05000260/LER-2012-005
| Docket Number Sequential Revmonth Day Year Year Month Day Yearnumber No. Bfn, Unit 3 05000296 | |
| Event date: | 10-18-2012 |
|---|---|
| Report date: | 12-17-2012 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition |
| 2602012005R00 - NRC Website | |
I. PLANT CONDITION(S)
On October 18, 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.
II. DESCRIPTION OF EVENT
A. Event
On October 18, 2012, at approximately 1314 hours0.0152 days <br />0.365 hours <br />0.00217 weeks <br />4.99977e-4 months <br /> Central Daylight Time (CDT), as a result of the National Fire Protection Association (NFPA) 805 transition reviews, it was determined that the current BFN Appendix R analysis for BFN, Units 2 and 3 does not adequately evaluate fire induced circuit damage.
In the event of an Appendix R fire, fire induced circuit damage could potentially create two conditions, an open circuit or a hot short, that would prevent the BFN, Units 2 and 3, Division II of the Residual Heat Removal System [BO], Low Pressure Coolant Injection (LPCI) inboard valves [FCV] from opening.
First, an open circuit on conductor 2C0 or 2C01 caused by fire damage to BFN, Unit 3 cable [CBL] 3ES3027-11, routed in Fire Zones 03-01 and 03-03, or cable 3ES3028-11, routed in Fire Zone 03-01, could prevent the valve open contactor from energizing which will inhibit the ability to open the credited BFN, Unit 3 Low Pressure Coolant Injection (LPCI) inboard valve, 3-FCV-074-0067.
Second, a hot short to conductor 2CC caused by fire damage to BFN, Unit 3 cable 3ES3027-11, routed in Fire Zones 03-01 and 03-03, or cable 3ES3028-11, routed in Fire Zone 03-01, could spuriously close the BFN, Unit 3 LPCI inboard valve, 3-FCV-074-0067, in a way that by-passes the close limit switch or torque switch.
This would allow the valve to over thrust and jam the valve closed to the point where the valve could not be opened either electrically or mechanically with the hand wheel. Therefore, fire damage to the cables discussed could result in a failure to open the credited LPCI inboard valve as written in Safe Shutdown Instruction, 0-SSI-3-1 or 0-SSI-3-3.
Similarly, a hot short to conductor 2CC caused by fire damage to BFN, Unit 2 cable 2ES3907-II, routed in Fire Zones 02-01 and 02-04, or cable 2ES3028-11, routed in Fire Zone 02-01, could spuriously close the BFN, Unit 2 LPCI inboard valve, 2-FCV-074-0067, in a way that by-passes the close limit switch or torque switch.
This would allow the valve to over thrust and jam the valve closed to the point where the valve could not be opened either electrically or mechanically with the hand wheel. Therefore, fire damage to the cable could result in a failure to open the credited LPCI injection valve as written in Safe Shutdown Instruction, 0-551-2-1 or 0-SSI-2-4.
On October 18, 2012, Appendix R Limiting Conditions for Operation (LCO) were established and a roving fire watch was established under Fire Protection Impairment Permits to mitigate each of the described conditions.
On November 30, 2012, a Design Change Notice was initiated to resolve the routing issues for cables 2ES3028-11, 2ES3097-11, 3ES3027-11, and 3ES3028-11 in Fire Zones 02-01, 02-04, 03-01, and 03-03.
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.
October 18, 2012, at 1314 hours0.0152 days <br />0.365 hours <br />0.00217 weeks <br />4.99977e-4 months <br /> CDT It was determined that in the event of an Appendix R fire, fire induced circuit damage could potentially prevent the BFN, Units 2 and 3, l_PC1 inboard valves from opening or could cause the valves to spuriously close.
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 an error associated with the resolution of cable routing associated with cables 3ES3027-11, 3ES3028-11, 2ES3097-11, and 2ES3028-11.
