Limerick Nuclear Generating Station operators shut down one of the reactors at the plant unexpectedly late Tuesday night because of a problem with the system that controls the flow of steam to the turbine.

The first alarm sounded at the Limerick, PA, plant at 11:24 p.m., said the Nuclear Regulatory Commisison (NRC).

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“At 11:24 p.m. Tuesday, the Limerick Unit 1 control room received alarms involving reactor feedwater heaters and the turbine. The operators in turn began lowering reactor power,” said NRC spokesman Neil Sheehan.

“When the level was at about 87 percent, at about 11:45 p.m., the operators manually scrammed the reactor, which means they inserted all control rods to halt the fissioning process and shut down the reactor,” Sheehan said.

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The NRC’s on-site inspector ended up summoned and reviewed the actions taken by the Exelon Nuclear employees.

Sheehan said initial information indicates the problem was with the “turbine electro-hydraulic control system,” which regulates the flow of steam from the reactor to the turbine. The steam spins the turbine to generate electricity.

“This event presents no threat to public health and safety and will not affect electrical service to customers. Limerick Unit 2 remains at full power,” plant spokeswoman Dana Melia said.

“Our initial assessment is that operator response to the condition and the event appears to have been appropriate, but our reviews are continuing,” Sheehan wrote.

He also noted the control system where the problem occurred “is due to be replaced with a digital EHC system during an upcoming refueling and maintenance outage.”

“Plant personnel will repair and test the valves before placing the unit back in service,” Melia said.

However, Melia said plant management could not yet say how long it will take to repair the problem, nor when Unit 1 will be back on online.

This is the first scram, or unscheduled reactor shutdown, of 2014 for the plant and the first since July 2012.

Exelon is seeking to renew the license for the nuclear plant, hoping to continue operations there for 20 more years. The plant’s operating licenses for its two reactors expire Oct. 26, 2024 (Unit 1) and June 22, 2029 (Unit 2).

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