05000390/LER-2012-005
| Watts Bar Nuclear Plant | |
| Event date: | 10-16-2012 |
|---|---|
| Report date: | 12-15-2012 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 3902012005R00 - NRC Website | |
All times are approximate and Eastern Daylight Savings Time (EDT) unless noted otherwise.
I. PLANT CONDITIONS:
Watts Bar Nuclear Plant (WBN) Unit 1 was in MODE 5, 0% Rated Thermal Power (RTP) following the 11th refueling outage (1R11). At the time of the event, WBN-1 was reliant on train A safety equipment.
II. DESCRIPTION OF EVENT:
Background:
The WBN-1 plant alternating current (AC) Electrical Power Distribution System sources consist of the offsite power sources (preferred power sources, normal and alternate(s)), and the onsite standby power sources (train A and B emergency diesel generators (EDGs)). The design of the AC electrical power system provides independence and redundancy to ensure an available source of power to the Engineered Safety Feature (ESF) systems.
The onsite Class 1 E AC Distribution System supplies electrical power to four power trains, shared between the two units, with each train powered by an independent Class 1 E 6.9 kV Shutdown Board (SDBD). Power trains 1A and 2A comprise load group A, and power trains 1B and 2B comprise load Group B. Two EDGs associated with one load group can provide all safety related functions to mitigate a loss-of-coolant accident (LOCA) in one unit and safely shutdown the opposite unit. The A and B train ESF systems each provide for the minimum safety functions necessary to shut down the plant and maintain it in a safe shutdown condition.
An EDG starts automatically on a safety injection (SI) signal or on a 6.9 kV SDBD degraded voltage or loss of-voltage signal. After the EDG has started, it will automatically tie to its respective 6.9 kV SDBD after offsite power is tripped as a consequence of 6.9 kV SDBD loss-of-voltage or degraded voltage, independent of or coincident with an SI signal. Following the trip of offsite power, a loss of-voltage signal strips (sheds) nonpermanent loads from the 6.9 kV SDBD. When the EDG is tied to the 6.9 kV shutdown board, loads are then sequentially connected to its respective 6.9 kV SDBD by the automatic sequencer. The sequencing logic controls the permissive and starting signals to motor breakers to prevent overloading the EDG by automatic load application.
A.� Event:
On October 16, 2012, at 2330, WBN-1 control room operators, in the conduct of scheduled maintenance activities, attempted a manual fast transfer of the 1B-B 6.9kV SDBD from the normal feeder breaker (preferred offsite power source) to the alternate feeder breaker (alternate preferred offsite power source). The transfer was unsuccessful and resulted in a momentary loss of power to the 6.9kV SDBD. The momentary loss of power caused the supplied loads to be shed and initiated a designed loss-of-voltage signal that resulted in the automatic start of the four EDGs [EllS Code EK].
Approximately 6 seconds after the 1B-B 6.9kV SDBD de-energized, the alternate feeder breaker closed-in and re-energized the 1B-B 6.9kV SDBD. The EDGs did not connect to the 1B-B SDBD because the momentary disruption of power did not allow sufficient time for the EDGs to reach nominal voltage and frequency and tie to the 1B-B SDBD.
II.C DESCRIPTION OF EVENT (continued):
Control room operators promptly entered the appropriate response procedure and re-established power to required loads and completed required testing to ensure operability of the 1B-B 6.9kV SDBD. The required safety systems functioned as designed.
B. Inoperable Structures, Components, or Systems that Contributed to the Event None C. Dates and Times of Major Occurrences DateC Time (EDT)C Event October 16, 2012C 23:30:00C Control room operators attempt to transfer the 1B-B 6.9kV SDBD to the alternate feeder breaker. Momentary loss of power to the 1 B-B 6.9kV SDBD occurs October 16, 2012C 23:30:01C EDGs started on loss of power to the 1B-B 6.9kV SDBD October 16, 2012C 23:30:03C Loads supplied by 1B-B 6.9kV SDBD are shed October 16, 2012C 23:30:06C Alternate feeder breaker re-energizes 1B-B 6.9kV SDBD October 16, 2012C 23:42:00C Control room operators enter the response procedure October 16, 2012C 23:57:49C Control room operators restart Train B spent fuel pool cooling per procedure October 17, 2012C 00:24:19C Control room operators secure EDG 1A-A October 17, 2012C 00:33:34C Control room operators secure EDG 2A-A October 17, 2012C 00:41:03C Control room operators secure EDG 1B-B October 17, 2012C 00:51:37C Control room operators secure EDG 2B-B October 17, 2012C 03:00:00C Control room operators re-connect loads to the 1B-B 6.9 kV SDBD.
