05000327/LER-2009-001
| Sequoyah Nuclear Plant Unit 1 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v), Loss of Safety Function 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition |
| 3272009001R00 - NRC Website | |
I. PLANT CONDITION(S)
Unit 1 and Unit 2 were operating at 100 percent power.
II. DESCRIPTION OF EVENT
A. Event:
On January 28, 2009, a review of the load breaker cables for the 6900-volt shutdown boards [EIIS Code EA] identified two fire areas where a postulated fire scenario could cause a loss of the shutdown board that is credited as part of the Appendix R safe shutdown strategy.
A postulated fire scenario in the elevation 669 Auxiliary Building corridor and the elevation 690 Auxiliary Building General Area could cause the breaker coordination to be defeated on the 6900-volt shutdown boards. Inside these two fire areas, the power cable is routed in close proximity to the 125-volt direct current (DC) [EIIS Code EJ] control circuit cable for the local control handswitch for pumps fed from the 6900 volt shutdown boards.
This event requires a complex sequence of events for a loss of the electrical board.
First, the load breaker must be closed and in most cases a spurious actuation of the control circuit cable must close the breaker. Second, the trip circuit fuses must be blown/opened such that over-current protection is defeated for the load breaker. Then the power cable must be damaged to cause an over-current fault. The feeder breaker opens to clear the fault and the 6900-volt shutdown board and electrical loads fed from that bus are lost.
B. Inoperable Structures, Components, or Systems that Contributed to the Event:
None.
C. Dates and Approximate Times of Major Occurrences:
2000-2001 SQN transitioned from a diagnostic to a prescriptive analysis for Appendix R safe shutdown strategy.
January 28, 2009 During an audit of the SQN Fire Protection Program, a postulated fire scenario was identified where the 6900V-shutdown board could potentially be lost that affects safe shutdown equipment in two fire areas.
January 28, 2009SSQN entered Fire Protection Report Limited Condition for at 1425 EST' Operation 3.7.12 for inoperable fire barriers and established hourly fire watches in the two areas affected.
D. Other. Systems or Secondary Functions Affected:
No other systems or secondary functions were affected by this condition.
E. Method of Discovery:
The condition was discovered during an audit of the SQN Fire Protection Program.
F. Operator Actions:
No operator actions were required.
G. Safety System Responses:
Not applicable - no safety system response was required.
CAUSE OF THE EVENT
A. Immediate Cause:
The immediate cause of the condition was failure to adequately protect the 6900 volt shutdown board from the effects of a postulated Appendix R fire.
B. Root Cause:
The cause of the event was that this fire scenario was not considered when SQN transitioned from a diagnostic to a prescriptive analysis for Appendix R safe shutdown strategy. The division of responsibilities between Engineering groups during the transition led to not performing adequate circuit analysis resulting in the load breaker and handswitch cable interaction not being adequately evaluated. There was not clear program ownership of the Fire Protection Program at the time of the transition.
IV.�ANALYSIS OF THE EVENT The cable issue identified at SQN during a Nuclear Assurance audit can be summarized as a potential cable interaction issue. The design change to remove this unanalyzed condition lifts the leads to the local handswitch to eliminate the potential cable interaction.
The original analysis of the local handswitch circuits did not address this potential cable interaction issue. An evaluation determined that cables in the control circuit trays (no separation) have the necessary neutral to cause the failure. These cables were not protected from the potential cable to cable faults in the trays.
A single cable fault will not impact the Appendix R safe shutdown strategy.
Initially, the pump(s) must be running (or spuriously started) that requires the breaker to be closed. Next, a cable to cable fault must blow the control fuse (disable the over-current protection) before tripping the breaker. Finally, the power cable for this breaker must have a phase to phase fault causing an over-current condition on the 6900-volt shutdown board.
This sequence will cause a loss of the 6900-volt shutdown board and electrical loads fed from that bus.
Plant cable routing methodology places the power cable trays very close to the ceiling of the room and one to three feet above the control cable trays. Failure of the power cable prior to the control circuit/over-current protection would prevent loss of the 6900-volt shutdown board.
Because of the vertical cable locations, a hot gas layer cannot cause the required sequence of failures to affect the board.
The identified condition requires specific, sequential, multiple cable faults. The likelihood of occurrence of this scenario is very small. Therefore, for most fire scenarios in the two subject fire areas the safe shutdown strategy would be unaffected.
The two subject fire areas have full area automatic detection and suppression. Additionally, SQN has a full onsite fire department. These two areas have low to moderate in-situ combustible loading and the site has implemented strict transient combustible control and housekeeping programs. The likelihood that a floor level Auxiliary Building fire could progress to encompass the multiple cable trays, sequentially including ceiling level power trays as described in the analysis above is very small.
Because of the factors influencing this analysis, it can be concluded that the level of effectiveness and reliability for the safe shutdown strategy is essentially unchanged. The implementation of the design change to remove the local handswitch circuit will restore the level of effectiveness and reliability for the safe shutdown strategy.
V. ASSESSMENT OF SAFETY CONSEQUENCES
Based on the above "Analysis of The Event," this event did not adversely affect the health and safety of plant personnel or the general public.
VI. CORRECTIVE ACTIONS
A. Immediate Corrective Actions:
Compensatory measures (fire watches) have been initiated in these two fire areas.
. Corrective Actions:
A modification has been initiated to remove the local control switch portion of the circuit to resolve this potential fire-related failure.
In addition, to ensure all fire scenarios are appropriately considered during various activities, the Fire Protection Program is being revised to clarify owner responsibilities.
VII. ADDITIONAL INFORMATION
A. Failed Components:
None.
B. Previous LERs on Similar Events:
A review of previous reportable events did not identify any previous similar events at SQN. The review of previous reportable events identified that this is an industry issue, which was previously identified at the Point Beach Nuclear Plant.
C. Additional Information:
None.
D. Safety System Functional Failure:
This event did not result in a safety system functional failure in accordance with 10 CFR 50.73(a)(2)(v).
E. Unplanned Scram with Complications:
This condition did not result in an unplanned scram with complications.
VIII. COMMITMENTS
None.