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Pilgrim Nuclear Power Station’s reactor automatically shut down late Saturday, avoiding a potentially dangerous situation when a valve that regulates the steam transfer from the reactor to the turbine closed when it should have remained open.

It marks the third time this calendar year the Plymouth, MA-based plant ended up forced to shut down due to mechanical failures.

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“When the valve closes, it causes reactor pressure and the reactor power to rise,” said Diane Screnci, spokeswoman for the Nuclear Regulatory Commission (NRC). “There are systems in place to shut the reactor down, and the operators can also do it manually.”

On Sunday morning, Entergy’s licensing manager, Chip Perkins, said plant operators spent the overnight scrutinizing engineering plans “to put together what we believed happened.”

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“We now have teams inside the primary containment looking at that valve and the seven others,” Perkins said.

Four lines carry steam from the reactor to the turbine, which generates the electricity. Each of those lines has two “main steam isolation valves.” The valves get their power by pressurized gas.

One of those valves remained closed Saturday, triggering an automatic shutdown of the reactor, which had been operating at full capacity at the time.

Perkins said the malfunction of a main steam isolation valve could not cause overpressurization of the reactor because of a “reactor protection system” — the mechanism referred to by Screnci — that detects any change in the balance of operations and triggers automatic shutdown when there’s an imbalance.

“If you are running at 100 percent and all the steam passing through four lines is then passing through three, the reactor system senses that,” Perkins said.

Pilgrim owner and operator, Entergy Corp. did not say when the plant will be back online, because the information is business-sensitive.

NRC spokesman Neil Sheehan said his agency’s senior resident inspector traveled to the plant Saturday and Sunday to “independently verify that plant operators properly handled the shutdown.”

“No immediate concerns were identified,” Sheehan said.

Entergy is currently awaiting the results of an appeal it made in July of a 68-page federal inspection report in which the plant received bad marks for performance during a forced shutdown during the Jan. 27 blizzard aggravated by a number of mechanical failures. After partial offsite power was lost, station response ended up complicated by the failure of a diesel-driven air compressor, leaking of a safety relief valve used to help depressurize the reactor, failure of one of the manually operated relief valves and failure of a high-pressure coolant system.

The NRC inspectors said the valve problem ended up discovered after a February 2013 shutdown, yet had not been addressed.

The plant also went into an unplanned shutdown May 22. Pilgrim was just powering up after a month of refueling and equipment repair that cost the company $70 million. Operators had to manually shut down the plant after reaching 15 percent power due to problems with the main condenser that uses water from Cape Cod Bay to cool the steam generated by the reactor and convert it back to water.

There are five categories in the Nuclear Regulatory Commission’s performance assessment. Pilgrim is currently in the “degraded cornerstone category,” a classification that puts it under closer scrutiny by federal regulators.

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