05000296/LER-2008-001
| Browns Ferry Unit 3 | |
| Event date: | 05-05-2008 |
|---|---|
| Report date: | 07-07-2008 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 2962008001R00 - NRC Website | |
I. PLANT CONDITION(S)
Unit 3 was in Mode 5, shutdown for a scheduled refueling outage. Units 1 and 2 were at 100 percent power (3458 Megawatts thermal) and unaffected by the event.
II. DESCRIPTION OF EVENT
A. Event:
On May 5, 2008, at approximately 0332 hours0.00384 days <br />0.0922 hours <br />5.489418e-4 weeks <br />1.26326e-4 months <br /> Central Daylight Time (CDT) Emergency Diesel Generators (EDGs) 3EC and 3ED [EK] auto-started and tied to their respective shutdown boards due to an under voltage condition. Operations was in the process of returning the Unit 3 4KV Unit Board 3B to the normal supply in accordance with Operating Instruction 0-OI 57A, Switchyard and 4160V AC Electrical System, when the board failed to transfer. The loss of power to Unit Board 3B resulted in a loss of power to 4KV Shutdown Boards 3EC and 3ED, 480V Reactor Motor-Operated Valve (RMOV) Board 3B [EC], and Reactor Protection System 3B (RPS) [JC] power supply. Due to the loss of power on the shutdown boards, EDGs 3EC and 3ED started and tied to their respective shutdown boards.
Unit 3 also received Primary Containment Isolation System (PCIS) [JE] Groups 3 and 6 isolations and actuations. A coincidental upscale trip of the 3A intermediate range monitor (IRM) [IG], which resulted in RPS Channel 3A half scram, in combination with the de-energizing of the RPS Channel 3B resulted in an unexpected full reactor scram. The Standby Gas Treatment (SGT) [BH] and Control Room Emergency Ventilation (CREV)[VI] systems initiated as expected.
By 0352 hours0.00407 days <br />0.0978 hours <br />5.820106e-4 weeks <br />1.33936e-4 months <br /> CDT the reactor scram and PCIS logic was reset, the SGT and CREV Systems were returned to standby readiness. By 0944 hours0.0109 days <br />0.262 hours <br />0.00156 weeks <br />3.59192e-4 months <br /> CDT power was restored to 4KV Shutdown Boards 3EC and 3ED; likewise, EDGs 3EC and 3ED were secured.
TVA is submitting this report in accordance with 10 CFR 50.73(a)(2)(iv)(A) as any event of condition that resulted in manual or automatic actuation of any system listed in paragraph 10 CFR 50.73 (a)(2)(iv)(B).
B. Inoperable Structures, Components, or Systems that Contributed to the Event:
None.
C. Dates and Approximate Times of Major Occurrences:
May 5, 2008 0332 hours0.00384 days <br />0.0922 hours <br />5.489418e-4 weeks <br />1.26326e-4 months <br /> CDT Emergency Diesel Generators 3EC and 3ED auto start and load to their respective 4KV Shutdown Boards.
May 5, 2008 0944 hours0.0109 days <br />0.262 hours <br />0.00156 weeks <br />3.59192e-4 months <br /> CDT Affected equipment is returned to standby readiness.
May 5, 2008 1102 hours0.0128 days <br />0.306 hours <br />0.00182 weeks <br />4.19311e-4 months <br /> CDT TVA makes an eight-hour non-emergency notification ito NRC in accordance with 10 CFR 50.72(b)(3)(iv)(A) and 10 CFR 50.72(b)(3)(iv)(B).
D. Other Systems or Secondary Functions Affected
None.
E. Method of Discovery
The main control room crew received alarms associated with the loss of power to the electrical boards, the start of the EDGs, and the reactor scram signal.
F. Operator Actions
The operator actions were as expected and did not contribute to the event.
G. Safety System Responses
All safety systems responded as designed during the event.
III. CAUSE OF THE EVENT
A. Immediate Cause
The initial investigation found that when the operator closed the normal supply breaker to 4KV Unit Board 3B, there was indication of disagreement between the demand breaker position and the actual breaker position. The Unit Supervisor directed the operator to transfer the board back to the alternate supply. However, the alternate supply breaker did not close resulting in the 4KV Unit Board being de-energized.
B. Root Cause
The root cause of this event was misalignment of the stationary breaker indicating (STA) switch mechanism in the normal feeder breaker for 4KV Unit Board 3B.
C. Contributing Factors
None.
IV. ANALYSIS OF THE EVENT
Unit 3 operations personnel were in the process of transferring the feed to 4KV Unit Board from the alternate to the normal power supply as part of scheduled refueling outage activates for return of Unit 3 to service. After transferring the 4KV Unit Board 3B to the normal feed, they had indication that there was a disagreement between the demanded breaker position and the actual breaker position. As discussed in the pre-job brief, the board was returned to the alternate power supply.
However, the alternate breaker failed to close. With no power on 4KV Unit Board 3B, the power was lost to all of the down stream loads which included 4KV Shutdown Boards 3EC and 3ED.
Approximately 5 seconds after the loss of power to the 4KV Shutdown Boards, EDGs 3EC and 3ED started to supply power to the boards.
A momentary upscale trip on I RM 3A was not expected when 4KV Unit Board 3B deenergized; however, noise issues with IRM's have previously resulted in a half scram. When power was lost to the 480V RMOV Board, the motor contactor for 3B RPS motor-generator set opened and the motor-generator started to coast (manual actions are required to place the RPS motor-generator back in service). When the RPS Bus frequency fell to less than 57 hertz; the RPS circuit protectors operated which generated a half scram on RPS Channel 3B. This coupled with the pre-existing RPS Channel 3A half scram provided the full reactor scram signal.
V. ASSESSMENT OF SAFETY CONSEQUENCES
The safety consequences of this event were not significant. All safety systems operated as required. Prior to the event, Unit 3 was in a shutdown condition with the control rods fully inserted.
The transient on the plant equipment was minimal. Therefore, TVA concludes that the health and safety of the public was not affected by this event.
VI. CORRECTIVE ACTIONS
A. Immediate Corrective Actions
Operations returned the effected equipment to the pre-event configuration in accordance with plant instructions.
B. Corrective Actions to Prevent Recurrence (1) The alignment issue with the STA switch was resolved. WA will revise, as needed, the breaker maintenance program to improve reliability.
VII. ADDITIONAL INFORMATION
A. Failed Components
None.
B. Previous LERs on Similar Events None.
C. Additional Information
Corrective action document for this report is PER 144272.
D. Safety System Functional Failure Consideration:
This event is not a safety system functional failure according to NEI 99-02.
E. Scram With Complications Consideration:
This event was not a complicated scram according to NEI 99-02.
VIII. COMMITMENTS
None.
(1) TVA does not consider the corrective action a regulatory requirement. The completion of the action will be tracked in TVA's Corrective Action Program.