Last Tuesday, August 25, 2015, The Pilgrim Nuclear Power Station came back online. This was three days after the plant experienced what plant officials said at the time was “an automatic shutdown due to the closure of a single main steam isolation valve (MSIV).”
However, plant officials have revised their original statement, attributing the cause of the scram to a “broken instrument control line that feeds one of the plant’s four main steam isolation valves (MSIVs).”
“The instrument line controls the opening and closing of these steam isolation valves,” Jerry Nappi, Entergy’s manager of nuclear communications further explained. “When the line failed, the valve appropriately closed, which then sent a signal to the reactor to automatically and safely shutdown.”
The plant’s main steam isolation valves are operated by gas, specifically nitrogen. When the half-inch line that supports the operation of one of these valves failed, the MSIV closed, resulting in the automatic shutdown. According to NRC publications, the main purpose of the MSIV is, in the event of an accident, to halt the flow of steam from the reactor to the turbine building and thereby help isolate radioactivity to the containment building surrounding the reactor.
NRC Public Affairs Officer Neil Sheehan has stated, “There will be further review by the NRC of the event. As of now, we are planning a normal review, by our on-site resident inspectors, for an unplanned shutdown.” Sheehan said the senior resident inspector at Pilgrim traveled to the plant Saturday to independently verify that operator actions in response to the scram were appropriate and that there were no other significant equipment issues during the shutdown.