05000362/LER-2007-001

LER-2007-001, Failure to declare Emergency Diesel Generator Inoperable and enter TS Action

Docket Number
Event date:	12-22-2007
Report date:	2-22-2008
Reporting criterion:	10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications
3622007001R00 - NRC Website

Background

San Onofre Unit 3 has two Emergency Diesel Generators (EDGs) [EK] to provide emergency AC power if normal AC power is unavailable. Technical Specifications (TS) 3.8.1 requires two EDGs to be operable in Modes 1-4. If one EDG is inoperable, TS 3.8.1, Action B.1 requires Southern California Edison (SCE) to verify correct breaker alignment and power availability for each required offsite circuit within 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> and once per 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> thereafter. TS 3.8.1, Action B.3. requires SCE to investigate and determine if the remaining EDG is not inoperable due to common cause failure or perform Surveillance Requirement (SR) 3.8.1.2 (start test) on the Operable EDG.

A routine monthly test, SR 3.8.1.3, requires verification that when each EDG is synchronized and loaded, it will operate for greater than or equal to 60 minutes at a load greater than or equal to 4450 kW and less than or equal to 4700 kW. Note 2 of this SR states "Momentary transients outside the load range do not invalidate this test.

Description of Event

On December 22, 2007, at about 1319 PST, during a routine monthly surveillance test of Unit 3 EDG 3G002, Control Room Operators noted a perturbation on the EDG load meter of approximately 900 kW. The SONGS data logging computer (R-Time) indicated a 4.6 MW to 3.4 MW output drop from the EDG for about 10 seconds. An Operator at the EDG also reported an audible indication of load swing from the EDG. At the time of the EDG surveillance, the EDG was paralleled to the grid.

Initial investigation noted that the Grid interchange meter/recorder between the Southern California Edison and the San Diego Gas and Electric systems indicated a load disturbance (about 20-30 MW) at about the same time as the EDG output perturbation. Although the Grid and EDG data were not time­ synchronized, SCE's Immediate Operability Determination (IOD) declared the EDG Operable on the basis that the grid disturbance caused the EDG output fluctuation and the TS note allowing momentary transients outside the load range.

To verify Operability, on December 23, 2007, SCE repeated the EDG surveillance with no grid disturbances present. Five of the six transducer outputs to the R-time computer had no indication of load swing. The sixth transducer (EDG megawatt output) displayed unsteady output which, at that time, was interpreted by SCE's engineering personnel to be a noisy transducer. SCE's follow-up Prompt Operability Determination (POD) concluded that no degraded condition existed.

SCE continued to investigate and on December 31, 2007 (Discovery Date), SCE retested the EDG with additional instrumentation installed. During the retest, load swings prompted the Control Room Operators to trip the EDG. SCE later determined the EDG output fluctuations were caused by a speed probe assembly connector that had failed due to inadequate solder. The speed probe is part of the control feedback loop of the EDG and is critical for EDG Operability. SCE repaired the speed probe connector and returned the EDG to Operable status on January 1, 2008.

Retrospectively, the EDG was failed on December 22, 2007. Although not known at that time, SCE should have taken the Actions required by TS 3.8.1 for an inoperable EDG. Consequently, SCE is reporting this occurrence in accordance with 10 CFR 50.73(a)(2)(i)(B) for not declaring the EDG Inoperable prior to December 31, 2007.

Cause of the event

SCE did not declare EDG 3G002 Inoperable prior to December 31, 2007, due to failure to identify the speed probe failure. This resulted in the TS violation of SR 3.0.1 and, subsequently, the failure to comply with the actions required by TS 3.8.1, Action B. A Root Cause Evaluation (RCE) is underway to identify the causes of the delay in identifying the speed probe failure and initiate appropriate Corrective Actions.

Corrective Actions r As noted above, SCE repaired the speed probe for EDG 3G002 and returned it to operable status on January 1, 2008. SCE also inspected the speed probes for the opposite train EDG at Unit 3 (3G003) and both EDGs at Unit 2 and verified all other speed probes were unaffected and remained operable.

The RCE of this occurrence is ongoing. SCE may implement additional corrective actions when the RCE is completed.

Safety Significance

During this occurrence, San Onofre remained connected to the offsite power grid and offsite power was operable. Additionally, the opposite train EDG, 3G003, remained operable and capable of performing its required safety function. Therefore, the delay in declaring EDG 3G002 inoperable had no safety significance.

Additional Information:

In the past three years, there have been no other instances of missed TS required Actions caused by a delay in flagging the EDG inoperable.