05000296/LER-2015-005
| Browns Ferry Nuclear Plant (Bfn), Unit 3 | |
| Event date: | 8-20-2015 |
|---|---|
| Report date: | 10-19-2015 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(B), System Actuation 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| Initial Reporting | |
| ENS 51333 | 10 CFR 50.72(b)(3)(iv)(A), System Actuation |
| 2962015005R00 - NRC Website | |
I. Plant Operating Conditions Before the Event
At the time of discovery, Browns Ferry Nuclear Plant (BFN) Unit 3 was operating in Mode 1 at approximately 100 percent rated thermal power. BFN Units 1 and 2 were unaffected by this event.
II. Description of Events
A. Event:
On August 20, 2015, at 1032 Central Daylight Time (CDT), while installing test equipment on the 3ED 4kV Shutdown Board (SD BD), for an online dynamic motor test of the 3D Residual Heat Removal (RHR)[BO] pump motor, the Unit 3 Control Room received degraded voltage alarms and under voltage alarms for the 3ED 4kV Shutdown Board (SD BD)[ECBD]. The 3ED 4kV SD BD normal feeder breaker [BKR] opened, and the 3D Diesel Generator (DG)[DG] fast started and tied onto the board.
BFN Unit 3 entered Technical Specification (TS) Limiting Condition for Operation (LCO) 3.8.1, AC Sources - Operating. This LCO requires, with one required offsite circuit inoperable: that power availability from the remaining operable offsite transmission network be verified within one hour and once every eight hours thereafter; that required feature(s) with no offsite power available be declared inoperable when the redundant required feature(s) are inoperable; and that the required offsite circuit be restored to operable status within seven days.
On August 20, 2015, at 1851 CDT, Event Notification (EN) 51333 was made to the NRC in accordance with Title 10 of the Code of Federal Regulations (10 CFR) 50.72(b)(3)(iv)(A).
On August 21, 2015, at 0136 CDT, Operations personnel secured 3D DG, and declared 3ED SD BD, 3D DG, and C Standby Gas Treatment (SGT)[BH] system inoperable.
On August 21, 2015, at 1945 CDT, troubleshooting was performed on the SD BD and on the DG. Two fuses [FU] were replaced on the SD BD, the normal feeder breaker was closed, offsite power to the SD BD was declared operable, and C SGT was declared operable. At 2300, 3D DG was declared operable.
B. Status of structures, components, or systems that were inoperable at the start of the event and that contributed to the event:
There were no systems, structures, or components inoperable at the start of this event that contributed to the event.
C. Dates and approximate times of occurrences:
August 20, 2015, at 1032 CDT 3ED 4kV SD BD de-energized 3D DG fast-started August 20, 2015, at 1851 CDT Provided eight-hour EN 51333 to the NRC.
August 21, 2015, at 0136 CDT 3D DG secured and declared inoperable 3ED 4kV SD BD declared inoperable C SGT declared inoperable August 21, 2015, at 1945 CDT Offsite power to 3ED 4kV SD BD declared operable 3ED 4kV SD BD declared operable C SGT declared operable August 21, 2015, at 2300 CDT 3D DG declared operable D. Manufacturer and model number (or other identification) of each component that failed during the event:
No component failures were identified that occurred during the event.
E. Other systems or secondary functions affected:
- D1 RHR Service Water (RHRSW)[CC] pump tripped.
- Control Bay Vent Board B was de-energized.
F. Method of discovery of each component or system failure or procedural error:
Degraded voltage for the 3ED 4kV SD BD resulted in multiple Control Room alarms.
G. The failure mode, mechanism, and effect of each failed component, if known:
There were no failed components related to this event.
H. Operator actions:
- Declared offsite power to 3ED 4kV SD BD inoperable.
- Removed 3D DG from service.
- Declared 3ED 4kV SD BD, C SGT, and 3D DG inoperable for corrective maintenance.
I. Automatically and manually initiated safety system responses:
3D DG started automatically upon opening of normal feeder breaker to 4kV SD BD.
III. Cause of the event
A. The cause of each component or system failure or personnel error, if known:
A definitive cause could not be identified for the clearing of the 3ED SD BD metering fuses.
B. The cause(s) and circumstances for each human performance related root cause:
A definitive cause could not be identified for the clearing of the 3ED SD BD metering fuses.
