According to a press release from Exelon Limerick Generating Station, the plant's Unit 2 needed an unplanned shutdown, or "scram," around noon for the replacement of a steam valve. 

"There are 14 steam valves on each unit that protect the main steam line," said communications manager Dana Melia. 

Melia said that plant operators are constantly monitoring all the parts of the system, and noticed a rise in the steam temperature.

"That indicated a potential failure [in the steam valve]," Melia said.

These types of repairs can only be done with the unit offline. The valve will be replaced and the system checked before the unit is restarted. 

The repairs to Unit 2 come less than 48 hours after the company announced that the plant's Unit 1 had returned to full power. Unit 1 was unexpectedly shut down last week following what the company and the Nuclear Regulatory Commission described as an electrical fault.

Unit 2 received extensive maintenance earlier this year during a scheduled outage that lasted about two weeks and brought hundreds of out-of-town workers to the facility.

The power station is in the middle of a relicensing application process that, if completed successfully, would allow the facility to remain in operation until the year 2049.