October 17, 2012C 05:46:00C Control room operators notify NRC (Event Number: 48414)
D. Other Systems or Secondary Functions Affected
None
E. Method of Discovery
This event occurred during the performance scheduled maintenance activities on the 1B-B 6.9kV SDBD. During the maintenance activity, control room operators observed that the transfer of the 1B-B 6.9kv SDBD from the normal breaker to the alternate breaker was unsuccessful and that the momentary loss of power caused supported loads to be shed and the four EDGs to start.
F. Operator Actions
Control room operators entered the appropriate procedure in response to the momentary loss of power to the 1B-B 6.9kV SDBD. The loads that were shed from the 1B-B 6.9kV SDBD during the failed transfer were restored and the four EDGs were secured.
II. DESCRIPTION OF EVENT (continued):
G.� Safety System Responses Following the loss-of-voltage to the 1B-B SDBD, nonpermanent loads were shed and the four EDGs started. The EDGs did not connect to the 1B-B 6.9 kV SDBD because the alternate supply connected before the EDGs reached nominal voltage and frequency.
III. CAUSE OF EVENT
The cause of this event was that control room operators did not ensure the alternate feeder breaker hand switch was held firmly in the "closed" position while initiating the fast board transfer.
IV. ANALYSIS OF THE EVENT
During this event, WBN-1 was in MODE 5 following a refueling outage and the 1B-B 6.9kV SDBD was not being relied upon for safety systems. Power supplied to the 1B-B 6.9kV SDBD was restored through the alternate feeder breaker six seconds after the failed transfer. The functionality of the 1B-B 6.9kV SDBD was restored after the loads were reconnected. Required testing to ensure operability of the 1B-B 6.9kV SDBD was successfully completed. A post event review did not identify any adverse conditions relating to the offsite power sources, the onsite standby power sources or the related control circuits.
WBN's investigation revealed the event was the result of an isolated human performance error. The board transfer process requires the control room operator to place and hold the hand-switch for the alternate feeder breaker firmly in the "closed" position, then to place the hand-switch for the normal feeder breaker in the "trip" position. The control room operator responsible for performing the transfer did not firmly hold the alternate feeder breaker hand-switch in the "closed" position during the tripping of the normal feeder breaker, resulting in the momentary loss of power to the 1B-B 6.9kV SDBD. Control room operators have re-performed the transfer without subsequent human performance issues. The investigation did not conclude that time pressure or situational pressure contributed to this event.
V. ASSESSMENT OF SAFETY CONSEQUENCES
During this event, WBN-1 was in MODE 5 following a refueling outage and the 1B-B 6.9kV SDBD was not being relied upon to support needed safety systems. Power supplied to the 1B-B 6.9kV SDBD was restored through the alternate feeder breaker six seconds after the failed transfer. The functionality of the 1B-B 6.9kV SDBD was restored after the loads were reconnected. Required testing to ensure operability of the 1B-B SDBD was successfully completed and a post event review did not identify any adverse conditions relating to the offsite power sources, the onsite standby power sources or the related control circuits.
There are three additional 6.9kV SDBDs at WBN. These three 6.9kV SDBDs passed surveillance requirement testing to demonstrate capability to automatically and manually transfer power from normal to alternate supply prior to the October 16, 2012 event and were capable of performing their safety function. This event did not adversely affect the safe operation of the plant or the health and safety of the public.
VI.�CORRECTIVE ACTIONS A. Immediate Corrective Actions The 1B-B 6.9kV SDBD was restored using the appropriate response procedure.
B. Corrective Actions to Prevent Recurrence The WBN procedure that describes generic equipment operating guidelines will be revised to provide special instructions on manipulation of switches for board transfers to ensure that switches are firmly held in place.
VII.�ADDITIONAL INFORMATION
A. Failed Components
None B. Previous LERs on Similar Events due to a loss of power to the 2B-B 6.9kV SDBD. The cause of the event was a misaligned breaker actuation arm which prevented the proper positioning of the fast transfer microswitch.
The previous event did not involve the same underlying concern or reason as this event, such as the same root cause, failure, or sequence of events.
C. Additional Information
None D. Safety System Functional Failure None E. Loss of Normal Heat Removal Consideration None
VIII. COMMITMENTS
None