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Information Notice No. 84-66: Undetected Unavailability of the Turbine-Driven Auxiliary Feedwater Train
SSINS No.: 6835 IN 84-66 UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF INSPECTION AND ENFORCEMENT WASHINGTON, D.C. 20555 August 17, 1984 Information Notice No. 84-66: UNDETECTED UNAVAILABILITY OF THE TURBINE- DRIVEN AUXILIARY FEEDWATER TRAIN Addressees: All pressurized water power reactor facilities holding an operating license (OL) or construction permit (CP). Purpose: This information notice is provided to alert recipients of a potentially significant problem pertaining to undetected unavailability of the turbine- driven auxiliary feedwater (AFW) train. It is expected that recipients will review the information for applicability to their facilities and consider actions, if appropriate, to preclude a similar problem occurring at their facilities. However, suggestions contained in this information notice do not constitute NRC requirements and, therefore, no specific action or written response is required. Description: Sequoyah 2 On January 12, 1982, the turbine-driven AFW pump at Sequoyah 2 failed to start on a safety injection actuation. Investigation revealed the electronic overspeed trip latch function for the stop valve had not been relatched following an earlier overspeed trip. The valve is normally relatched from the control room. The operator had apparently not held the valve hand switch for the necessary 10 seconds, and there was a design deficiency in that no indication was provided in the control room to determine that the valve had been reset. As corrective actions, the licensee has modified procedures to require local visual verification of latching, improved valve checklists, and posted precautionary labels or signs both in the control room and locally at the stop valve. Salem 1 On August 11, 1983, the turbine-driven AFW pump failed to start following a low-low steam generator level signal because the pump turbine trip valve was in the tripped position. Although it could not be substantiated, the valve had apparently been tripped and left in that position following maintenance and testing activities on August 3, 1983. It appeared that when the pump was removed from service, the control room "trip" pushbutton was inadvertently depressed instead of the "stop" pushbutton, resulting in activation of the trip valve. In addition, the valve trip position limit switch was out of adjustment causing the "trip" indication in the control room to be inoperable. A design 8408100055 . IN 84-66 August 17, 1984 Page 2 of 3 change request was initiated to provide a positive control room indication of a trip valve latched condition. Until this change is complete, the trip valve will be verified to be in the latched position by daily observation. Salem 2 At approximately 1800 hours, October 5, 1983, during routine full power operation, the control room operator noticed that the trip indication light for the turbine-driven AFW pump was illuminated. Subsequent investigation revealed that the valve was in the tripped position and had apparently been left in that position following completion of routine pump surveillance testing at 1700 hours, October 5, 1983. When the problem was identified, the AFW pump trip valve was latched. The redundant electrically driven pumps were operable throughout the occurrence and the turbine-driven pump was restored to operability within the time period specified in the action requirements. Investigation of the incident revealed that the operator who relatched the pump trip valve at the time of the surveillance had turned the valve handwheel to set the valve linkage but had not completed the latching operation by turning the handwheel back to the starting position. This resulted in the valve remaining in the tripped position. No local mechanical position indication existed on the valve, thus detailed knowledge of the valve operating linkage was required to ensure that the valve was in the proper position. As a corrective action, the licensee installed local valve position indication, and the licensee is planning to install control room indication of the "latched" condition. San Onofre 3 On October 31, 1983, at 1925 hours, Unit 3 was manually tripped from 62% reactor power in response to a loss of main feedwater. An emergency feedwater actuation signal (EFAS) was received when the unit was tripped. However, the turbine-driven AFW pump failed to start. Both electrically driven AFW pumps started and remained operable during this event. The operator investigated the failure of the turbine-driven pump, found the pump turbine in a tripped condition, and manually reset the pump turbine's steam admission valve. The cause of the pump trip was unknown. The pump had previously been satisfactorily tested on October 30, 1983, at 1820 hours. Although control room instrumentation was available to signal when the pump turbine tripped on turbine overspeed, troubleshooting of the instrumentation subsequent to the plant trip on October 31, 1983, indicated that there were intermittent failures of the instrumentation to signal when the pump turbine was tripped. These intermittent failures were investigated and corrected. The San Onofre AFW pump turbine steam admission valve is normally closed and opens when a EFAS signal is received. When the AFW pump turbine trips on overspeed, the condition is alarmed in the control room. However, it is possible to relatch the trip mechanism in such a way that the overspeed trip is reset but the valve is not and the condition is not indicated in the control room. . IN 84-66 August 17, 1984 Page 3 of 3 Ginna On December 28, 1983, with the reactor at 100% power, the plant entered a 7-day Limiting Condition for Operation (LCO) Action Statement when it was discovered that the turbine driven AFW pump was inoperable. The cause was that the throttle on the pump had been tripped. The licensee felt that a contract person, working in the area of the pump, bumped the lever into the tripped position earlier in the day. The limit switch did not make contact; as a result, there was no indication in the control room of the pump being tripped. The valve was reset. The licensee investigation found that the valve operated sluggishly because of insulation debris in the external valve linkages. The valve limit switch had not been activated because of the limited valve stroke. Discussion: In five events at operating reactors (1982-1983), the turbine-driven AFW pumps were unavailable because the steam supply was isolated (trip and throttle valve was not latched). The condition was noted either during routine inspections or as the result of an investigation of failure of the turbine-driven AFW pump to respond to an engineered safety features actuation signal. Three of these five events were failures to return a safety system to an operable condition, including failure to verify, and maintenance was a significant contributing factor in four of the events. On the basis of AEOD evaluations, each of these events were found to have limited safety significance because (1) the motor-driven AFW pumps were operable and available in all cases, and (2) the LCO time period was generally not exceeded. However, because events involving undetected unavailability of the turbine-driven AFW train could be significant at other plants or under other circumstances, the NRC recommends that recipients of this notice review these events and the various preventive actions summarized in Table 1 for applicability to their plants. No written response to this information notice is required. If you need additional information about 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 Contact: Ray Smith, IE (301) 492-7190 Attachments: 1. Table 1: Summary of Various Actions Taken by Facilities to Prevent Undetected Unavailability of the Turbine Driven AFW Train 2. List of Recently Issued IE Information Notices . TABLE 1 Summary of Various Actions Taken By Facilities to Prevent Undetected Unavailability of the Turbine-driven AFW Pump 1. Design change to provide a positive control room indication of a trip valve "latched" condition. 2. Regular adjustment and testing of the limit switches to ensure operability (where limit switches are used to provide the control room with the status of trip valves). 3. Local verification of position after resetting the trip valve. 4. Visual verification daily or once per shift to see that the valve is not tripped. 5. Local mechanical valve position indication installed and permanent tags attached to the valve providing instructions for operation. 6. On-shift training in operation of the trip valve for all personnel who are required to operate the valve. 7. Improved housekeeping to prevent fouling external valve linkages. 8. Warning sign installed near the trip lever.
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