B. Root Cause
The root cause was determined to be legacy human performance errors that occurred during the resolution of Appendix R cable routing failures associated with cables 3ES3027-11, 3ES3028-11, 2E53907-11, and 2ES3028-11. The human performance error prevention process designed to identify and address these types of errors was not in existence during the time of the Appendix R evaluation.
C. Contributing Factors
No contributing causes were identified.
IV. ANALYSIS OF THE EVENT
The Tennessee Valley Authority is reporting these conditions in accordance with 10 CFR 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, Unit 3 LPCI inboard valve, 3-FCV-074-0067, is credited in Fire Zones 03-01 and 03-03. The associated Appendix R cable, 3ES3027-11, is routed in Fire Zones 03-01 and 03-03. Similarly, the associated Appendix R cable, 3ES3028-11, is routed in Fire Zone 03-01. The Appendix R Computerized Separation Analysis recognizes the above listed cables are routed in these Fire Zones and the fire failures are resolved in the analysis with a manual action to use the Appendix R Emergency Switch [HS], 3-HS-074-0067C, to open the valve locally at the 480V Reactor Motor-Operated Valve (RMOV) Board 3E [ED]. However, in the event of an Appendix R fire, fire induced circuit damage could potentially cause two conditions, an open circuit or a hot short, that may affect the opening of valve 3-FCV-074-0067.
First, the current analysis for BFN, Unit 3 does not take into consideration that the emergency switch circuit does not by-pass the potential cable damage to conductors 2C0 and 2C01 from limit switch 4 in cables 3ES3027-11 and 3ES3028-11. Limit switch 4 breaks open the contactor circuit when the valve has stroked full open. An open circuit on conductor 2C0 or 2C01 would prevent the valve open contactor from energizing to electrically open the valve 3-FCV-074-0067.
Second, the current analysis for BFN, Unit 3 does not take into consideration that a hot short to conductor 2CC, in cable 3ES3027-11 or 3ES3028-11, could spuriously close valve 3-FCV-074-0067 in a way that by-passes the close limit switch or torque switch. This may allow the valve to over thrust and jam the valve closed to the point where the valve could not be opened either electrically or mechanically.
Therefore an open circuit or hot short caused by fire damage to these cables could result in a failure to open the credited LPCI inboard valve using the emergency switch as written in Safe Shutdown Instruction, 0-551-3-1 or 0-SSI-3-3.
The BFN, Unit 2 LPCI inboard valve, 2-FCV-074-0067, is credited in Fire Zones 02-01 and 02-04. The associated Appendix R cable 2ES3907-II is routed in Fire Zones 02-01 and 02-04. Similarly, the associated Appendix R cable 2ES3028-11 is routed in Fire Zone 02-01. The Appendix R Computerized Separation Analysis recognizes the above listed cables are routed in these Fire Zones and the fire failures are resolved in the analysis with a manual action to use the Appendix R Emergency Switch, 2-HS-074-0067C, to open the valve locally at the 480V RMOV Board 2E. However, in the event of an Appendix R fire, fire induced circuit damage could potentially cause a hot short that may affect the opening of valve 2-FCV-074-0067.
The current analysis for BFN, Unit 2 does not take into consideration that a hot short to conductor 2CC, in 2ES3907-II or 2ES3028-11, could spuriously close the valve in a way that by-passes the close limit switch or torque switch. This may allow the valve to over thrust and jam the valve closed to the point where the valve could not be opened either electrically or mechanically. Therefore, fire damage to these cables could result in a failure to open the credited LPCI injection valve using the emergency switch as written in Safe Shutdown Instruction, 0-551-2-1 or 0-SSI-2-4.
To mitigate the chances for each of the above potential conditions, Appendix R LCOs have been established and a roving fire watch was established under Fire Protection Impairment Permits.
The issues identified extend back to the initial recovery and start up of BFN, Units 2 and 3 and identifies errors made with respect to the technical evaluations performed on cable separation to meet Appendix R requirements. The human performance error prevention process designed to identify and address these types of errors was not in existence during the time of the Appendix R evaluation.
