PA Nuke Down after Valve Leak

Thursday, February 26, 2015 @ 05:02 PM gHale

A problem with a steam valve forced the shutdown of one of the two reactors at Limerick Generating Station in Pottstown, PA at 10 p.m. Monday night, officials said.

Called a “hot shutdown,” the reactor for Unit 1 “automatically shut down at about 9:40 p.m.,” according to statement released by Exelon Nuclear Tuesday.

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Unit 2 remained unaffected and continued to operate generating electricity.

The grid is stable and Exelon reported the shutdown will not affect electrical service, said officials at the Nuclear Regulatory Commission (NRC).

The Unit 1 shutdown occurred because of the unexpected closure of one of the “main steam isolation valves” in the line sending steam to the unit’s electrical generator.

The valve closed due to a leak in the nitrogen supply line, the gas used in the hydraulic system to operate the valve, according to NRC spokesman Neil Sheehan.

“During an accident, the (main steam isolation valves) would be closed to prevent the release of radioactivity from the containment building, which houses the reactor,” Sheehan said.

As a result of the valve’s closure, pressure in the reactor began to rise “exceeding the reactor protection system setpoint of pressure,” according to the NRC’s initial incident report on the matter.

This caused the “hot shutdown,” during which “control rods” insert amid the fuel rods to slow or stop the nuclear reaction that generates the heat used to create the steam that spins the turbines and creates electricity.

“A ‘hot shutdown’ means the reactor and the reactor coolant system remain heated and pressurized, thus allowing for a fairly quick restart once the problem is resolved,” Sheehan said.

“It essentially means the reactor is idling until being placed back into service,” Sheehan added. “But no fissioning is taking place and therefore there is no greater risk of a radiation release than if the plant was in ‘cold shutdown,’ which means the reactor and reactor coolant system have been cooled down and depressurized. A “cold shutdown” occurs when a plant will be down for at least several days.

“The shutdown was normal and the plant is stable in hot shutdown with normal pressure control via the main steam bypass valves to the main condenser and normal level control using the feedwater system,” according to the NRC preliminary report.

“Plant equipment responded as designed during the shutdown. Station operators responded appropriately,” according to Exelon.

The NRC received word of the “scram” quickly Monday night and the plant’s two resident NRC inspectors will oversee the repairs, Sheehan said.

To repair the problem, Exelon crews will have to “go into the plant’s drywell, the enclosed area surrounding the reactor, to search for the source of the leakage and then make any needed repairs,” Sheehan said.

“The forced outage will be counted in the plant’s Performance Indicator for Unplanned Scrams per 7,000 Hours of Operation,” Sheehan said.

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