The US Nuclear Regulatory Commission (NRC) will begin a special inspection at the Palo Verde Nuclear Generating Station to review circumstances surrounding the failure of an emergency diesel generator during testing on 15 December 2016.

The plant, in Wintersburg, Ariz., is operated by Arizona Public Service Co.

“The purpose of this special inspection is to better understand the cause of the failure and to review the adequacy of corrective actions proposed by the licensee,” NRC Region IV Administrator Kriss Kennedy said.

Emergency diesel generators are used to supply power to safety-related systems in the event of a loss of off-site power. NRC requires that each of the three reactors at Palo Verde have two emergency diesel generators that must be tested monthly to ensure operability.

During a scheduled test, a piston failed on one of the emergency diesel generators at Unit 3, prompting the licensee to declare an Alert, the second lowest of four levels of nuclear emergency. No site personnel were injured and the Alert was terminated about two hours later. There was no danger to the public and the event had no effect on plant operations.

NRC inspectors will spend about a week on site. The team will also determine whether there are any potential generic implications for the industry and determine whether plant operators’ response to the event was appropriate. An inspection report documenting the team’s findings will be publicly available within 45 days of the end of the inspection.