One March 31, 2013, workers at Arkansas Nuclear One, were removing a stator, a large part of a diesel generator, from the generator building. During the loading process, the stator slipped and fell to the floor of the building. One worker was killed, and eight others were injured, four seriously.
The falling stator ruptured a water pipe, spilling water into the switchgear, knocking out offsite power to Unit 1 and one train of Unit 2. Unit 1 was offline for refueling at the time of the accident, but the failure of one train of offsite power caused an automatic shutdown to Unit 2.
As expected, the owners of Arkansas Nuclear One issued a reported downplaying the safety significance of the accident. They characterized the accident as taking place “in a non-radiation area, and there was no risk to public health and safety.”
The Nuclear Regulatory Commission has completed its post-accident assessment, and reached a different conclusion. As Reuters reported today:
The U.S. Nuclear Regulatory Commission (NRC) said on Tuesday that after a series of investigations, it has declared a 2013 incident at an Arkansas nuclear power plant to be of substantial safety significance, the penultimate of four emergency classifications.
The NRC report says that the initial accident assessment was incorrect:
The accident was initially classified as an unusual event, the lowest of four emergency classifications, because the incident caused a small explosion inside electrical cabinets.
The new NRC assessment says that the damage and the danger were worse than initially reported:
Emergency diesel generators were relied upon for six days to supply power to cooling systems.
If the diesel generators would have failed, there was no alternate means of providing electrical power to key safety-related systems at the plant at that time, due to the damaged electrical cables and equipment.
The Nuclear Regulatory Commission also found that proper safety precautions were not followed in removing the stator:
“The NRC determined that the lifting assembly collapse resulted from the plant’s failure to adequately review the assembly design and ensure an appropriate load test in accordance with its procedures or approved standards,” the NRC statement said.
Following its review, the NRC “determined that ‘substantial safety significance’ was appropriate to characterize the risk significance of the event for both Unit 1 and 2.”
The NRC is looking for at increased agency oversight for the Arkansas Nuclear One facility as a result of the new assessment.