05000296/LER-2005-001
| Browns Ferry Unit 3 | |
| Event date: | 02-11-2005 |
|---|---|
| Report date: | 04-12-2005 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 2962005001R00 - NRC Website | |
I. PLANT CONDITION(S)
During this event Unit 3 was in Mode 1 at approximately 3458 megawatts thermal (100 percent reactor power). Unit 1 was shutdown and defueled and was unaffected by the event. Unit 2 was also in Mode 1 at approximately 3458 megawatts thermal (100 percent power) and was unaffected by this event.
II. DESCRIPTION OF EVENT
A. Event:
At 1629 hours0.0189 days <br />0.453 hours <br />0.00269 weeks <br />6.198345e-4 months <br /> Central Standard Time on February 11, 2005, the Unit 3 reactor scrammed from 100% power. The scram was caused by a simultaneous false trip signal generated to the main generator circuit breaker 234, switchyard circuit breakers 5264 and 5268, and a main generator trip. The trip occurred when a PK block (disconnect device 26W), which had been pulled as part of a clearance for breaker 5264, was re-inserted as part of switching order No. 93 for returning the breaker to service. When the PK block was inserted (out of sequence of the switching order), the associated current transformer (CT) circuit was momentarily grounded resulting in a false differential signal. The correct sequence of the switching order was to actuate the trip cutout switches for the differential trip functions prior to inserting any of the PK blocks.
The generator trip resulted in a turbine trip and opening of the output breakers caused a power-load unbalance trip. The control valve fast closure caused the reactor SCRAM.
All rods inserted. Reactor water level lowered, as expected, and was recovered by normal feed water flow. All expected Primary Containment Isolation System (PCIS) isolations [Group 2 (Residual Heat Removal (RHR) Shutdown Cooling (S/D Cooling)), Group 3 Reactor Water Cleanup (RWCU), Group 6 (Ventilation), and Group 8 Traversing Incore Probe (TIP)] were received along with the auto start of Control Room Emergency Ventilation (CREV), and the three Standby Gas Treatment (SGT) trains.
This report is submitted in accordance with 10 CFR 50.73(a)(2)(iv)(A), as an event that resulted in an automatic actuation of the systems listed in paragraph (a)(2)(iv)(B) (i.e., Reactor Protection System including: reactor scram or reactor trip, and general containment isolation signals affecting containment isolation valves in more than one system).
B. Inoperable Structures, Components, or Systems that Contributed to the Event:
None C. Dates and Approximate Times of Major Occurrences:
February 11, 2005 1600 hours0.0185 days <br />0.444 hours <br />0.00265 weeks <br />6.088e-4 months <br /> CST Unit Supervisor (Utility Licensed), responsible for areas out side of the powerhouse, was given switching order No. 93, initiated by the WA Load Dispatcher in Chattanooga, to perform.
C. Dates and Approximate Times of Major Occurrences (continued) February 11, 2005 1629 hours0.0189 days <br />0.453 hours <br />0.00269 weeks <br />6.198345e-4 months <br /> CST Unit Supervisor (Utility Licensed) replaced PK block disconnect device (26W) Relays 387T A and C Phases tripped due to main transformer differential Relay 386tx tripped Turbine tripped (breaker 5268 and breaker 234 tripped) Power-load unbalance trip Reactor Scram due to control valve fast closure February 11, 2005 1658 hours0.0192 days <br />0.461 hours <br />0.00274 weeks <br />6.30869e-4 months <br /> CST Scram reset
D. Other Systems or Secondary Functions Affected
None
E. Method of Discovery
This event was immediately apparent to the operating crew through numerous indications and alarms in the Unit 3 Control Room.
F. Operator Actions
Operations personnel responded to the event in accordance with applicable plant procedures.
G. Safety System Responses
All rods inserted. Reactor water level lowered, as expected, and was recovered by normal feed water flow. All expected Primary Containment Isolation System (PCIS) isolations [Group 2, Residual Heat Removal shutdown cooling (RHR S/D Cooling)), Group 3, Reactor Water Cleanup (RWCU), Group 6 (Ventilation), and Group 8, Traversing Incore Probe (TIP)] were received along with the auto start of Control Room Emergency Ventilation (CREV) and the three Standby Gas Treatment trains.
