05000255/LER-2010-003
| Palisades Nuclear Plant | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition |
| 2552010003R00 - NRC Website | |
At the time of discovery, the plant was coasting down in power in preparation for a refueling outage scheduled to start on October 3, 2010. Approximate power level was 92%.
EVENT DESCRIPTION
On October 1, 2010, during a corrective action program extent of condition review, a postulated Appendix R fire scenario was identified in three fire areas that could potentially result in the loss of safety-related 2400 volt alternating current (VAC) bus 10 and/or bus 1D, with subsequent loss of equipment credited for Appendix R compliance to support safe shutdown in the event of such a fire. As part of the Appendix R common power supply analysis, coordination between load breakers and feeder breakers is required to protect the power supply because of a fire-generated fault on the load power cable. The fire scenario identifies how this coordination could be defeated on the safety-related 2400 VAC buses. The scenario occurs .when the load breaker's control circuit is damaged by a fire which could potentially open the control circuit fuses and disable the breaker's trip circuit containing the over-current relay protection. The same fire could subsequently damage the 2400 VAC power cables, causing a cable fault. The load breaker cannot clear the fault with the trip circuit control power disabled. The clearing of the cable fault would propagate upstream to the next coordinated breaker, which would result in the bus feeder breaker opening, causing a loss of the 2400 VAC buses and associated electrical loads.
The three fire areas are 1-C switchgear room, screenhouse room and component cooling water pump room. The breakers having control cables routed in the same fire areas as the associated power cables are 152-103 (service water pump P-7B), 152-205 (service water pump P-7C), and 152-208 (component cooling water pump P-52B).
Breaker 152-103, having both control and power cables present in the screenhouse room, fire area (FA) 9, has potential to result in the loss of 1C bus for a fire in the screenhouse room. The Appendix R safe shutdown analysis credits both 1C and 1D buses remaining available for a fire in FA9. Breaker 152-205, having both control and power cables present in the screenhouse room (FA9) and in the 1C switchgear room (FA4), has potential to result in the loss of 1C and 1D buses for a fire in the screenhouse room or the 1C switchgear room. The Appendix R safe shutdown analysis credits both 10 and 1D buses remaining available for a fire in FA9 and credits 1D bus remaining available for a fire in FA4. Breaker 152-208, having both control and power cables present in the component cooling water room (FA16) has the potential to result in the loss of 1D bus for a fire in the component cooling water room. The Appendix R safe shutdown analysis credits both 1C and 1D buses remaining available for a fire in FA16.
U.S. NUCLEAR REGULATORY COMMISSION
CONTINUATION SHEET
Inadequate procedural guidance and the failure to sufficiently implement operating experience precluded the identification of the hypothetical failure.
In 1995, as part of the Appendix R Program Enhancement Plan, analysis and identification of postulated failures was performed. The hypothetical failure mechanism did not consider the specific sequence of events that had to occur to cause the failures. The guidance in 1995 resulted in electrical coordination reviews that identified failures to control cables and the resultant effects, and failures to power cables and the resultant effects.
In March, 2010, a focused self assessment was performed to verify the adequacy of safe shutdown component selection and associated circuit analysis regarding assumptions, methodology, and criteria applied to the safe shutdown equipment and cables, considering the potential for multiple concurrent spurious actuations. During the focused self assessment, it was determined that should both control cable and power cable for safe shutdown'equipment be routed in the same fire area, a failure could prevent credited safe shutdown bus/es from performing their safe shutdown function. An extent of condition review was initiated to determine the configuration of control and power cabling for both safe shutdown and non-safe shutdown equipment that could adversely affect safe shutdown pathways.
On October 1, 2010, during the extent of condition review, a postulated Appendix R fire scenario was identified in three fire areas that could potentially result in the loss of safety-related 2400 VAC bus 1C and/or bus 1D, with subsequent loss of equipment credited for Appendix R compliance to support safe shutdown in the event of such a fire.
CORRECTIVE ACTIONS TAKEN
°Hourly fire watch tours were implemented as compensatory measures in the 1C switchgear room, screenhouse room and component cooling water room.
- A standing order was issued to immediately sound the fire alarm and call out the fire brigade for a fire alarm indicating a fire in any of the three affected fire areas.
CORRECTIVE ACTIONS TO BE TAKEN
- Determine if the condition is allowed to be addressed as part of the National Fire Protection Association (NFPA) Standard 805 Transition project and disposition as appropriate.
- Revise plant procedure EGAD-EP-08, "Appendix R Fire Safe Shutdown Analysis," to ensure consistency with current industry guidance for circuit analysis referencing Nuclear Energy Institute (NEI) 00-01, "Guidance for Post Fire Safe Shutdown Circuit Analysis.
suppression systems, hourly fire watch tours and a trained on-site fire brigade, makes it highly unlikely that a credible fire would result in the circuit failure combinations in the required sequence for the hypothetical scenarios to occur. The hourly fire watch tours and immediate call out of the on-site fire brigade provide compensatory measures to promptly identify and extinguish a fire to minimize the fire impact and provide assurance that the identified scenarios will not jeopardize post-fire safe shutdown capability during the interim time frame prior to implementation of actions required for permanent issue resolution.
PREVIOUS SIMILAR EVENTS
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