Washington — Entergy's 728-MW Pilgrim nuclear reactor, in Plymouth, Massachusetts, shut at 2:09 pm EST (1908 GMT) Thursday, when one of two 345-kV transmission lines that supply the plant with electricity from the grid "became unavailable," according to a US Nuclear Regulatory Commission spokesman.
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The unit continues to receive offsite power from a second 345-kV line and a 23-kV line, Neil Sheehan, a spokesman for NRC Region I, said in an email. The unit was operating at 100% of capacity early Thursday, according to NRC's daily reactor status report.
Patrick O'Brien, a Pilgrim spokesman, said in an email: "We are working to determine the cause of the line loss."
O'Brien said: "We will take this opportunity to conduct preventive maintenance that we could not otherwise perform with the plant operating at full power. When Pilgrim will return to 100% power is considered business sensitive, and we do not disclose that information."
He added that "Pilgrim had been safely operating for 227 consecutive days following the completion of our most recent refueling outage in May 2017."
Pilgrim was shut for 12 days through February 7, 2015, after the two 345-kV transmission lines became inoperable during a snowstorm, according to NRC reports.