However, a possible failure mode was identified that was related to human performance, shorting between two terminals when attempting to attach the clip.
IV. Analysis of the event:
The Tennessee Valley Authority (TVA) is submitting this report in accordance with 10 CFR 50.73(a)(2)(iv)(A), as any event or condition that resulted in manual or automatic actuation of any of the systems listed in paragraph (a)(2)(iv)(B) of 10 CFR 50.73 (including DGs), except when:
1. The actuation resulted from, and was part of, a pre-planned sequence during testing or reactor operation 2. The actuation was invalid and occurred while the system was properly removed from service, or was invalid and occurred after the safety function had been already completed.
Because this event resulted in a valid, unplanned actuation of 3D DG, it is reportable under 10 CFR 50.73(a)(2)(iv)(A).
The safety objective of the Standby Alternating Current Power System is to provide a self- contained, highly reliable source of power, as required for the Engineered Safeguards System, so that no single credible event can disable the core standby cooling functions or their supporting auxiliaries. The DGs are designed to support the electrical load of a 4kV SD BD upon loss of supply from a 4.16-kV shutdown bus. During this event, 3D DG adequately performed its safety function of automatically assuming the load of 3ED 4kV SD BD upon a loss of supply to the board.
Troubleshooting of the DG and SD BD determined that the failure mode for this event was the clearing of one primary and one secondary 3ED SD BD metering fuses. The secondary fuse that cleared supplies the RHR Motor 3D Watt-hour meter. The primary fuse that cleared was one of the 3D SD BD potential transformer fuses that supplies power to the 3D RHR motor watt- hour meter and the 3ED shutdown board under-voltage relays.
An inspection of test equipment by the vendor identified four potential failure mechanisms for this equipment. Two possible failure modes were potentially caused by mechanical failure.
These included a short circuit between two test points or between the first test point and the ground. Potential human error failure modes included shorting between two terminals when attempting to attach the clip. No definitive cause was identified for the clearing of fuses which led to this event. Therefore, corrective actions will be implemented that address the four most probable causes and minimize the likelihood of recurrence.
V. Assessment of Safety Consequences
This event resulted in an unplanned automatic start of 3D DG. The diesel generators are designed to automatically start upon a loss of offsite power. 3D DG successfully performed its safety function during this event; therefore, TVA has concluded there was no significant increase in risk to the health and safety of the public or plant personnel due to this event.
A. Availability of systems or components that could have performed the same function as the components and systems that failed during the event:
During this event, all four Unit 3 DGs remained available, and all four Unit 3 4kV SD BDs were operable. D2 RHRSW pump was started when D1 RHRSW pump tripped.
B. For events that occurred when the reactor was shut down, availability of systems or components needed to shutdown the reactor and maintain safe shutdown conditions, remove residual heat, control the release of radioactive material, or mitigate the consequences of an accident:
This event did not occur when the reactor was shut down.
C. For failure that rendered a train of a safety system inoperable, an estimate of the elapsed time from discovery of the failure until the train was returned to service:
Offsite power to the 3ED 4kV SD BD was inoperable for approximately thirty-three hours from the time the SD BD de-energized on August 20, 2015, at 1032 CDT, to the time power was restored on August 21, 2015, at 1945 CDT.
VI. Corrective Actions:
Corrective Actions (CA) are being managed by TVA's Corrective Action Program under Condition Report (CR) 1073157. The following CAs are in progress:
1. Perform inspections on circuits used for online dynamic motor testing of motors.
2. Remove potentially faulty test equipment from service.
3. Remove online dynamic motor testing from 4kV motor preventative maintenance. This action will be taken in order to prevent future human performance errors similar to the type of error which may have caused this event.
VII. Additional Information:
A. Previous Similar Events at the same plant:
A review of BFN Licensee Event Reports and Corrective Action Program documents for the past three years did not identify any relevant equipment trips caused by the online dynamic motor test equipment.
B. Additional Information:
There is no additional information.
C. Safety System Functional Failure Consideration:
This event is not considered to be a Safety System Functional Failure in accordance with NUREG-1022.
D. Scram with Complications Consideration:
This event did not result in a reactor scram.
VIII. COMMITMENTS
There are no new commitments.