05000348/LER-2001-003
| Docket Number | |
| Event date: | |
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| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 3482001003R00 - NRC Website | |
Westinghouse -- Pressurized Water Reactor Energy Industry Identification Codes are identified in the text as [XX]
Description of Event
On October 29, 2001 with the unit in cold shutdown (mode 5), the routine B train loss of offsite power (LOSP) test was performed. This test, done on an 18 month frequency, checks the proper function of the emergency diesel generators (EDG)[EK] and associated circuitry for a simulated loss of offsite power event. During this test the EDG sequencer that loads the needed B train equipment onto the emergency power buses is checked. The LOSP sequencer associated with the 1B E.,IDG failed to operate as required. The 4160-volt emergency bus was re- energized per design, but all required loads were not automatically restarted. At the time of the surveillance test unit 1 was in cold shutdown (mode 5) for a refueling outage that began October 06, 2001 and the 1B EDG was not required to be operable. Plant conditions at the time of the test were such that no adverse operational transient resulted from this failure.
Cause of Event
Investigation determined that a mechanically operated contacts (MOC) switch, containing 16 contact pairs, mounted in the 1B EDG output breaker cubicle failed to operate properly. A pair of contacts ("b" contacts) in the switch that allows operation of the LOSP sequencer failed to achieve the required electrical continuity when the 1B EDG output breaker was in the open position. This failure did not prevent closure of the EDG output breaker to re-energize the emergency bus, but prevented the B train LOSP sequencer from operating to start the required equipment. This is a condition prohibited by Technical Specifications 3.3.1 "AC Sources-Operating" and could have prevented the completion of the safety function of the EDG. Investigation determined that the MOC switch had likely been installed with a bent contact on August 2, 2000. This failure was not detected during post maintenance testing (PMT) conducted after the switch was replaced.
This event was caused by inadequate PMT in that specific post maintenance guidance was not adequately specified in the applicable breaker maintenance procedure or work sequence. As a result the MOC switch was installed without being fully tested. The contact pairs were not tested for continuity and plant conditions at the time of the MOC switch replacement were not compatible with performance of the LOSP test, therefore, an existing defect went undetected.
Safety Assessment A LOSP event did not occur during the time when the B train LOSP sequencer was inoperable. The safety injection (SI) sequencer was unaffected by this event and therefore, had a SI occurred, the required B train equipment would have started automatically. This failure only impacted equipment operation for a LOSP without a concurrent SI. The manual equipment loading functions were unaffected by this condition. In addition, plant procedures provide for manually starting any required equipment that does not start automatically. If an LOSP occurs, plant operators procedurally check for proper equipment operation and if needed manually start required equipment. Normally the station blackout diesel was also available to back up the 1B EDG and, if started, would have automatically sequenced on LOSP loads. Therefore the health and safety of the public were unaffected by this event.
During the time period from August 2, 2000 to October 29, 2001, while the B Train LOSP sequencer for 1B DG was inoperable, some A train equipment needed for an LOSP response was removed from service for routine maintenance. During these periods some B train equipment would have required manual actuation by operators had an LOSP occurred.
This event is a Safety System Functional Failure of the auto sequencing function for an LOSP without SI. The SI sequencer function was unaffected.
Corrective Action The procedure has been revised to include an as-left continuity check of all pairs of contacts on any reinstalled or replaced MOC switches. The procedure has also been revised to include electrical verification of proper MOC switch mechanical alignment following MOC switch maintenance.
All other completed MOC switch maintenance work packages conducted since the last integrated safeguards test on both Units have been reviewed to determine if additional as-left checks on MOC switches are required. Two MOC switches were identified for additional checks and have been tested satisfactorily.
Additional Information
The following LERs have been submitted in the past 2 years on inadequate procedure: