Crane accident had ‘substantial’ safety significance, reports US regulator

2 July 2014

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.

 



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