Exelon: "Still Working" on Nuke Plant Unit 2 [UPDATED]

A representative for Exelon Nuclear today said there were "no updates" on the situation at the company's , where one of two nuclear reactors at the facility, Unit 2, unexpectedly shut down early Sunday morning after what the company called "scheduled testing and maintenance on an electrical system in the non-nuclear side of the plant."

Personnel at the plant were "still working the plan to return the unit to service," said April Schilpp, senior manager of communications with Exelon Nuclear.

Neil Sheehan, a spokesperson for the Nuclear Regulatory Commission (NRC), provided additional details on the shutdown, or "unplanned scram," as it's known in nuclear energy parlance.

"There was leakage taking place at the time involving the main turbine control valve. Plant power was reduced to allow repair work on the valve. Following maintenance work on that valve, an isolate valve was opened," Sheehan said in an email.

"For reasons Exelon will need to determine, pressure in the plant's Electro-Hydraulic Control system (which regulates turbine speed) dropped, causing the Reactor Protection System (a computerized system that protects the reactor) to initiate an automatic shutdown of the reactor," Sheehan said.

A single vapor plume rose from the plant's westernmost cooling tower this morning, as opposed to the twin plumes that typically signify the operation of both generating units.

The plant received  and is thus subject to "baseline" inspections from the agency.

Sheehan said the average number of "unplanned scrams" per 7,000 hours of operation (about 9 1/2 months) is one of the measures by which the NRC tracks plant performance. If a plant has more than three "unplanned scrams" in that period, it receives additional NRC oversight.

"The shutdown yesterday will count as a hit on that performance indicator," Sheehan said, but noted that the Limerick site's rolling average of "unplanned scrams" stood at 0.8 as of the end of the first quarter of 2011.

"This shutdown is not expected to ... result in more NRC oversight," Sheehan said.

Sheehan said two full-time NRC inspectors are permanently assigned to the site.

"Our Senior Resident Inspector for Limerick responded to the site to ensure the reactor was safely shutting down and that operators were taking the proper steps. He found that to be the case," Sheehan said.

"The plant safely shut down, with all safety systems operating as designed. The control room operators also properly responded to the event," Sheehan said.