05000259/LER-2012-009
| Browns Ferry Nuclear Plant (Bfn) | |
| Event date: | 10-31-2012 |
|---|---|
| Report date: | 12-28-2012 |
| 2592012009R00 - NRC Website | |
I. PLANT CONDITION(S)
On October 31, 2012, when the conditions were identified, Browns Ferry Nuclear Plant (BFN), Units 2 and 3, were in Mode 1 at approximately 100 percent rated thermal power.
The BFN, Unit 1, was in Mode 5 at zero percent rated thermal power.
II. DESCRIPTION OF EVENT
A. Event
On October 31, 2012, at approximately 1253 hours0.0145 days <br />0.348 hours <br />0.00207 weeks <br />4.767665e-4 months <br /> Central Daylight Time (CDT), as a result of the National Fire Protection Association (NFPA) 805 transition reviews, it was determined that in the event of an Appendix R fire in Fire Zone 01-03, 01-05, 02-03, 02-04, or 03-03, or Fire Area 16, the ability to provide power to equipment needed to achieve and maintain safe shutdown may be adversely impacted.
In certain fire zones and fire areas, feeder breakers [BKR] for the 480 Volt Shutdown Boards [ED] are credited for backup control operation using the emergency transfer switches [HS] which isolate the breaker controls from circuits going to the control bay and allow for local operation of the breaker. The emergency transfer switches were added to resolve Appendix R separation criteria issues. An error was discovered in crediting the backup control operation of the 480 Volt Shutdown Boards during postulated Appendix R fires in the above listed fire zones and fire area.
Fire damage to the Main Control Room [NA] 480 Volt Shutdown Board transfer switch cables [CBL] could cause the control circuit fuse [FU] for the credited breaker to clear prior to the use of the emergency switches used for backup control operation. This would inhibit the ability to isolate the 480 Volt Shutdown Board feeder breakers for circuits going to the control bay using the associated emergency transfer switches and thereby inhibit local operation of the breakers. Additionally, cable fire damage could also cause the normal or alternate feeder breakers or both to spuriously trip. Therefore, Safe Shutdown Instruction steps to use the emergency switches to perform local breaker operation to supply power to safe shutdown equipment may not work as written during a postulated Appendix R fire.
B. Inoperable Structures, Components, or Systems that Contributed to the Event There were no inoperable structures, components, or systems that contributed to the event.
C. Dates and Approximate Times of Major Occurrences Calendar Years 1987 to 1991� Appendix R Separation Analysis was performed for the BFN, Unit 2 restart effort.
Calendar Years 1991 to 1995� Appendix R Separation Analysis was performed for the BFN, Unit 3 restart effort, which included BFN, Unit 2, Appendix R Separation Analysis.
Calendar Years 2003 to 2007 Appendix R Separation Analysis was performed for the BFN, Unit 1 restart effort, which included BFN Units 2 and 3, Appendix R Separation Analysis.
October 31, 2012, at 1253 hours0.0145 days <br />0.348 hours <br />0.00207 weeks <br />4.767665e-4 months <br /> CDT It was determined that in the event of an Appendix R fire, the ability to provide power to equipment needed to achieve and maintain safe shutdown may be adversely impacted.
D. Other Systems or Secondary Functions Affected
There were no other systems or secondary functions affected.
E. Method of Discovery
These conditions were discovered during NFPA 805 transition reviews.
F. Operator Actions
There were no operator actions.
G. Safety System Responses
There were no safety system responses.
III. CAUSE OF THE EVENT
A. Immediate Cause
The immediate cause was a human performance error in the technical resolution of circuit failure postulated as a result of fire damage to control circuits for credited breakers.
B. Root Cause
The root cause was determined to be a legacy human performance error that occurred during the technical resolution of Appendix R fire protection analyses associated with cable separation and operator manual actions.
C. Contributing Factors
No contributing causes were identified.
IV. ANALYSIS OF THE EVENT
The Tennessee Valley Authority is reporting this condition in accordance with Title 10 of the Code of Federal Regulations 50.73(a)(2)(ii)(B), as any event or condition that resulted in the nuclear power plant being in an unanalyzed condition that significantly degraded plant safety.
The BFN, Units 1, 2, and 3, Appendix R Computerized Separation Analysis and an Appendix R Operator Manual Action (OMA) calculation credits operation of the 480 Volt Shutdown Boards during postulated Appendix R fires. In these analyses, an OMA using the emergency transfer switches is credited for isolating the 480 Volt Shutdown Board feeder breaker controls from circuits going to the control bay, allowing local operation of the breakers. However, the analysis does not address the potential for the control circuit fuse for the respective credited breakers clearing prior to the use of the emergency transfer switch, thereby preventing local operation of the breaker. Therefore, fire damage to the Main Control Room 480 Volt Shutdown Board transfer switch cables could inhibit the ability to perform the required actions in the Safe Shutdown Instructions.
