05000325/LER-2011-001
| Brunswick Steam Electric Plant (Bsep), Unit 1 | |
| Event date: | 04-07-2011 |
|---|---|
| Report date: | 06-02-2011 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident |
| Initial Reporting | |
| 3252011001R00 - NRC Website | |
Energy Industry Identification System (EIIS) codes are identified in the text as [XX].
Introduction Initial Conditions At the time of the event, Unit 1 was in Mode 1, at approximately 100 percent of rated thermal power (RTP).
Unit 2 was operating in Mode 4 for a scheduled refueling outage.
Reportability Criteria This event is being reported in accordance with 10 CFR 50.73(a)(2)(v)(D), as an 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. The NRC was initially notified of this event on April 8, 2011 (i.e., Event Number 46740).
Event Description
On April 7, 2011, the plant was performing OMST-DG13R, "DG-3 Loading Test." During performance of this test, the 480 VAC Emergency Bus E-7 [EB] main feeder breaker tripped unexpectedly. As a result, the Control Room Emergency Ventilation (CREV) system [VI] emergency makeup damper, 2-VA-2J-D-CB, closed on loss of power. This resulted in two CREV subsystems required by Technical Specification (TS) 3.7.3, "Control Room Emergency Ventilation (CREV) System," being declared inoperable at approximately 1740 hours0.0201 days <br />0.483 hours <br />0.00288 weeks <br />6.6207e-4 months <br /> Eastern Daylight Time (EDT). Only Unit 1 was required to enter TS 3.7.3, Required Action C.1, for two CREV subsystems inoperable (i.e., be in Mode 3 within 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br />). Unit 2 was operating in Mode 4 for a scheduled refueling outage (i.e., B220R1) and did not meet any applicability conditions for TS 3.7.3.
Approximately 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> and 51 minutes later, following restoration of the CREV emergency makeup damper, operability of the two CREV subsystems was restored and TS 3.7.3, Required Action C.1, was exited at 1931 hours0.0223 days <br />0.536 hours <br />0.00319 weeks <br />7.347455e-4 months <br />.
Spring 2011 refueling outage (i.e., B220R1) as part of a normally scheduled preventive maintenance activity. A new solid state trip unit was installed on the refurbished breaker as part of an extent of condition action in response to a transistor corrosion issue. The new trip unit had been tested and burned-in by an outside vendor prior to installation on the breaker. The breaker was then tested again at BSEP with no problems found.
Following the testing, the refurbished breaker with the new trip unit was installed in emergency bus E-7, on March 18, 2011.
Approximately three weeks later, on April 7, 2011, the breaker tripped during testing of the diesel generator per OMST-DG13R. Based on computer data traces, the breaker appears to have tripped immediately after the diesel generator output breaker was closed and the bus reenergized. The bell alarm switch on the breaker was actuated (i.e., indicating an overcurrent trip) and the associated control room annunciator alarmed (i.e., UA-17-4-4). The breaker is equipped with long-time and short-time trip functions, with a target provided for each function, but neither of the targets was flagged after the breaker tripped.
Event Description (continued) The breaker, including the solid state trip unit, was removed and sent to Progress Energy's Crystal River plant breaker shop for testing, to the extent possible, and analysis. There was no evidence of a fault, and the trip unit was found to be operating within specifications during secondary current injection. No problems occurred during actual trip testing that would cause a spurious trip. The solid state trip unit was then sent to Progress Energy's Harris Environmental and Energy (HE&E) Center. No problems were noted in the visual examination, including checks of the solder connections and inspection of the transistors for corrosion.
Discreet component checks were performed on all diodes and transistors with no failures noted. The trip unit was then sent to ABB (i.e., the original equipment manufacturer) for additional testing. The trip unit passed the secondary current injection test and the silicon control rectifier (SCR) leakage was verified to be acceptable. ABB confirmed that there have been no design changes in the trip units between 1992 and 2010. ABB could not find any problem with the trip unit. No spurious trips occurred during any of this testing and no problems related to a spurious trip were found.
Based on the evidence and analysis performed in this investigation, the spurious trip of the E-7 main breaker is attributed to a malfunction of the new solid state trip unit. However, this malfunction could not be duplicated during testing and no specific root cause could be determined.
The BSEP breaker in question is an ABB K-3000S model; which is widely used throughout the industry.
A review of external Operating Experience (OE) identified other spurious trips of this model breaker. The cause of a number of these breaker trips was attributed to the trip unit, however, testing and analysis could not ultimately identify a problem with the trip unit. Thus, spurious trips with an unknown cause attributed to the trip unit have been known to occur. The typical industry corrective action for solid state trip unit failures with undetermined causes is replacement of the individual unit.
The BSEP breaker (i.e., with its original trip unit) which had been removed from E-7 during B220R1 was reinstalled on April 8, 2011, and the substation reenergized. No subsequent problems occurred with the breaker.
Event Cause The direct cause of the E-7 breaker trip event was a spurious actuation of the solid state trip unit. No problems could be found in the post-event testing or during repeat of the loss of offsite power/loss of coolant accident (LOOP/LOCA) test. Therefore, the root cause of this event could not be determined.
Safety Assessment The safety significance of this event is considered minimal. The condition existed for approximately one hour and 51 minutes, and plant staff took immediate action to return the equipment to service.
Corrective Actions
The following corrective actions will be taken.
- The E-7 breaker was replaced and the substation reenergized. This action is complete.
- The tripped breaker that was removed will be refurbished, which will include replacement of the trip unit, prior to reuse. This action is currently scheduled to be completed by October 20, 2011.
Previous Similar Events
A review of LERs and corrective action program nuclear condition reports (NCRs) for the past three years identified the following similar previous occurrence.
NCR 350664, dated August 16, 2009, identified a similar failure where the feeder breaker to lighting panel 1A tripped and no problem could be found. The breaker was reinstalled and has not had an additional trip. The trip unit was later replaced on May 11, 2010. The old trip unit was inspected and corrosion was observed on transistors. This event could not have reasonably been expected to prevent the condition reported in this LER. (Note that for the E-7 breaker trip identified in this LER, the new solid state trip unit that was installed on the refurbished breaker was part of an extent of condition action in response to the transistor corrosion issue identified in NCR 350664).
air conditioning systems. The direct cause of the loss of control air to the control room ventilation system was blockage of air flow through the Control Building instrument air dryer, due to freezing of condensate within the cooling coil. This event could not have reasonably been expected to prevent the condition reported in this LER.
isolate. The select cause of this event was determined to be failure to identify appropriate Preventive Maintenance (PM) routes for the CREV system components. The corrective action was to develop PM tasks for the Control Building ventilation system dampers, actuators, limit switches, and relays. This event could not have reasonably been expected to prevent the condition reported in this LER.
Commitments No regulatory commitments are contained in this report.