05000286/LER-2011-002
| Docket Number | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(v), Loss of Safety Function |
| 2862011002R00 - NRC Website | |
Note: The Energy Industry Identification System Codes are identified within the brackets {}.
DESCRIPTION OF EVENT
On October 13, 2010, at approximately 12:58 hours, while at 100% steady state reactor power, during performance of 3-PT-W013 (Station Battery Weekly Inspection), the as- found float voltage was found to be low (127.6 VDC) but within the operability span (125.7 - 139.8 VDC). Float voltage adjustment could not be performed due to a faulty float potentiometer on the 31 Battery Charger (BC) {BYC}. This condition was recorded in the Indian Point Energy Center (IPEC) Corrective Action Program (CAP) as Condition Report CR-IP3-2010-03092. At approximately 16:18 hours, the Control Room (CR) {NA} received a 31 Static Inverter (SI) {INVT} auto transfer alarm (ALM). A Nuclear Plant Operator (NPO) was dispatched to investigate the condition and reported to the CR that the 31 SI reverse transfer local alarms "Inverter Undervoltage" and "Load Supplied by Alternate Source" were lit. The 31 SI had auto transferred back to the normal power supply (Forward Transfer). The alarms were reset which cleared the local alarms and the CR alarm. This condition was recorded in the CAP as CR-IP3-2010-03098.
On October 13, 2010, at approximately 21:35 hours, the 35 BC was placed in service and the 31 BC shutdown. A troubleshooting plan was prepared for the 31 battery charger (BC) based on the applicable vendor manual section. Troubleshooting the 31 SI was not considered necessary as the 31 SI alarms reset and there were no indications of a blown fuse. The 31 SI was verified to be in forward transfer and carrying instrument bus loads. It was suspected that the low input voltage from the 31 BC may have caused the 31 SI swap. Troubleshooting of the 31 BC determined the 31 BC was current limited (approximately 105 amps) and as this current value was approached the voltage would drop. On October 14, 2010, Instrumentation and Control (I&C) personnel verified the 31 BC float potentiometer was faulty and the potentiometer was replaced along with the Float/Equalize switch. The 31 BC was re-energized and proper operation of the new potentiometer and switch was verified while unloaded. On October 15, 2010, at approximately 8:40 hours, the 31 BC was started and the load transferred off the 35 BC.
A post maintenance test (PMT) was performed to demonstrate the effectiveness of the repairs. The PMT was unsatisfactory because the voltage could not be set within the required range. At 9:25 hours, on October 15, 2010, the 35 BC was placed back in service. Further troubleshooting was performed and on October 19, 2010, a full load test was conducted. During initiation of the load test, performance personnel identified that the 31 BC output exhibited voltage oscillations. The BC voltage under load is normally steady but was fluctuating. Engineering concluded the BC output was abnormal and the BC control module (A1) was replaced. Subsequent testing showed that the control module replacement did not resolve the fluctuating output voltage. I&C personnel reinstalled the original control module and replaced the gate drive board (A14) and the 31 BC was successfully retested. On October 21, 2010, at approximately 10:22 hours, the 31 BC was returned to service and 3-PT-W013 satisfactorily performed.
The 31 BC is part of the 125 VDC electric distribution system (EDS) {EJ} that provides DC power to equipment and components during normal operation and equipment that requires power in the event of a loss of all AC power. The 125 VDC EDS also supplies power to the 118 VAC instrument buses via the SIs. The 125 VDC EDS is composed of Station Batteries (31, 32, 33, 34, 36), Power Panels (31, 32, 33, 34, 36), Distribution Panels (31, 32, 33, 34, 31A, 32A), and Battery Charges (31, 32, 33, 34, 35, 36). The BCs are used to keep the batteries in a fully charged condition, perform periodic equalizing charges and to supply the DC power panels under normal operating conditions.
The 35 BC is an installed spare and can supply BC loads for anyone of the 31-34 BCs.
Cause of Event
The direct cause of the SI transferring to the alternate power source was due to an unstable DC input voltage from the 31 BC due to degradation of the Gate Driver Board (A14). The Gate Driver Board (A14) intercepts the silicon controlled rectifier (SCR) gate lead wiring from the Control Module (Al). If the Gate Driver Board (A14) corrupts the signal to the SCR, then the SCR will misfire and cause the voltage to oscillate.