As such, the OMA using the emergency transfer switches to ensure alternate local operation of the 480 Volt Shutdown Board feeder breakers does not resolve the Appendix R condition identified in the analyses.
To mitigate the chances of this potential inability for local breaker operation, an Appendix R Limiting Condition for Operation (LCO) was entered and a roving fire watch was established under a Fire Protection Impairment Permit.
The issues identified extend back to the initial recovery and start up of BFN, Units 1, 2, and 3 and identifies errors made with respect to the technical evaluations performed for OMAs using emergency transfer switches. It was determined that the deficient technical evaluations were caused by legacy human performance errors that occurred between 1987 and 2007.
The human performance error prevention process designed to identify and address these types of errors did not exist during the time of the failure evaluation. The inadequate analysis occurred in the 1987 to 2007 timeframe before the implementation of a program for technical human performance tools.
Since the time of the BFN, Units 1, 2, and 3 evaluations, BFN has made human performance program improvements to ensure that human errors are minimized to the maximum extent possible. The station implemented and maintains a human performance program and appropriate individuals are trained on the.use of human performance tools. Corrective actions for this event will improve the program for technical human performance tools.
Extent of Cause The extent of cause was determined to be other legacy programs or calculations that are knowledge intensive and are comparable to the significance of Appendix R Separation Analysis.
A snapshot self assessment will be performed in order to determine the overall health and the extent of the errors of the specified calculations or programs identified by the extent of cause. The self assessment will conduct a review of a cross section of calculations within each of the listed programs, review department human performance errors for the past five years to determine if they were associated with these programs or calculations, and conduct a review of the documented observations to determine the percentage that contain at-risk behaviors.
V. ASSESSMENT OF SAFETY CONSEQUENCES
For a fire that occurs in Fire Zone 01-03, 01-05, 02-03, 02-04, or 03-03, or Fire Area 16, the 480 Volt Shutdown Boards, 1A, 1B, 2A, 2B, 3A, and 3B are credited for Appendix R Fire Safe Shutdown. There are cables in these fire zones and fire area that are connected to the normal and alternate supply breakers in these boards. Therefore, a fire in any of these zones could result in a complete loss of power to the 480 Volt Shutdown Boards.
The Safe Shutdown Instructions for these Fire Zones contain tables for suggested alternate equipment in the event of credited equipment failure. Fire Zones 01-03, 01-05, 02-03, 02-04, and 03-03 in the BFN, Units 1, 2 and 3 Reactor Buildings have detection and automatic suppression to reduce the chances of a serious fire occurring and causing significant fire damage resulting in the loss of the 480V Shutdown Boards. Fire Area 16 (Control Building) has detection and automatic suppression in most areas and the Control Room is continually manned which will reduce the chances of a serious fire occurring and causing significant fire damage resulting from the loss of the 480V Shutdown Boards.
These 480V Shutdown Boards provide power to components required for safe shutdown. There are no alternate paths identified as available for recovering the potential loss of the 480V Shutdown Boards in the Safe Shutdown Instructions to ensure BFN, Units 1, 2, and 3, can be safely shutdown for an Appendix R fire. Based on the evaluation above, this condition could significantly challenge the ability to safely shutdown the affected Units during a fire event in the subject fire zones and fire area.
The credited 480V Shutdown Boards would potentially be unable to perform their required function. Therefore the condition is an unanalyzed condition for Appendix R that significantly degrades safety.
The general approach used to determine the risk associated with the referenced Appendix R compliance issue is a staggered approach. The issue is first evaluated to determine its impacts. If individual targets are assigned to the listed issue, these targets are queried in the Appendix R Safe Shutdown Analysis Software to determine the applicable fires that could damage the subject targets. The identified source ignition frequencies are then taken from the scoping fire modeling calculation performed in support of the BFN Fire Probabilistic Risk Assessment (PRA).
If the fire frequency that impacts the identified issue is less than 1.00E-4 per year, then the issue is determined to be less than red. If the issue is red, additional evaluations are performed.
The fire frequency for a given cable, component, basic event, or fire area is calculated based on three factors; the ignition source frequency, severity factor and non-suppression probability. The values for these factors are pulled from the weekly BFN Fire PRA Quantification report. Then, for each scenario identified, these factors are multiplied together. This calculation will result in the fire frequency for that scenario.
