The US Nuclear Regulatory Commission (NRC) has revealed the cause of a fatal industrial accident that took place during refurbishment work last year at Entergy's Arkansas Nuclear One plant. In a letter to the utility, it said that an inappropriate lift assembly had been used.
|Arkansas Nuclear One (Image: Entergy)
Unit 1 of the two-unit plant was taken offline in early 2013 for refuelling and refurbishment work which required the removal of the generator stator. In late-March, the gantry used to remove this from the turbine hall collapsed, sending the 525-tonne component crashing through a hole in the floor to a heavy transport vehicle below. One contractor was killed and eight other workers were injured.
The dropped stator ruptured a water pipe in the transport vehicle bay, causing flooding in unit 1. Certain electrical systems shared by both reactors were damaged and unit 2 automatically shut down. Unit 2 remained without power for some four minutes, with normal grid power supply restored after about ten hours. Unit 1 was without offsite power for six days.
The NRC has now concluded that Entergy "approved a design for the temporary hoisting assembly that was not supported by detailed drawings, specifications, evaluations and/or certifications." In addition, Entergy failed to review associated calculations to ensure that the assembly was designed to support the projected load. It also failed to carry out a load test on the lifting assembly in all configurations for which it would be used.
It also determined that Entergy "did not ensure adequate supervisory and management oversight of the contractors and other supplemental personnel involved with the stator lift, and this contributed to the event."
The NRC inspected the Arkansas Nuclear One plant immediately after the accident and said it had no safety concerns about the plant. However, after a follow-up inspection this February, it determined a "high safety significance" finding related to the accident for unit 1 and one with "substantial safety significance" for unit 2. At Entergy's request, a regulatory conference was held on 1 May to discuss these findings.
The NRC has informed Entergy that both findings have now been classified as being of "substantial safety significance."
Last September, the US Department of Labor's Occupational Safety and Health Administration cited 26 safety violations against a number of companies involved in the stator replacement and issued fines totalling $175,000.
Researched and written
by World Nuclear News