The NRC evaluates regulatory performance at commercial nuclear plants with a color-coded process that classifies inspection findings as green, white, yellow or red in order of increasing safety significance. The NRC has preliminarily determined that the incident had high safety significance, or was red, for Unit 1, and had substantial safety significance, or was yellow, for Unit 2.
Workers were moving a 525-ton component out of the plant’s turbine building during a maintenance activity when a temporary heavy equipment assembly collapsed on March 31, 2013, causing the component to fall, damaging plant equipment, killing one person and injuring eight others.
Unit 1 was in a refueling outage at the time, with all of the fuel still in the reactor vessel, safely cooled.
Entergy officials declared a Notice of Unusual Event, the lowest of four emergency classifications used by the NRC, because the accident caused a small explosion inside electrical cabinets. The damaged equipment caused a loss of off-site power.
Emergency diesel generators were relied upon for six days to supply power to heat removal systems. The falling turbine component damaged electrical cables needed to route power from an alternate AC power source to key plant systems at both units. This condition increased risk to the plant because alternate means of providing electrical power to key safety-related systems was not available using installed plant equipment in the event the diesels failed.
Unit 2, which was operating at full power, automatically shut down when a reactor coolant pump sensed vibrations as the heavy component fell and hit the turbine building structure. A preliminary yellow finding is being issued for Unit 2 because the impact of the incident was less significant. Specifically, Unit 2 never completely lost offsite power, and means existed to provide emergency power using the diesel generators.
NRC Resident Inspectors responded to the site the day the incident occurred. The NRC conducted an Augmented Team Inspection, prepared a detailed chronology of the event, evaluated the adequacy of licensee actions in response to the incident, and assessed the factors which may have contributed to the incident. Worker safety issues are the responsibility of the Occupational Safety and Health Administration, which conducted an independent inspection of the incident.
The NRC determined that the lifting assembly collapse resulted from the licensee’s failure to adequately review the assembly design and ensure an appropriate load test in accordance with its procedures or approved standards.
The Augmented Team Inspection report documented information gathered and identified areas for further inspection follow-up. The NRC held a public meeting in Russellville on May 9, 2013, to discuss the team’s findings. From its follow-up inspections, the NRC identified the preliminary red and yellow findings documented in the latest NRC inspection report.
No decision on the final safety significance of the findings or any additional NRC actions will be made at the May 1 conference. That decision will be announced at a later time.