Document: NUREG-0800
Document ID: 61198d64-419c-45ea-aebb-cf34c3b171fe
Document Type: srp
Title: Revision 0 – July 2015
Source: NUREG-0800
Source URL: https://www.nrc.gov/docs/ML1505/ML15057A085.pdf
Revision Date: 2023-06
Chapter: 8
Section ID: 8
CFR Part: 
CFR Title: 

Content:
on the two 6.9-kV nonsafety-related RCP buses and the two 4.16-kV ESF buses. Some ESF loads that were energized relied on equipment protective devices to prevent damage from an unbalanced overcurrent condition. The phase overcurrent condition actuated relays to trip several ESF loads. Approximately 8 minutes after the reactor trip, the control room operators diagnosed the loss of Phase C condition and manually tripped circuit breakers to separate the unit buses from the offsite power source. When the operators opened the SAT feeder breakers to the two 4.16-kV ESF buses, the loss of voltage relays started the emergency diesel generators (EDGs) and restored power to the ESF buses. If the condition had been allowed to persist for an additional few minutes, damage to the RCP seals could have occurred due to loss of RCP seal cooling water. This in turn could have resulted in a loss of coolant from the RCP seals in the containment building. A second event also occurred at Byron Station Unit 1 on February 28, 2012. This event was also initiated by a failed inverted porcelain insulator that resulted in an open phase as well as a phase-to-ground fault on the line side of the circuit. In this event, the fault current was high enough to actuate protective relaying on the 345-kV system. The 4.16-kV ESF buses experienced a loss of voltage due to the opening of 345-kV system breakers, which resulted in separation of the SATs from the 4.16-kV buses. The two EDGs started and energized the 4.16-kV ESF buses as designed. A review of other operating experience identified design vulnerabilities associated with single-phase open circuit conditions at South Texas, Unit 2 (See Licensee Event Report (LER) 50-499/2001-001, ADAMS Accession No. ML011010017); Beaver Valley Power Station, Unit 1 (See LER 50-334/2007-002, ADAMS Accession No. ML080280592); and a single event that affected Nine Mile Point, Unit 1 (See LER 50-220/2005-04, ADAMS Accession No. ML060620519) and the neighboring James