United States Nuclear Regulatory Commission - Protecting People and the Environment

W17  Loss of Safety Functions – Undetected Open Phase(s) in Balanced Three-Phase Offsite Power System

The panel will discuss NRC and industry response to the Byron Station Unit 2 loss of single phase event and other related operating events and the implications to operating and new nuclear power plants.

The session will cover:

(1) existing NRC requirements and staff positions

(2) status of NRC evaluation of licensee responses to NRC Bulletin 2012 01, “Design Vulnerability in Electric Power System”

(3) NRC interactions with external stakeholders (e.g., industry, public)

(4) corrective actions and interim compensatory measures taken by licensees

(5) industry initiatives and schedules for resolving the open phase(s) issue

(6) status of plant modifications and design analyses

(7) NRC proposed regulatory actions, and Bulletin resolution/closeout for each plant

On January 30, 2012, an electrical insulator failed in the Byron 345 kilovolt (kV) switchyard that resulted in a Unit 2 automatic reactor trip and Notice of Unusual Event emergency declaration. An open circuit condition was created that caused a loss of phase on the two 6.9 kV reactor coolant pump buses and the two 4.16 kV engineered safety features (ESF) buses. The ensuing overload condition caused several safety related loads to trip. Approximately 8 minutes after the reactor trip, the control room operators diagnosed the loss of phase condition and manually tripped breakers to separate the unit buses from the offsite power source. When the station auxiliary transformer feeder breakers to the two 4.16 kV ESF buses were opened, the loss of ESF bus voltage caused the emergency diesel generators to automatically start and restore power to the ESF buses.




  • Sergiu Basturescu, Electrical Engineer, Division of Engineering, NRR/NRC tel: 301-415-1237, e-mail: Sergiu.Basturescu@nrc.gov
Page Last Reviewed/Updated Friday, May 23, 2014