05000277/LER-2014-001
| Peach Bottom Atomic Power Station Unit 2 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition |
| Initial Reporting | |
| ENS 50128 | 10 CFR 50.72(b)(3)(ii)(B), Unanalyzed Condition |
| 2772014001R00 - NRC Website | |
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Peach Bottom Atomic Power Station Units 2 and 3 14 001 00 Unit Conditions Prior to the Event Units 2 and 3 were both at approximately 100% power when this condition was discovered. There were no structures, systems or components out of service that contributed to this event.
Description of the Event
On May 21, 2014 at approximately 1100 hours0.0127 days <br />0.306 hours <br />0.00182 weeks <br />4.1855e-4 months <br />, it was determined that an unanalyzed condition existed that potentially impacted the ability to mitigate an Appendix R fire postulated to occur in the control room and cable spreading room (CSR). Broken wires leading to the alternate fuse for the 125vdc control power supply had been previously identified in the breaker enclosures for three 4 kV safety-related breakers (EIIS: BKR). Breakers for the following circuits were impacted:
- E-2 Emergency Diesel Generator (EDG) feed to the E-23 4kV emergency bus (referred to as the E-23 breaker)
Broken wires were identified in the E-23 breaker on March 6, 2014, and repaired during an E-2 EDG planned maintenance outage. During this time the EDG (EIIS: DG) was inoperable as allowed by Technical Specifications 3.8.1, Condition B (14 day completion time). To determine if the issue was isolated to the E-23 breaker, the decision was made to look at the wiring in the E-43 breaker during the next E-4 EDG planned maintenance outage. On May 8, 2014, broken wires were identified in the E-43 breaker and it was determined that additional inspections were appropriate to determine the extent of condition. A total of 13 breakers were inspected during non-maintenance outage periods and one additional breaker (the 3D RHR breaker) with broken wires was identified on May 21, 2014. Based on the information available from the three breakers, it was determined that it was likely that the broken wires existed prior to the maintenance outages. As a result, the breakers would not have been able to perform their intended function in the event of an Appendix R fire and, therefore, an unanalyzed condition existed.
Initial notification of this event was made to the NRC (EN# 50128) in accordance with 10CFR 50.72(b)(3)(ii)(B), as an unanalyzed condition that significantly degrades plant safety. This follow-up report is being made in accordance with 10CFR 50.73(a)(2)(ii)(B).
Analysis of the Event
Peach Bottom Fire Safe Shutdown Analysis 10CFR50 Appendix R, Section III.G requires one train of systems necessary to achieve and maintain hot shutdown conditions to be free of fire damage. Peach Bottom's Fire Protection Plan describes how this is accomplished for each specific fire area. Fire Area 25 includes the control room and the CSR. For the purposes of an Appendix R analysis, a fire in this area is conservatively assumed to affect all equipment in the entire fire area at time T=0, including producing short circuits that may affect equipment outside of the fire area. To ensure a safe shutdown can be achieved and maintained after a fire in Fire Area 25, an alternate shutdown capability is provided, which relies on Alternative Control Stations (ACSs). Each ACS includes transfer/isolation switches that are used to transfer control of the components from the control room to the ACS. The transfer/isolation switches also provide electric circuit isolation between alternate shutdown circuits and circuits in the control room that could be affected by a fire.
This event is associated with the emergency switchgear ACS. The 125vdc control power to these breakers is used for functions such as indication lights, alarms, interlocks, and to remotely open and close the breaker. The control power supply is protected by two primary 35 amp fuses. An alternate fuse is in parallel with the primary fuse and is connected to the circuit with the transfer/isolation switch.
