PA Nuke Shut Down Not So Smooth

Thursday, April 16, 2015 @ 05:04 PM gHale

As it is with quite a few nuclear plants this time of year, Exelon Nuclear prepared to shut down the Unit 2 reactor at the Limerick Generating Station Monday, but that is when a small issue ensued which resulted in a scram.

A “scram” is a reactor trip during which all control rods rapidly insert into the core of the reactor to stop the nuclear reaction.

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On Monday, this scram procedure was on the schedule as part of the reactor re-fueling and maintenance outage at the Pottstown, PA, facility. The operators then undertook a planned “scram reset procedure,” said Neil Sheehan, a spokesman for the Nuclear Regulatory Commission (NRC). That is when the issue presented itself.

“The operators were able to reset the ‘2A’ Reactor Protection System (RPS) channel but were unable to reset the ‘2B’ RPS channel,” Sheehan said. “By procedure, operators were required to insert a second manual scram for this condition.”

“Exelon determined that the second manual scram was reportable to the NRC because it was not part of pre-planned testing,” he said.

“The bottom line is this was more a reset than the way most people think of a scram,” Sheehan said.

The problem was due to the failure of a “nuclear monitoring instrument drawer,” he said.

“There are probes that are inserted into the reactor core to measure power levels,” Sheehan said. “They are part of the Reactor Protection System. The ‘drawers’ in this case are part of the electrical support system for these instruments.”

There are several “redundant drawers” and the malfunctioning drawer ended up bypassed and will undergo repair during the outage, Sheehan said.

While the unit is offline, technicians will replace nearly one-third of the reactor’s fuel and perform thousands of inspections and maintenance activities. They cannot perform most of the work while the unit is online, Exelon officials said.

“In addition to the safety inspections and maintenance tasks underway, we are upgrading key components to ensure long-term reliability,” said Site Vice President Tom Dougherty.

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