05000266/LER-2012-003
| Point Beach Nuclear Plant | |
| Event date: | 06-06-2011 |
|---|---|
| Report date: | 08-28-2012 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident |
| 2662012003R00 - NRC Website | |
Description of the Event
On June 6, 2011, while Unit 1 was in MODE 1 at 100% power and Unit 2 was in MODE 4, the feeder breaker [EB] to safeguards 480V bus, 2B-04, opened on overcurrent.
The feeder breaker tripped on overload after energizing a pressurizer heater group. This trip resulted in de-energizing electrical bus 2B-04. No other breakers tripped during this event. The results of an investigation determined the overload trip was set low.
As part of the casual analysis it was determined that a spare feeder breaker was setup with an overload setting not appropriate for use as the feeder breaker for bus 2B-04. As part of a PM activity, the spare breaker was staged for swapping into the feeder breaker cubicle for bus 2B-04 and subsequently installed.
The breaker was not loaded above the overload setting until the Unit 2 heat-up in MODE 4 when a pressurizer heater group was energized and the overload trip occurred. After the breaker trip, the feeder breaker was replaced with a spare breaker that had satisfactorily completed breaker maintenance and had a correctly set overload.
A technical assessment for reportability was completed. NextEra determined that bus 2B-04 was inoperable for specific time periods from March 23, 2011 through June 7, 2011 for Unit 1 and Unit 2. Technical Specification LCOs and associated TSACs for TS 3.8.9 Distribution System - Operating (TSAC 3.8.9 A and B), TS 3.8.10 Distribution Systems - Shutdown (TSAC 3.8.10 A), and TS 3.5.3 ECCS - Shutdown (TSAC 3.5.3 A and B) were not met during specific time periods. Therefore, pursuant to 10 CFR 50.73(a)(2)(i)(B), the event is reportable as a condition prohibited by Technical Specifications.
The improperly set bus 2B-04 supply breaker, in conjunction with a postulated MODE 4 LOCA, may have caused a simultaneous loss of low head injection and containment sump recirculation capability.
This resulted in TS 3.5.3 ECCS - Shutdown (TSAC 3.5.3 A and B) not being met for Unit 2. Therefore, pursuant to 10 CFR 50.73(a)(2)(v)(D) this event is being reported as a safety system functional failure for any event or condition that could have prevented the fulfillment of the safety function of structures or systems that are needed to mitigate the consequences of an accident.
Cause of the Event
The cause of the event was the breaker's incorrect overload trip setting.
Analysis of the Event
The 480V system buses and switchgear are supplied by the 4.16 kV system through the 4160/480 VAC station service transformers (X-11 through X-14) and diesel generator building transformers (X-06). The 480 volt safeguards equipment is connected to buses B-03 and B-04. Power for safeguards valve motors is supplied from the motor control centers B-32 and B-42 which in turn are served from buses B-03 and B-04, respectively. Since the normal source of power for these buses is the 345 kV system (via station auxiliary transformers X-03 and X-04; 4160 volt buses A-03, A-05 and A-04, A-06; and station service transformers X-13 and X-14).
Under certain plant configurations of operating equipment, the lower amperage setting may have caused the feeder breaker to trip thus removing power from bus 2B-04 and making it incapable of performing its safety function. An engineering evaluation was performed to determine the time periods when bus 2B-04 may not have been capable of performing its safety function. For the periods of time that the feeder breaker with the improper setting was installed in bus 2B-04, the operating conditions were Unit 1 in MODE 1 and Unit 2 in MODE 4, 5, 6, or Defueled.
The feeder breaker was placed in service with the improper setting on March 23, 2011. A breaker with the correct setting was installed on June 7, 2011. Between those dates NextEra determined that bus 2B-04 was inoperable for specific time periods. Technical Specification Limiting Conditions for Operation (LCOs) and associated Action Conditions (TSACs) for TS 3.8.9 Distribution System - Operating (TSAC 3.8.9 A and B), TS 3.8.10 Distribution Systems - Shutdown (TSAC 3.8.10 A), and/or TS 3.5.3 ECCS - Shutdown (TSAC 3.5.3 A and B) were not met during specific time periods.
Analysis of Safety Significance During the time periods when bus 2B-04 may have been incapable of supplying its required loads for Unit 1, the operating loads and those loads expected to be started in response to a Unit 1 event, could have caused the incorrectly set feeder breaker to bus 2B-04 to trip open. The loss of bus 2B-04 may have an impact on Unit 1 risk due to the common/shared loads normally powered by bus 2B-04, such as K-3B Service Air Compressor, P-32D and P-32E Service Water Pumps, P-38B Standby Steam Generator Feedwater Pump, K-2B Instrument Air Compressor, and the D-08 Battery Charger. NextEra determined the event to be of very determined that there was no SSFF because redundant equipment remained in service during the event.
Unit 2 was in Modes 4, 5, 6, or defueled during this time period. A deterministic, defense-in-depth, shutdown safety assessment process was used to evaluate the significance of this issue. Time periods when bus 2B- 04 was determined to be inoperable was used in the assessment. Based on the assessment, the issue was considered to be of very low safety significance for Unit 2. A review for SSFFs for Unit 2 determined that there was a SSFF because the improperly set bus 2B-04 supply breaker, in conjunction with a postulated MODE 4 LOCA, may have caused a simultaneous loss of low head injection and containment sump recirculation capability.
Corrective Actions
A feeder breaker with the proper overload setting was installed in electrical bus 2B-04.
A root cause evaluation was completed and corrective actions were developed to address organizational and programmatic issues.
Similar Events None
Failed Components
None