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It is one thing for a nuclear reactor to shut down, but for it to scram twice in less than two days, it quite another.

That is what happened at Exelon’s Limerick Generating Station. The same reactor that shut down unexpectedly early Sunday morning did so again Monday, said an official at the Nuclear Regulatory Commission (NRC).

“As the plant was being started up, two reactor re-circulation pumps tripped,” said Neil Sheehan, a spokesman for the NRC office in King of Prussia. “The reactor was at zero-percent power when this occurred,” Sheehan wrote.

According to NRC documents, the second scram occurred at 11:50 a.m.

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“Licensee is investigating a potential issue with the relay logic associated with the scram bypass feature. Offsite power circuits and emergency diesel generators are operable and available. There was no increase in plant risk associated with this event,” read an update in status posted on an NRC daily log of events.

It was the second time in less than 36 hours the reactor shut down unexpectedly.

Unit 2 shut down at 5:02 a.m. Sunday “after the turbine tripped following scheduled testing and maintenance on an electrical system in the non-nuclear section of the plant,” said Exelon officials on Sunday.

The company has not yet released any complete explanation for the first scram.

Sheehan said when a reactor scrams, the NRC begins to analyze if a unit has more than three scrams in its 7,000 hours of operations in a service quarter.

“I’m told this would not count as a hit on the plant’s Performance Indicator for Unplanned Scrams per 7,000 Hours of Online Operation because it occurred while the reactor was subcritical (at zero-percent power),” Sheehan said.

In February, the same reactor, Unit 2, shut down unexpectedly due to problems with systems related to recirculating pumps. That shutdown lasted for more than two days.

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