05000391/LER-2023-001, Unanalyzed Condition Related to Loss of the 2A Emergency Diesel Generator During a Postulated Appendix R Fire
| ML23201A109 | |
| Person / Time | |
|---|---|
| Site: | Watts Bar  |
| Issue date: | 07/20/2023 |
| From: | Anthony Williams Tennessee Valley Authority |
| To: | Office of Nuclear Reactor Regulation, Document Control Desk |
| References | |
| WBL-23-031 LER 2023-001-00 | |
| Download: ML23201A109 (1) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition |
| 3912023001R00 - NRC Website | |
text
U.S. Nuclear Regulatory Commission WBL-23-031 Page 2 July 20, 2023
Enclosure: LER 391/2023-001-00, Unanalyzed Condition Related to Loss of the 2A Emergency Diesel Generator During a Postulated Appendix R Fire
cc (w/Enclosure):
NRC Regional Administrator - Region II NRC Senior Resident Inspector - Watts Bar Nuclear Plant NRC Project Manager - Region II
ENCLOSURE Tennessee Valley Authority Watts Bar Nuclear Plant Unit 2
LER 391/2023-001-00, Unanalyzed Condition Related to Loss of the 2A Emergency Diesel Generator During a Postulated Appendix R Fire
Abstract
On May 25, 2023 at 1345 Eastern Daylight Time (EDT), operations personnel were verbally notified that a fire barrier for area 737-A1B was not installed, and would render the 2A Emergency Diesel Generator (EDG) not operable in the event of a fire on the Unit 2 side of elevation 737 in the Auxiliary Building. The 2A EDG is the credited power source for fire safe shutdown for a fire located in this area. Without the credited source of power, this placed Watts Bar Nuclear Plant (WBN) Unit 2 in an unanalyzed condition. A fire watch was established in the area until the issue was resolved.
The cause is a legacy design issue which failed to recognize the Normal and Alternate Feeders share the same breaker on the associated 480V shutdown board (2-BD-212-A1-A and 2-BD-212-A2-A). Permanent corrective action by implementation of a design change was completed on June 10, 2023.The design change disconnected the Alternate Feeder Cable for the Diesel Auxiliary Boards from the associated 480V Shutdown Board. Therefore, a fire in fire area 737-A1B would no longer trip the common breaker and the 2A EDG would remain operable.
I. Plant Operating Conditions before the Event
Both Watts Bar Units 1 and 2 were at 100 percent Rated Thermal Power (RTP).
II. Description of Event
A. Event Summary
On May 25, 2023 at 1345 Eastern Daylight Time (EDT), operations personnel were verbally notified that a fire barrier for area 737-A1B was not installed, and would render the 2A Emergency Diesel Generator (EDG) [EIIS:DG] not operable in the event of a fire on the Unit 2 side of elevation 737 in the Auxiliary Building. The 2A EDG is the credited power source for fire safe shutdown for a fire located in this area. Without the credited source of power, this placed Watts Bar Nuclear Plant (WBN) Unit 2 in an unanalyzed condition.
This event is being reported to the NRC under 10 CFR 50.73(a)(2)(ii)(B) as an unanalyzed condition that significantly degraded plant safety.
B. Status of structures, components, or systems that were inoperable at the start of the event and that contributed to the event
None.
C. Dates and approximate times of occurrences
Dates and Occurrence Approximate Times 5/25/2023 at 1345 Operations personnel were notified a fire barrier was not EDT installed.
5/25/2023 at 1358 Condition Report 1858921 documents issue with loss of EDT 480v Diesel Auxiliary Boards 2A1-A and 2A2-A during an Appendix R fire.
5/25/2023 at NRC Event Notification 56540 submitted to NRC.
1701 EDT 6/10/2023 at 1338 WBN Units 1 and 2 implement modification WBN-23-034 to EDT address Appendix R issue associated with the DG Auxiliary Boards.
D. Manufacturer and model number of each component that failed during the event
N/A
E. Other systems or secondary functions affected
None.
F. Method of discovery of each component or system failure or procedural error
On May 25, 2023, WBN engineering personnel submitted CR 1858921 as a result of findings while performing a Fire Probabilistic Risk Assessment (PRA) for WBN.
G. The failure mode, mechanism, and effect of each failed component
No components failed associated with this event.
