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There will be additional inspections at the Oyster Creek Nuclear Plant, after the discovery of past problems with electromatic relief valves which help to keep reactor fuel covered and cool during a plant shutdown, said Nuclear Regulatory Commission (NRC) officials.

The NRC today issued a “yellow” classification about problems with the valves, which indicates “substantial safety significance.” The federal agency also issued a “white” inspection finding — of low to moderate safety significance — dealing with the maintenance of an emergency diesel generator at the plant in Forked River, NJ.

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“These enforcement actions underscore the need for plant owners to be vigilant when it comes to maintaining essential safety equipment,” NRC Region I Administrator Dan Dorman said. “In the case of these issues, two components that can play an important role during a reactor shutdown either experienced or may have experienced material failures that could have prevented them from performing their functions when needed.”

Exelon, which is Oyster Creek’s owner, has corrected the equipment problems, but the NRC will do more inspections to make certain the problems ended up addressed, according to an NRC press release.

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The electromatic relief valves depressurize the reactor during a pipe break to allow coolant to inject into the reactor core, in order to keep the reactor covered and cooled during a shutdown.

“Even though the violation involving the EMRVs has been classified as “yellow,” the NRC has determined it represents an old design issue,” the NRC said. “That is, the issue stems from an inspection finding involving a past design-related problem and does not reflect a current performance deficiency associated with existing programs, policies or procedures used by the company.”

Exelon found an alignment problem during a shutdown in June 2014 while doing refurbishment work on two EMRVs removed from the plant in 2012. When the valves ended up tested, they did not open, according to the NRC.

Once the issue ended up identified, Exelon immediately tested five of its then-installed EMRVs. All five actuated successfully. The company also installed redesigned actuators for the valves during a refueling and maintenance outage at the plant in October 2014.

The repair work will keep Oyster Creek out of the NRC’s “Degraded Cornerstone Column” Action Matrix. But the NRC will perform a team inspection to review the company’s evaluation and corrective actions.

The NRC’s “white” finding involved one of the plant’s emergency diesel generators, where the back-up power source may not have been available to operate because of the degradation of its cooling fan drive shaft.

Alarms went off on July 28, 2014 during a bi-weekly test of the generator. Oyster Creek personnel discovered the belt-driven cooling fan drive shaft had sheared in two pieces, which left the generator inoperable, the NRC said.

The fan keeps the generator from overheating while in use. The fan shaft ended up replaced. It was later determined that fatigue caused the shaft to fail, according to the NRC.

NRC inspectors learned in May 2005, Exelon changed the method for tensioning the generator’s cooling fan belt, but did not verify the acceptance criteria for the change, according to the NRC.

“The company did not detect that the change left the shaft subject to fatigue and, as a result, the failure occurred on July 28, 2014,” the NRC said. “Further, the period of inoperability for the generator exceeded its allowable outage time in violation of the plant’s technical specifications. Unlike the EMRV design issue, the tensioning issue was identified by the NRC.”

Exelon performed a failure analysis on the broken shaft, examined the cooling fan drive shaft for the plant’s other emergency diesel generator and corrected the tensioning method.

Exelon officials provided written responses for both problems on March 13.

“The company disagreed with some aspects of the NRC’s analysis of the EMRV issue, concluding that the issue was less significant than characterized by the agency,” according to the NRC. “The NRC considered the points raised by Exelon and concluded that the EMRV issue was appropriately characterized as yellow.” For the EDG “white” finding, the company accepted the finding and its significance determination.”

The NRC will perform team inspections in response to both inspection findings when Exelon notifies the agency it’s ready for the reviews, NRC officials said.

Oyster Creek was already under additional NRC oversight in 2014 because of a “White” (low to moderate safety significance) performance indicator that stemmed from four unplanned shutdowns, or scrams, in 2013 and 2014.

The NRC’s performance indicator for unplanned scrams for each 7,000 hours of operation changes from green to white if a nuclear plant has more than three unplanned shutdowns. Oyster Creek crossed the green/white threshold July 11, 2014, when the plant had a fourth unplanned shutdown, the NRC has said.

The NRC conducted a supplemental team inspection at Oyster Creek from Dec. 8 through Dec. 11.

The NRC found Exelon had performed an adequate root cause evaluation for the four unplanned shutdowns and a collective root cause evaluation for all of the events. But it had not reached a final decision on the yellow or white findings.

Oyster Creek is the oldest nuclear plant in the United States. It went online on Dec. 23, 1969.

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