Ill. CAUSE OF THE EVENT
A. Immediate Cause
The immediate cause of this event was the insertion of the PK block (disconnect device 26W) out of sequence from the instructions given in switching order No. 93 provided by the WA load dispatcher.
13 � Root Cause Personnel error, in that the Operator (Utility Licensed) failed to follow the task sequence identified in the switching order (procedure noncompliance).
C. Contributing Factors
Infrequency of task performance Less rigor placed on switching orders than on station generated clearance orders, including:
- No formal Pre job Briefing that discuss risk significance of work to be performed
- Peer checker not assigned Significant differences between WA Nuclear's clearance order process and WA Load Dispatcher's switching order process, i.e., TVAN's clearances do not require pulling all tags prior to restoring equipment — switching orders do.
Identification, discussions, and the addition of barriers has not been managed adequately to prevent errors during the performance of critical evolutions.
IV. ANALYSIS OF THE EVENT
At 1629 hours0.0189 days <br />0.453 hours <br />0.00269 weeks <br />6.198345e-4 months <br /> Central Standard Time on February 11, 2005, the Unit 3 reactor scrammed from 100% power. The scram was caused by a simultaneous false trip signal generated to the main generator circuit breaker 234, switchyard circuit breakers 5264 and 5268, and a main generator trip. The trip occurred when a PK block (disconnect device 26W), which had been pulled as part of a clearance for breaker 5264 was re-inserted as part of switching order No. 93 for returning the breaker to service. When the PK block was inserted (out of sequence for the switching order) the associated CT circuit was momentarily grounded resulting in a false differential. The correct sequence of the switching order was to actuate the trip cutout switches for the differential trip functions prior to inserting any of the PK blocks.
The generator trip resulted in a turbine trip and opening of the output breakers caused a power-load unbalance trip. The control valve fast closure caused the reactor SCRAM.
All rods inserted. Reactor water level lowered, as expected, and was recovered by normal feed water flow. All expected PCIS isolations, Group 2 (RHR S/D Cooling), Group 3 (RWCU), Group 6 (Ventilation), and Group 8 (TIP) were received along with the auto start of CREV and the three Standby Gas Treatment (SGT) trains.
All other plant responses, such as the responses of the feedwater level control system, turbine control system, etc. were as expected.
V. ASSESSMENT OF SAFETY CONSEQUENCES
The safety consequences of this event were not significant. Reactor scrams are an analyzed transient for which the plant is designed. Control rod insertion occurred as designed. Makeup water was recovered by normal feed water flow. The health and safety of the public was not affected by this event.
VI. CORRECTIVE ACTIONS
A. Immediate Corrective Actions
- Appropriate personnel action taken with involved personnel
- Interim Action - Operations Shift Manager to conduct the Pre-job Brief for switching orders
- Reinforce lessons learned from this event during startup simulator training.
C. Corrective Actions to Prevent Recurrence)
- Evaluate future switching order activities as potential high risk/loss of generation evolutions.
- Reinforce requirement to perform formal pre-job briefings for switching order activities.
- Reinforce differences between clearance order and switching order processes during operator training.
- Nuclear Assurance will perform observations on quality and rigor of pre-job briefings site-wide.
- Develop action plan that will address and improve defense-in-depth barriers for critical evolutions/activities.
- Develop a Nuclear Safety/Generation Worksheet as a tool to help identify activities that challenge nuclear safety and generation and identify additional barriers to prevent error/plant transients.
VII. ADDITIONAL INFORMATION
A. Failed Components
None B. Previous LERs on Similar Events None
C. Additional Information
Browns Ferry Corrective Action Program document PER 76599 D.�Safety System Functional Failure Consideration:
No safety functions were compromised as a result of this event. Therefore, this event is not considered a safety system functional failure in accordance with NEI-99-02 in that functional capability of the overall system was not jeopardized.
(1) TVA does not consider these corrective actions regulatory commitments. The completion of these actions will be tracked in TVA's Corrective Action Program.
E. Loss of Normal Heat Removal Consideration:
The condenser remained available, providing a normal heat removal path following the reactor scram. Accordingly, this event did not result in a Scram with a loss of Normal Heat Removal as defined in NEI 99-02.
VIII.COMMITMENTS None