The US Nuclear Regulatory Commission (NRC) has launched a special inspection at the Davis-Besse NPP that will focus on two separate issues: multiple diesel generator failures during testing and maintenance; and a complicated reactor trip on 8 July. The six-person inspection team will review the company’s response to each diesel generator failure, including the company’s cause analysis, extent of condition reviews, maintenance practices and system design. The team will also focus on the circumstances affecting the recent complicated automatic reactor shutdown, which was triggered by a turbine trip, assessing equipment performance and operator response.

In the past 24 months, Davis-Besse experienced four failures of emergency diesel generators to operate in accordance with specifications during NRC-required testing and one failure of a station blackout diesel generator during maintenance. Davis-Besse has two emergency diesel generators, which would provide emergency power during the loss of offsite power, and one station blackout diesel generator, which would provide power in case both emergency diesel generators failed. One diesel generator is sufficient to enable the plant to safely shut down and remain in a stable condition. Normal plant operations were not affected by the diesel failures during testing and maintenance, and public safety was maintained.

While NRC was planning the special inspection on diesel failures, Davis-Besse experienced an unplanned reactor trip. During the plant’s response to the trip, certain pieces of equipment did not function as designed. Operators took action to address the equipment issues, and the reactor was shut down safely and placed in a stable condition. After making the necessary repairs, the reactor returned to power. Based on the complications of the trip, NRC chose to expand the special inspection to better understand equipment performance issues and operator response. Upon completion of the special inspection, NRC inspectors will document their findings on both issues in a publicly available inspection report.

The 925 MW Davis-Besse Nuclear Power Station is a two-loop Babcock & Wilcox pressurised water reactor that entered commercial operation in July 1978. The reactor's current operating licence expires in 2017, but First Energy Nuclear Operating Co submitted a 20-year life extension application in 2010, which was granted by NRC in 2015.

Photo: Davis-Bess (Source, FENOC)