05000282/LER-2009-008
| Prairie Island Nuclear Generating Plant Unit 1 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(A), Loss of Safety Function - Shutdown the Reactor 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition |
| Initial Reporting | |
| ENS 45567 | 10 CFR 50.72(b)(3)(ii)(B), Unanalyzed Condition |
| 2822009008R00 - NRC Website | |
FACILITY NAME (1) � DOCKET NUMBER (2) LER NUMBER (6) � PAGE (3) � 05000 282 YEAR 09 -- 008 -- 0
EVENT DESCRIPTION
On December 11, 2009, Unit 1 was operating at 100 percent power. It was identified that fire induced damage to cable 1C-333 in FA32 could cause a spurious lockout of the 1RY transformer', the alternate offsite power source to Safeguards Bus 162, which is credited to power equipment required to achieve safe shutdown in the event of a fire in Fire Area (FA) 32. Prior to this time, cable 1C-333 was not included in the FA32 analysis. It was noted that cables for the D2 emergency diesel generator3 are also routed though FA32, but the tray that they are routed through is protected with a thermal shield as approved in a NRC fire protection safety evaluation report dated May 4, 1983. Therefore a temporary procedure change was initiated to credit the D2 emergency diesel generator as a source to Bus 16.
On December 14, 2009 it was identified that cables for the D2 Emergency Diesel Generator exit the protected tray and run through the ceiling unprotected for approximately four feet in FA32. An automatic wet pipe sprinkler system is installed in the area with fire detection but there is not twenty feet free of intervening combustibles between the redundant power supplies to Bus 16 in FA32 as required by 10 CFR 50 Appendix R Section III.G.2.b. Because the required degree of separation between power supplies to Bus 16 and the redundant safeguards train (Bus 15) was found to be missing, an 8-hour report (event number 45567) was made in accordance with 10 CFR 50.72.
EVENT ANALYSIS
Fire induced cable damage to cable 1C-333 in FA32 could cause a spurious lockout of the 1RY transformer, the alternate offsite power source to Safeguards Bus 16, which is credited to power equipment required to achieve safe shutdown in the event of a fire in FA32. Because a fire in FA32 could damage cables for Bus 16 and Bus 15, both trains of safe shutdown equipment, this condition is reportable as an unanalyzed condition under 10 CFR 50.73(a)(2)(ii)(B).
Without compensatory measures in place, this condition could have resulted in a loss of safety function (equipment required to achieve safe shutdown) in the event of a fire. Consequently, this event is reportable per 10CFR 50.73(a)(2)(v)(A) as a Safety System Functional Failure.
SAFETY SIGNIFICANCE
The unprotected condition of 1C-333 could have affected the credited offsite power source to Bus 16. Bus 16 is credited to power equipment required to achieve safe shutdown for Unit 1 in the event of a fire in FA32. A fire in FA32 could damage cables for both trains of safe shutdown equipment. FA32 has fire detection and an automatic wet pipe fire suppression system.
No fire occurred and there was no actual consequence to the public as a result of this unanalyzed condition.
EIIS System Code: FK 2 EIIS System Code: EB 3 EIIS System Code: EK FACILITY NAME (1 DOCKET NUMBER 2 MZEIZLER NUMBER 61 EM 0 05000282 YEAR 09 -- 008 -- 0
CAUSE
The Appendix R circuit analysis package did not include the sudden pressure switch trip (1C-333 cable) due to inattention to detail and the lack of a detailed procedure to perform Appendix R circuit analysis. A contributing cause was that the sudden pressure trip switch is a unique feature of the offsite power transformers; most other 4KV breakers do not have a sudden pressure trip switch. However, there are several 4KV load breaker circuit analysis packages that could have been referenced when performing the circuit analysis.
CORRECTIVE ACTION
An hourly fire watch was previously in place for Fire Area 32 as a compensatory measure and will continue until the cable protection issue has been resolved.
A procedure for Appendix R circuit analysis will be implemented. The procedure will include examples such as the unique feature of the offsite power transformers.
The site is transitioning to National Fire Protection Association (NFPA) 805, which is a performance-based, risk-informed licensing basis for fire protection. NFPA 805 endorses the use of state-of-the-art fire modeling techniques and fire probabilistic risk assessment to determine the best solution to properly protect safe shutdown capability. Cables for the credited power source to Bus 16 will be provided the required fire protection to meet the safe shutdown requirements in NFPA 805.
PREVIOUS SIMILAR EVENTS
A review of the site's Corrective Action Program found two instances where cables were not included in their respective analysis fire areas. However, the equipment that the cables supported was not relied upon to be free of fire damage in those particular fire areas.
A review of LERs for Unit 1 and Unit 2 since 2006 identified LER 1-07-02 and LER 1-09-01 as being related to Appendix R non-compliance. These LERs reported unanalyzed conditions for fire areas due to non compliance with Appendix R in the use of operator manual actions.