Since the time of the BFN, Units 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 will improve the program for technical human performance tools.
Extent of Condition The extent of condition is the Appendix R computerized separation resolutions for cable separation issues in BFN, Units 1, 2, and 3.
The actions to address the extent of condition will include validation that the analysis of identified cable separation issues is adequate for all three units using a graded approach, which is based on risk and consequence. This approach will identify the resolutions of cable separation issues in the Appendix R Computerized Separation Analysis that result in operator manual actions (OMAs) of 25 minutes or less.
This will be accomplished using a sampling plan to perform a technical review of a sample of cable separation issues in the Appendix R Computerized Separation Analysis that result in an OMA of 25 minutes or less with bias toward those OMAs performed outside the Main Control Room [NA] or Auxiliary Instrument Rooms [NF].
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 indentified 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
The Safe Shutdown Instructions for Fire Zones 02-01, 02-04, 03-01, and 03-03 contain tables for suggested alternate equipment in the event of credited equipment failure.
These four Fire Zones have detection and automatic suppression [KP] to reduce the chances of a serious fire occurring which could cause significant fire damage resulting in cable damage affecting the valves.
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. These identified source ignition frequencies are then taken from the scoping fire modeling calculation performed in support of the BFN Fire Probabilistic Risk Assessment (PRA). This evaluation is limited to the zones which the subject issues adversely impacts the credited Appendix R safe shutdown strategy.
If the creditable fire frequency that impacts the identified issue is less than 1.0E-4 per year, then the issue is determined to be less than red. If the issue is red, additional evaluations are performed. Currently, the scoping fire modeling for BFN is conservative, given it does not generally consider individual targets. Therefore, considerations of fire frequency reduction techniques are limited.
The BFN, Unit 3 cables 3ES3027-11 and 3ES3028-11, and Unit 2 cables 2ES3907-II and 2ES3028-11 were queried in the Appendix R Safe Shutdown Analysis software to determine the fire scenarios which contain the subject cables. The tables included the mitigation credits that are in the current BFN fire PRA analysis. These include fire severity, probability of the fire propagation outside the cabinet and manual suppression.
No credit is given to manual suppression for the risk evaluation. The fire frequency that contains the subject cables is less than 1.0E-4 per year. Therefore, this condition is considered to be less than red.
The risk impact is significantly lower, if additional credit is given for operator actions to use alternate means to achieve the safety function.
However, there are no alternate paths identified as available for the BFN, Unit 2 LPCI inboard valve, 2-FCV- 074-0067, or the BFN, Unit 3 LPCI inboard valve, 3-FCV-074-0067, in the Safe Shutdown Instructions to ensure the Units 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 Fire Zones 02-01, 02-04, 03-01 and 03-03. The credited LPCI inboard valves would potentially be unable to perform their required function for Fire Zones 02-01, 02-04, 03-01 and 03-03 during Appendix R fires. Therefore, these conditions are unanalyzed conditions that significantly degrade safety.
VI. CORRECTIVE ACTIONS - The corrective actions are being managed by TVA's corrective action program.
A. Immediate Corrective Actions On October 18, 2012, Appendix R LCOs were established and a roving fire watch was established under Fire Protection Impairment Permits.
On November 30, 2012, a Design Change Notice was initiated to resolve the routing issues for cables 2ES3028-11, 2ES3097-11, 3ES3027-11, and 3ES3028-11 in Fire Zones 02-01, 02-04, 03-01, and 03-03.
B. Corrective Actions to Prevent Recurrence The corrective actions to prevent recurrence are to implement a fleet wide technical human performance procedure and revise the human performance tools procedure to incorporate the five INPO 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 spurious operation of plant equipment due to fire induced circuit damage that was discovered during NFPA 805 transition. 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 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. The similar PERs related to this condition are PERs 358612, 405118, 409312, 422371, 493807, and 499047.
C. Additional Information
The corrective action documents for this report are PERs 626885 and 626886.
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.