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Resolution of Generic Safety Issues: Issue 46: Loss of 125 Volt DC Bus ( NUREG-0933, Main Report with Supplements 1–35 )


Historical Background

This proposed generic issue has its origin in a January 1981 event at Millstone Unit 2.394 An operator inadvertently opened a 125V DC main feeder breaker, causing the loss of one of the two redundant emergency systems which led to a reactor trip from 100% power. The loss of this DC system precluded the main turbine from tripping automatically as designed and it was manually tripped 30 seconds later. The trip of the turbine coupled with the inoperability of this DC system caused the loss of offsite power to one of the two redundant AC systems and the automatic starting of both redundant diesel generators. Subsequently, both of the diesel generators tripped automatically as a result of an inherent design trip feature in the control circuits of one of the diesel generators and a mechanical failure in the other.

Safety Significance

The safety significance of the January 1981 Millstone 2 event and the licensee's corrective actions were evaluated by AEOD.395 The AEOD evaluation did not identify any generic safety concerns or the need for any further action by the NRC other than those being considered in Item A-30, "Adequacy of Safety Related DC Power Systems," and USI A-44, "Station Blackout." According to the AEOD report, the event offered lessons to be learned in connection with operator training and plant procedures. The AEOD findings were referred to DHFS for evaluation in connection with operator training and plant procedures.396


The issue is covered in Issue 76.


0394.Memorandum for S. Hanauer et al. from D. Eisenhut, "Operating Reactor Event Memorandum No. 81-31: Loss of Direct Current (DC) Bus at Millstone Unit 2," March 31, 1981. [8104100493]
0395.Memorandum for H. Denton from C. Michelson, "Millstone Unit 2—Reactor Trip Following De-Energization of a 125 V DC Bus," November 5, 1981. [8112010276]
0396.Memorandum for C. Michelson from H. Denton, "AEOD November 1981 Report on the Millstone Unit 2 Loss of 125 V DC Bus Event," January 4, 1982. [8202040017]