Once all scenarios are determined, each fire frequency is summed up to get the total fire frequency for that cable, component, basic event or fire area.
To prevent a loss of power to the 480 Volt Shutdown Boards, both cables are required.
Therefore, the failure of either cable could lead to a power loss. To identify the total final list of scenarios, the lists of scenarios for both cables are added together. The total list of scenarios can now be reduced by removing scenarios in fire areas that do not impact the 480 Volt Shutdown Board Breakers.
The total fire frequency for all these scenarios does not include manual suppression.
BFN has a mixture of non-qualified and qualified cables. The non-qualified cables which are routed in cable trays are protected with a flame-retardant coating; therefore possible cable damage due to a fire would be delayed. Additionally, fire growth and cable fire spread would provide time for manual suppression activities as well. A ten minute time to damage is assumed for the identified cables. The non-suppression probability for ten minutes is 0.3. The 0.3 was determined to be appropriate given this credit is applied to all fires without existing suppression credit. This includes transient, cable and fixed sources.
The fire frequencies that contain the subject cables are less than 1.00E-4 per year.
Therefore, these conditions are considered to be less than red.
VI. CORRECTIVE ACTIONS - The corrective actions are being managed by TVA's corrective action program.
A. Immediate Corrective Actions On October 31, 2012, an Appendix R LCO was entered and a roving fire watch was established under a Fire Protection Impairment Permit.
B. Corrective Actions
1. A Design Change Notice (DCN) will be initiated to replace the BFN, Units 1 and 2, Appendix R credited breakers with new breakers that will provide an alternate for local breaker operation in the event of an Appendix R fire in the subject Fire Zones and Fire Area. Local breaker operation will be available through the use of the readily accessible manual spring charging and mechanical breaker trip or close capability on the new breakers.
Implementation of the DCNs to replace the credited breakers on BFN, Units 1 and 2 will result in a revision to the associated Appendix R Computerized Separation Analysis calculation, the associated Appendix R OMA calculation and the associated Safe Shutdown Instruction steps.
2. A Post Issuance Change will be issued to incorporate the revision to the associated Appendix R Computerized Separation Analysis, the associated Appendix R OMA calculation and the associated Safe Shutdown Instruction steps for BFN, Unit 3, to utilize the manual spring charging and mechanical breaker trip or close capability on the new Appendix R credited breakers that were replaced in 2012 by a previous DCN.
C. Corrective Actions to Prevent Recurrence
The corrective action to prevent recurrence is to revise the human performance tools procedure to incorporate the five Institute of Nuclear Power Operations Technical Conscience Principles, focus technical task risk factors, mitigation strategies, and decision making.
VII. ADDITIONAL INFORMATION
A. Failed Components
There were no failed components.
B. Previous Similar Events
A search of BFN LERs for Units 1, 2, and 3 for approximately the past five years identified the following similar events involving potential adverse plant conditions due to fire induced circuit damage that were discovered during NFPA 805 transition review. These similar events are a result of legacy issues with Appendix R Separation Analysis and are being discovered due to the ongoing NFPA 805 transition, and as such, the corrective actions in place are still being implemented.
1. LER 50-259/2012-001-00, Unanalyzed Conditions Discovered during NFPA 805 Transition Review 2. LER 50-259/2012-002-00, Fault Propagation during a Postulated Appendix R Event could result in an Inability to Close Motor Operated Valves 3. LER 50-259/2012-003-00, Reactor Protection System Circuit could Potentially Remain Energized during an Appendix R Fire 4. LER 50-259/2012-004-00, Fire Damage to Cables in Fire Areas Could Cause a Residual Heat Removal Service Water Pump to Spuriously Start 5. LER 50-259/2012-007-00, Cable Routing Error Found in the Appendix R Separation Analysis 6. LER 50-260/2012-005-00, Unanalyzed Conditions Discovered during National Fire Protection Association 805 Transition Affecting Division II of the Residual Heat Removal System A search was performed on the BFN corrective action program. The previous problem evaluation reports (PERs) associated with the above similar LERs are PERs 229734, 245385, 259787, 424389, 503304, 521739, 561101, 577583, 626885, and 626886.
Additional similar PERs related to this condition are PERs 358612, 405118, 409312, 422371, 493807, and 499047.
C. Additional Information
The corrective action document for this report is PER 635230.
D. Safety System Functional Failure Consideration
In accordance with NEI 99-02, these conditions are not considered a safety system functional failure.
E. Scram With Complications Consideration
These conditions did not include a scram.
VIII. COMMITMENTS
There are no commitments.