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Wolf Creek nuclear power plant is now seeing yellow after an off-site power event had substantial safety significance to bring additional oversight.

Wolf Creek Nuclear Operating Corp. operates the Burlington, KS-based plant.

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On Jan. 13, operators at the plant declared an unusual event, the lowest of four levels of nuclear emergency, after the failure of a main generator electrical breaker, followed by an unexplained loss of power to a transformer, said officials at the Nuclear Regulatory Commission (NRC).

This caused the switchyard to lose power, which removed the plant’s connection to the electrical power grid. All safety systems responded as expected and emergency diesel generators automatically powered safety-related equipment.

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The NRC conducted an Augmented Inspection and determined that actions by the licensee set the stage for the incident because the company failed to provide adequate oversight of contractors while they performed work that could affect safety-related equipment in April 2011.

As a result, the licensee failed to identify that electrical maintenance contractors had improperly connected wires on an electrical component, the NRC said. This allowed an electrical short to prevent transfer of power to a transformer Jan. 13.

The NRC held a public meeting March 6 in New Strawn, KS, to discuss the preliminary results of the Augmented Inspection. Following further review, the NRC staff has concluded the inspection finding is “yellow,” which the licensee accepted.

Under the NRC reactor oversight process, inspection findings end up evaluated using a significance determination process and assigned a color indicating its safety significance. Findings with very low safety significance are “green.” “White” findings have low to moderate safety significance, “yellow” findings have substantial safety significance, and “red” findings have high safety significance.

The “yellow” finding moves Wolf Creek into the “degraded cornerstone” column of the NRC action matrix, resulting in a higher level of NRC oversight. This is the third highest level of NRC oversight and Wolf Creek joins six other U.S. nuclear units in that column.

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