05000423/LER-2002-002
| Event date: | 12-26-2001 |
|---|---|
| Report date: | 04-11-2002 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 4232002002R00 - NRC Website | |
1. Event Description On March 14, 2002, Dominion Nuclear Connecticut, Inc. (DNC) determined that circumstances involving an event which occurred on December 26, 2001, were reportable as operation in a condition prohibited by Technical Specifications (TS). On December 26, 2001, with the unit in Mode 1 at 100 percent power, the "B" Emergency Diesel Generator (EDG) [DG][EK] was inoperable due to the EDG output breaker [BKR] closing springs failure to charge. This condition went undetected until approximately 2046 on December 27, 2001, at which time the on duty plant equipment operator (PEO) noted the condition of the breaker. TS 3.8.1.1 requires a verification of offsite sources within 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> and at eight hour intervals, and completion of a common cause investigation within 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />. The condition existed for approximately 29 hours3.356481e-4 days <br />0.00806 hours <br />4.794974e-5 weeks <br />1.10345e-5 months <br /> before identification, during which time the requirements of TS 3.8.1.1 were not met. This is considered operation prohibited by the plant's TS, and is being reported in accordance with 10 CFR 50.73(a)(2)(i)(B).
On December 27, at 2121, the "A" EDG breaker indications were verified to be normal, and Operations concluded that the "A" EDG was operable, and unaffected by this condition. The "B" EDG output breaker was racked down and up. The closing springs were recharged. The "B" EDG was then declared available but not operable. At 2137, verification of offsite power sources as required by TS 3.8.1.1, were complete. After a successful installation of a spare output breaker to the "B" EDG, on December 28, 2001, at 0457 hours0.00529 days <br />0.127 hours <br />7.556217e-4 weeks <br />1.738885e-4 months <br />, the "B" EDG was returned to an operable status.
2. Cause The breaker's closing springs did not recharge due to its closing latch not returning to the latch position. This rendered the "B" EDG inoperable. The faulty breaker was returned to General Electric (GE) for testing. Verbal results from GE indicate that they were unable to duplicate the failure. The apparent cause for the closing latch not resetting could not be determined.
The delay in detecting that the "B" EDG was inoperable has been attributed to both work practice and the omission of relevant information in procedures. Specifically, a) the surveillance procedure used for monthly load testing of the EDGs did not verify breaker charging springs status following completion of the test. Routine operations and surveillance testing has not required verification that closing springs are charged after vital breaker operations, and b) the PEO rounds performed on shift following the breaker being closed on December 26, 2001, failed to initially recognize that the vital breaker white indicating light for the 'B" EDG breaker cabinet was not lit.
PEO round sheets also do not specifically address verification of the vital breaker white indicating lights.
3. Assessment of Safety Consequences The EDGs will mitigate the consequences of an accident by providing a source of power in the event of a loss of offsite power. This source of power supplies the equipment necessary to mitigate design basis accidents. The "B" EDG was inoperable for approximately a 37 hour4.282407e-4 days <br />0.0103 hours <br />6.117725e-5 weeks <br />1.40785e-5 months <br /> period, and its unavailability was not recognized for about 29 hours3.356481e-4 days <br />0.00806 hours <br />4.794974e-5 weeks <br />1.10345e-5 months <br /> into this period. However, this condition had low safety significance, and there was no loss of safety function for the AC electrical power sources. This EDG was only one of a possible four power supplies to the vital 4160 volt buses. As discussed in a recent NRC inspection report (NCV 50-423/01-14-01), this condition has been evaluated using the NRC Significance Determination Process and concluded to be of very low significance (Green) To prevent recurrence of this condition from causes associated with work practice and omission of relevant information in procedures, the Unit 3 PEO round sheet was updated to include the checking of vital bus breaker indicating lights. A briefing to communicate the work practices surrounding this condition will be completed with the PEOs. Additionally, where determined necessary by review, a restoration step in surveillance procedures that operate the EDG breakers will be included to verify that the EDG breaker springs are charged.
An investigation has been completed and the corrective actions described above will be addressed in accordance with the Millstone Corrective Action Program.
5. Previous Occurrences No previous similar events/conditions were identified.
Addtional Information Circuit breaker 15G-15U-2 was found racked up with the closing springs not charged.
Energy Industry Identification System (EIIS) codes are identified in the text as [XX].
because the TS provided for an allowed outage time of 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />, because the AC electrical offsite sources and redundant onsite sources remained operable during the period of "B" EDG unavailability, and because no plant work was done to compromise the unit's risk configuration during the time that the EDG was inoperable.
4. Corrective Action Immediate corrective action was completed on December 28, 2001, to return the "B" EDG to an operable status by installation of a spare 4160 volt breaker. The original "B" EDG breaker that had failed was removed and sent to GE for evaluation. The cause for the breaker's closing latch failure to reset could not be determined.