05000482/LER-2006-001
| Docket Numbersequential Revmonth Day Year Year Month Day Yearnumber No. 05000 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition |
| 4822006001R00 - NRC Website | |
Plant Conditions Prior to the Event:
MODE -1 . Power — 100 percent Normal Operating Temperature and Pressure
Event Description:
NRC Information Notice (IN) 92-18, "Potential For Loss Of Remote Shutdown Capability During A Control Room Fire," requested licensees to review their facilities for the potential of a control room fire causing damage to motor operated valve (MOV) control circuits. Based on the IN, a fire in the control room could cause hot shorts that would energize the open or close relays and bypass the torque switch, such that the torque switch would be ineffective in protecting the valve and motor against mechanical damage.
IN 92-18 was evaluated by Wolf Creek Nuclear Operating Corporation (WCNOC) in 1992. The condition was determined to not be applicable to Wolf Creek Generating Station (WCGS) based on the original license basis. The WCGS response plan for a fire in the control room, as presented to the NRC prior to licensing, addressed spurious signals and hot shorts through the use of isolation switches, cutting wires, opening disconnects, removing arid replacing fuses, installation of jumpers and the manual realignment of selected valves. The evaluation cone uded no action needed to be taken at that time.
In 1999, the NRC raised questions during an inspection at another nuclear plant regarding their response to the IN.
This question concerned the potential for valve failure prior to manipulation of the isolation switches. WCNOC initiated a corrective action document to re-evaluate the concerns raised in IN 92-18. At the time, the industry and the NRC were involved in an initiative to determine the potential for the specified hot short cable failure to occur in the event of fire. WCNOC evaluated that it was acceptable to hold decisions on modifications until the industry/NRC initiative was complete.
During the NRC Triennial Fire Protection Inspection in November 2005, a concern was identified on how IN 92-18 was addressed by WCNOC. While evaluating that concern on February 10, 2006, it was identified that in the event of a fire in the control room, 40 motor operated valves (MOVs) credited for post-fire safe shutdown could potentially fail in an unanalyzed condition. Of those 40 MOVs, failure of 8 of the valves could potentially prevent achieving and maintaining safe shutdown conditions.
In the 40 MOV circuits, an intra-cable hot short between one conductor on the hot side of the indication circuit and another conductor on the load side of the control room hand switch could bypass the torque switch and energize either the open or close coil. If this occurs, the open or close contactor will close and the motor will operate n either the open or close direction until the motor stalls, possibly resulting in damage to the valve such that it cannot be manually operated.
Basis for F:eportability:
10CFR50 Appendix R Section III.L.7 requires "The safe shutdown equipment and systems for each fire area shall be known to be isolated from associated non-safety circuits in the fire area such that hot shorts, open circuits, or shorts to ground in the associated circuits will not prevent operation of the safe shutdown equipment." Based on this information, WCNOC made an eight hour Emergency Notification System call in accordance with 10 CFR 50.72(b)(3) (ii)(B)
- This condition is also reportable pursuant to 10 CFR 50.73(a)(2)(ii)(B) for any event or condition that resulted in the nuclear power plant being in an unanalyzed condition that significantly degraded plant safety.
Root Cause:
Less than adequate management oversight and monitoring resulted in inadequate organizational response to correcting latent design deficiencies.
Corrective Actions:
An hourly fire watch was established in the Control Room due to a previous condition identified on November 16, 2005. This condition was included on the hourly fire watch.
Engineering will complete a design modification on the 8 valves to prevent fire-induced spurious actuation prior to plant startup following Refueling Outage 16.
The potent al spurious valve operation, due to a fire, will be reviewed for the remaining thirty-two valves to determine if additional procedure changes are required to provide long-term corrective measures or whether a design modificatioi is required to addreks the torque switch concern. This will be completed by December 31, 2006 Based on that review,4engineering will complete any required design modification(s) for the remaining valves to address the torque switch concern prior to plant startup following Refueling Outage 16.
Safety Significance:
The safety significance of this deficiency is minimized by the very low probability of the "hot short" circuit failure mode occurring for the duration necessary to cause valve damage with no other interrupting circuit failures, such as shorts to ground or open circuits, occurring. In addition, the likelihood of a fire in the Control Room, which is contirmusly manned, of sufficient magnitude to cause such cable damage is extremely unlikely.
Present system operability is not affected as there has been no occurrence of a fire in the Control Room and - compensatory actions are in place to detect and mitigate the effects of a fire in the Control Room.
Operating Experience/Previous Events:
None.