During review of
10CFR50 Appendix R, III.G.2 compliance, it was discovered that a fire in a divisional switchgear room affecting maintenance tie breakers 64T or 65T, may result in the opposite division diesels [tripping]. The postulated fire could affect cables associated with a maintenance tie breaker which could in turn cause associated bus tie breakers in the opposite division to close (B9, C9, E9, F9). The III.G.2 scenario would also include a Loss of Offsite Power event which would require the [Emergency Diesel Generators] (
EDG's) to be running. Postulating multiple
hot shorts could result in both 9 breakers in a single division closing and paralleling the
EDG's out of synch resulting in potential damage to the
EDG's.
Compensatory action: Eliminated this failure mechanism by racking out the four tie breakers (B9, C9, E9, F9) that could be affected. This does not affect operability of the EDG's.
The licensee notified the NRC Resident Inspector.
