Federal regulators considered the stuck valve a prominent example of similar problems dogging the plant. Around the same time, plant workers installed a bearing backward and failed to correctly diagnose other damaged equipment. The reactor had to be unexpectedly taken offline more frequently than others in the industry.
During the three-part inspection, NRC officials checked whether other valves could be affected, reviewed the rigor of the TVA's testing and maintenance programs and conducted hundreds of interviews to delve into the plant's safety culture, a review that finished in May.
NRC inspectors say they now believe the Browns Ferry plant is operating safely, has increased its overall performance and is working through an improvement plan. One of the TVA's major tasks has been catching up with maintenance work that slipped, contributing to an increase in equipment failures.
"What was happening was there (were) thousands of preventive maintenance activities that they had not performed over several years on the safety system, which created a significant backlog," Guthrie said. "By not performing that preventative maintenance, it was directly affecting the reliability of those systems."
Browns Ferry has a troubled history. In 1975, a worker using a candle to check for air leaks started a fire that was considered the most serious industry accident until a meltdown happened four years later at Pennsylvania's Three Mile Island plant. TVA voluntarily shuttered its entire nuclear fleet in 1985 to address safety and performance issues. The Unit 1 reactor at Browns Ferry reopened in 2007.
TVA supplies electricity to about 9 million people in Alabama, Georgia, Kentucky, Mississippi, North Carolina, Tennessee and Virginia.