expected that recipients will review the information herein for applicability to their facilities. No specific action or response is required. Description of Circumstances: Calvert Cliffs On May 20, 1980, Calvert Cliffs Unit 1 sustained a loss of both service water system (SWS) redundant trains. The SWS became air bound after a service water heat exchanger (SWHX) was returned to operation, following routine maintenance. During the time that the SWHX was not in operation, the heat exchanger outlet valve was closed and air accumulated on the shell side of the heat exchanger. The source of air was a failed tube in the instrument air compressor (IAC) aftercooler. When the SWHX was brought back on line, the trapped air was swept into the SWS. Shortly thereafter, high temperature alarms for components cooled by service water (SW) were observed. Simultaneously, low pressure was also observed in both SW headers and low amperage was being drawn by the two SW pumps that were running. These indications signaled that SW pumps were cavitating. The reactor was manually tripped because of increasing main turbine and feed pump turbine bearing temperatures. It was subsequently found that the air was swept into the common inlet header for the SW pumps, causing pump cavitation and disabling both safety-related subsystems. Although the SWS is provided with a number of constant vent valves, their relieving capacity was exceeded by the sudden influx of the large quantity of air that had accumulated in the SWHX, while it was out of service. After 8311010015 . IN 83-77 November 14, 1983 Page 2 of 3 the IAC aftercooler discharge valve was shut, open vents began discharging solid streams of water and the two SW subsystems were both operating soon thereafter. McGuire On February 12, 1982, McGuire Unit 1 experienced a loss of high head safety injection emergency boration and reactor coolant makeup capability. Hydrogen from the positive displacement pump (PDP) suction dampener entered the common suction of the charging system, causing both centrifugal charging pumps and the PDP to be inoperable. The system was restored within 30 minutes. The unit was in Mode 1, at 50% of full power at the time. This event was described in more detail in Information Notice No. 82-19. San Onofre During preoperational testing on March 14, 1982 at the San Onofre Nuclear Generating Station, Unit 2, the shutdown cooling system was inoperable for 90 minutes when the low pressure safety injection (LPSI) pumps became nitrogen bound. The event resulted from an improper valve alignment during nitrogen backflushing of a purification filter in the chemical and volume control system. Backflushing consists of passing nitrogen gas at 350 psig through the isolated purification filter and discharging the gas and collected contaminants into the filter backflush storage tank. In this instance, as a result of either a system malfunction or operator error, the gaseous nitrogen passed through the purification line into the suction of the LPSI pumps which were being used for shutdown cooling. Flow from the operating LPSI pump fell from 4000 gpm to zero as the pump became gas bound and attempts to establish flow with this pump or the alternate LPSI pump were unsuccessful. The pumps and piping high points were vented and shutdown cooling flow was reestablished. St. Lucie On October 23, 1982, with St. Lucie Unit 1 in hot standby during recovery from a reactor trip, the three operating positive displacement charging pumps stopped injecting coolant to the reactor coolant system because the volume control tank (VCT) was pumped dry. The reactor had tripped on a low steam generator water level signal after a loss of feedwater flow to the steam generator. The VCT was empty although its two liquid level sensors indicated an acceptable liquid inventory and hence an apparently acceptable inflow/outflow balance from the VCT. The hydrogen cover-gas blanket of the VCT entered the suction of each pump. The false liquid level indication was caused by an empty reference leg that was shared by both liquid level sensors. The pumps were restored to operation by repeated venting after filling the VCT to a high level. . IN 83-77 November 14, 1983 Page 3 of 3 The aforementioned events are intended to be illustrative. Hence, licensees are cautioned that the types of system inoperability resulting from air or gas entrainment vary. Moreover, redundant safety-related trains or components can be affected as shown by the events cited above. The serious consequences that may result from such system or component impairment cannot be overemphasized. For example, although the actual consequences of the loss of shutdown cooling flow at San Onofre-2 were minimal because the event occurred prior to initial criticality, the event could have been more serious if the plant had been operating at full power for an extended period of time prior to the event. If you have any questions regarding this matter, please contact the Regional Administrator of the appropriate NRC Regional Office, or this office. Edward L. Jordan Director Division of Emergency Preparedness and Engineering Response Office of Inspection and Enforcement Technical Contacts: R. M. Young, IE 49-27275 E. V. Imbro, AEOD 49-24495 Attachment: List of Recently Issued IE Information Notices .
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