H. Operator actions
Operations instituted a fire watch following review of CR 1858921.
I. Automatically and manually initiated safety system responses
None.
III. Cause of the event
A. Cause of each component or system failure or personnel error
The cause is a legacy design issue which failed to recognize the Normal and Alternate Feeders share the same breaker on the associated 480V shutdown board (2-BD-212-A1-A and 2-BD-212-A2-A). Design Change 54192 issued in 2010 during Unit 2 completion rerouted and wrapped the Normal Feeder but did not include the Alternate Feeder.
No human performance cause is attributed to this event.
IV. Analysis of the event
A legacy issue was discovered in the Unit 1 and 2 Appendix R Safe Shutdown Analysis. This legacy issue stems from a resolution applied to the Normal Feeder Cables [EIIS:CBL] and Alternate Feeder Cables for the 480V Diesel Auxiliary Boards 2A1-A and 2A2-A (2-MCC-215-A001-A and 2-MCC-215-A002-A)[EIIS:SWGR]. The analysis failed to recognize the normal and
alternate feeders share the same breaker on the associated 480V shutdown board (2-BD-212-A001-A and 2-BD-212-A002-A). The resolution was applied to the Normal and Alternate Feeder Cables covering fire area 737-A1B (Unit 2 side of elevation 737). The resolution was incorrectly applied to the alternate cables with no protection in fire area 737-A1B. This results in an associated circuit that would result in loss of 480V Diesel Auxiliary Boards 2A1-A and 2A2-A by tripping the common breaker on a short to ground fault during a fire in 737-A1B. This would result in the loss of the 2A EDG, which is the credited power source for an Appendix R fire in this area supporting Unit 2 safe shutdown. In the event the 2A EDG was lost, the existing station batteries could support safe plant operation for several hours until a FLEX DG could be started and aligned to the 2A 6.9kV Shutdown Board (SDBD).
V. Assessment of Safety Consequences
For an Appendix R fire in the area of concern, Unit 2 would have lost offsite power and its credited onsite AC power source, the 2A EDG. Cooling for Unit 2 for this event is provided by the Turbine Driven Auxiliary Feedwater Pump (TDAFWP)[EIIS:P]. Motor operated valving for the Unit 2 TDAFWP is powered by station batteries. The plants response would therefore have been similar to that for a station blackout (SBO). Several hours would have been available to start and load an installed FLEX diesel to the 2A 6.9 kV SDBD normally powered by the 2A EDG, allowing for safe plant shutdown.
A. Availability of systems or components that could have performed the same function as the components and systems that failed during the event
No components failed as a result of this issue. In the event of an Appendix R fire in the location identified, WBN had the capability to load a FLEX diesel generator to the 2A 6.9kV SDBD.
B. For events that occurred when the reactor was shut down, availability of systems or components needed to shutdown the reactor and maintain safe shutdown conditions, remove residual heat, control the release of radioactive material, or mitigate the consequences of an accident
Appendix R events are analyzed with the plant at power, so this is not applicable.
C. For failure that rendered a train of a safety system inoperable, estimate of the elapsed time from discovery of the failure until the train was returned to service
Not applicable.
VI. Corrective Actions
This event was entered into the TVA Corrective Action Program and is being tracked under Condition Report 1858921.
A. Immediate Corrective Actions
Immediate corrective actions were to institute a fire watch until the identified issue was corrected.
B. Corrective Actions to Prevent Recurrence or to reduce the probability of similar events occurring in the future
Design change WBN-23-034 was implemented by Work Order (WO) 123783604 on June 10, 2023. The design change disconnected the Alternate Feeder Cable for the Diesel Auxiliary Boards from the associated 480V Shutdown Board. Therefore, a fire in fire area 737-A1B would no longer trip the common breaker. The normal feeder remains protected in this area.
VII. Previous Similar Events at the Same Site
Multiple Licensee Event Reports (LERs 390-2013-005-00, 390-2014-004-02, 390-2015-002-01, and 390-2015-004-01) were submitted to NRC in 2014 and 2015 for Unit 1 related to unanalyzed conditions associated with Appendix R. These issues were identified during the licensing of WBN Unit 2. This LER is the first item identified for WBN Unit 2.
VIII. Additional Information
None.
IX. Commitments
None.