In the event of an Appendix R fire in the control room and cable spreading room, with a fire-induced short circuit that results in a blown primary fuse, the broken wires would prevent the alternate fuse from being connected to the circuit, resulting in the loss of control power to the breaker. Along with the loss of indication lights on the front of the breaker enclosure, this would result in the loss of function of a handswitch on the front of the enclosure that is used to close the breaker. Post-fire safe shutdown procedures call for the use of this switch to power the 4kV emergency bus for the E-23 and E-43 breakers, and to power the 3D RHR pump for the 3D RHR breaker. The broken wires to the alternate fuse would therefore impact the ability to reach and maintain hot shutdown by the method described in the plant's fire safe shutdown analysis.
Fire Protection Features Assumptions used for the site analysis of an "Appendix R fire" (all equipment in the given fire area is lost and short circuits occur at time T=0) are not realistic, but are used in a bounding analysis to account for all fires that could occur in Fire Area 25. An actual fire in this area would need time to propagate, which is influenced by the following fire protection features:
1. Administrative control of transient combustible materials.
2. The use of non-combustible and fire resistant materials.
3. Electrical separation and fire barriers.
4. Detection and suppression including a trained fire brigade that is onsite at all times.
In the unlikely event that a fire occurs in the control room or CSR, its propagation rate would be limited such that it is likely that it would be detected in its early stage and extinguished. If not immediately extinguished and the decision is made to evacuate the control room, procedure SE-10, "Plant Shutdown from the Alternate Shutdown Panels," directs an operator to the ACS. Operation of the transfer/isolation switch prior to the existence of a short circuit would result in the primary fuse still being available, the switch would isolate the control room circuitry, and the broken wires would have no impact.
Ability to Manually Close Breaker or Replace the Primary Fuse If a short circuit exists when an operator arrives at the emergency switchgear room, the operator would have the option of replacing the primary fuse or manually closing the breaker. Manually closing the breaker is performed by opening the breaker enclosure door and pushing the prominently labeled manual close button. Procedure AO 54.2, "4kV Breaker Manual Operation," provides direction for manually closing the breaker. Although AO 54.2 was not referenced in SE-10, there is high confidence that an operator would successfully perform the simple alternate action of opening the enclosure door and pushing the manual close button. SE-10 has been revised to specifically reference AO 54.2.
Safety Significance
This event would not impact the ability to reach and maintain a safe shutdown condition in the event of an actual fire based on the following:
a) Separation of electrical panels along with fire barriers in the control room and the CSR reduce the likelihood that a single fire could affect multiple circuits.
b) The use of non-combustible and fire resistant material limits the initiation and spread of a fire, which provides adequate time for detection and suppression activities.
c) Given the importance of the control room and CSR, significant detection and suppression capabilities exist, including a fire brigade that is onsite at all times.
d) Operation of the transfer/isolation switch prior to the existence of a short circuit would result in the broken wires having no impact.
e) Manual action to close the breaker is a simple task for the operator to perform. After opening the breaker enclosure door, the manual close button is prominently labeled. This action is familiar to operators and there is high confidence that it would be successfully performed.
It is highly likely that at least one of the above factors would result in a positive outcome and the broken wires would have no impact on the post-fire safe shutdown of the plant.
Cause of the Event
The cause of the broken wires has been determined to be due to an installation error during a plant modification in 1985. Wires were installed between the transfer/isolation switch on the door of the breaker enclosure and to a terminal block inside of the enclosure. The wires were installed with a stress relief loop to allow the breaker enclosure door to be opened without causing localized stress in the wires. For the breakers with the failed wires, the stress relief loop was not properly secured during installation. As a result, when the enclosure door was opened, the wire flexed at its connection to a terminal block instead of within the loop. This caused localized stress in the wire when the enclosure door was opened and eventually caused the wire to fail.
Corrective Actions
Wires in the affected breakers were repaired or replaced. Other actions are being evaluated and performed as documented in the site's corrective action program.
Previous Similar Occurrences safely shutdown following an Appendix R fire due to a failed wire for the power supply to the High Pressure Coolant Injection (HPCI) ACS. It was determined that the broken wire occurred during maintenance activities performed on May 5, 2003 and was not detected during post maintenance testing.