The apparent cause could not be determined but the most likely cause was random failure. The degraded Gate Driver Board (A14) was not considered age related as the A14 board for the 31 BC was replaced in 2005.
Corrective Actions
The following corrective actions have been performed under Entergy's Corrective Action Program to address the cause:
- Troubleshooting to identify the cause of the low 31 BC output voltage and oscillations, replacement of the 31 BC Gate Driver Board (A14).
- Satisfactory testing of the 31 BC and return to service.
- The BC vendor was contacted to determine what may have caused the A14 board to malfunction and to provide recommendations. The vendor responded that there was no history of failures of this component.
Event Analysis
The event is reportable under 10CFR50.73(a)(2)(i)(B). The licensee shall report any operation or condition which was prohibited by the plants TS. On October 13, 2010, at approximately 12:58 hours, during performance of the weekly station battery inspection, the 31 BC as-found float voltage was found to be low and adjustments could not be performed to increase the voltage. Subsequently at 16:18 hours on October 13, 2010, the 31 SI swapped to reverse transfer and then back to forward transfer. It was concluded the 31 SI was operating normally and that low input voltage from the 31 BC may have caused the SI swap.
On October 19, 2010, during load testing, the 31 BC voltage output was identified as oscillating and the Gate Driver Board (A14) was replaced. Testing was performed and the 31 BC was returned to service on October 21, 2010. At the time of discovery (October 13, 2010), the as-found voltage of 127.6 volts was outside its required float voltage range (129.8-132.8) but within the TS Surveillance Requirement (SR) 3.8.4.1 specification of equal to or greater than 125.7 volts. Operations concluded that the 31 BC and 31 battery were operable. Based on the operability determination the event was not considered reportable. Subsequent questions on the operability of the BCs were recorded in CR-IP3-2011-00098 on January 10, 2011. Engineering assessed the condition for past operability and determined that troubleshooting at the time of the event found the 31 BC was current limited (approximately 105 amps) and as this voltage was approached the voltage would drop. An engineering calculation provides the minimum current for the 31 BC to be operable as 280 amps based on re-charging the 31 BC within 15 hours1.736111e-4 days <br />0.00417 hours <br />2.480159e-5 weeks <br />5.7075e-6 months <br /> while carrying a continuous DC load. Because the 31 BC was found to be current limited at a current value below its required operability criteria, engineering concluded the 31 BC was inoperable while that condition existed. Engineering concluded on February 23, 2011, that the 31 BC was inoperable on October 13, 2010 from 12:58 hours to 21:25 hours when it was removed from service for an inoperable period of 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> and 27 minutes. Subsequent periods when the 31 BC was in-service was for troubleshooting and testing but were performed within the TS allowed completion time.
TS 3.8.4 has an allowed outage time of 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> for one DC electric power subsystem (31 or 32 or 33) inoperable. This event meets the reporting criteria because the inoperable condition during past operation exceeded the 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> allowed completion time for TS 3.8.4 and the required actions were not performed.
There was no safety system functional failure reportable under 10CFR50.73(a)(2)(v) as the minimum required safeguards components were available to perform the function during the time the 31 BC was inoperable. In accordance with reporting guidance in NUREG-1022, an additional random single failure need not be assumed in that system during the condition
Past Similar Events
A review was performed of the past three years of Licensee Event Reports (LERs) for events that involved a TS violation for an inoperable BC or SI. No LERs were identified.
Safety Significance
This event had no significant effect on the health and safety of the public. There were no actual safety consequences for the event because there were no accidents or transients during the period of inoperability. With one BC inoperable, the remaining BCs and EDS were operable and available to perform their safety function.
Additionally, the BCs have an installed spare in BC-35 which can supply BC loads for one of the 31-34 BCs. Also, failure to supply power to the vital instrument bus would cause the associated SI automatic static transfer switch to transfer to its alternate source of power. A loss of offsite power could result is stripping the SI power supply but they would be re-energized in accordance with plant procedures.
A risk assessment was performed of the condition assuming the 31 BC was unavailable for 8.5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br />. The baseline Core Damage Frequency (CDF) is 1.466E-5 per year. With BC unavailable for 8.5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br />, the CDF increased to 2.365E-5 per year which results in an incremental CDF (ICDF) of 9.0E-6 per year. Considering 8.5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br /> BC unavailability results in an incremental core damage probability (ICDP) of 8.7E-9. This impact is not considered significant.