A March 2013 crane accident at Entergy's Arkansas One nuclear power plant that killed one worker, injured eight others, damaged the plant and required days of backup emergency diesel power was judged to be of substantial safety significance by the US nuclear regulator.
Workers were moving the 525-ton main generator stator out of the plant's turbine building during maintenance when a temporary lifting assembly collapsed, causing the component to fall, damaging plant equipment, killing one person and injuring eight others.
Unit 1 was in a refuelling outage at the time, with all of the fuel still in the reactor vessel, safely cooled.
The stator fell on and extensively damaged portions of the Unit 1 turbine deck and subsequently fell over 30 feet into the train bay. The stator drop resulted in a Unit 1 loss of offsite power for 6 days and a Unit 2 reactor trip and loss of offsite power to one vital bus. The dropped stator ruptured a common fire main header in the train bay, which caused flooding in Unit 1 and water damage to the electrical switchgear for Unit 2. The alternate alternating current diesel generator (station blackout) electrical supply cables to both units were pulled out of the electrical switchgear and the diesel was therefore not available to either unit, according to a 24 March 2014 follow-up inspection report (NRC document ML14083A409).
Unit 2, which was operating at full power, automatically shut down when a reactor coolant pump tripped due to vibrationscaused by the heavy component hitting the turbine building floor when it fell. Unit 2 never completely lost off-site power, and means existed to provide emergency power using the diesel generators, said the regulator in a press release.
In September 2013, the US government's Occupational Safety and Health Administration cited Entergy and three contractors, Precision Surveillance Corp, Bigge Crane and Rigging and Siemens Power Generation for 26 safety violations.
The nuclear regulator blamed Entergy for approving a temporary gantry design not supported with adequate documentation, for failing to identify load deficiencies in the vendor's calculations, failing to identify a component of the wrong size in the north tower. It also said that the tower was not designed for 125% of the load, and inadequate load testing (including a 125% proof load test) was carried out before the lift.
Initially, the regulator assessed the risk significance of the event at the highest level, red, for unit 1, and at the next lower level, yellow, for unit 2, but reduced the unit 1 significance to yellow after Entergy gave evidence in a 9 May conference arguing that there was a high likelihood of success (90%) for recovering electrical power to cooling pumps before water in the core boiled away, exposing fuel.
An Entergy spokeswoman said: "Entergy is dedicated to the safe operation of Arkansas Nuclear One, and we take the NRC's findings very seriously. We are committed to learning from this tragic incident, sharing our knowledge with the industry, and ensuring that it never happens again."
As of early July, the nuclear regulator was still determining what its response to the incident would be.
Pictured: Arkansas Nuclear One. Image from Entergy.