PLYMOUTH — Pilgrim Nuclear Power Station, already one step from being forced by federal regulators to close down based on poor performance, remained powered down for a third day Tuesday as crews tried to repair a malfunctioning valve designed to prevent radioactivity from leaking into the environment during a nuclear accident.

Patrick O'Brien, a spokesman for plant operator Entergy, said inspectors discovered during routine test last week that one of the valves was not closing fast enough. The plant had already been lowered to 47 percent power last Tuesday for maintenance and was powered down completely on Sunday to allow crews to repair the valve.

This is the second time in a year that a problem in the valve system has shut down the reactor. Critics say it is further evidence that parts are wearing out and not being replaced in time at the 44-year-old reactor because owner-operator Entergy Corp. plans to permanently shutter the plant in mid-2019.

“Even with increased federal oversight, repetitive failure of critical safety equipment is yet another serious warning that Pilgrim's ongoing degradation continues to threaten our region,” Diane Turco, co-founder of the watchdog group Cape Downwinders, wrote in an email. “Pilgrim is an accident waiting to happen. Closure should be now, not in 2019.”

Eight “main steam isolation valves” regulate the flow of steam through four large lines that connect the nuclear reactor to the turbine, which spins and produces electricity.

Federal standards require that the valves close at a certain speed to prevent radioactive leaks in an emergency. Operators found during testing Tuesday that the valve at issue was closing too slowly.

This is not the first time the steam isolation valve system has malfunctioned. The plant went into automatic emergency shutdown in August 2015 to prevent a buildup of reactor pressure after a steam isolation valve closed when it should have remained open.

Just weeks after that incident, federal regulators shifted Pilgrim into its so-called Column 4 performance category — as low as a plant can go without being ordered to simply shut down.

Neil Sheehan, spokesman for the Nuclear Regulatory Commission, said the valve problem last summer was “one of the factors that contributed to the plant being moved to Column 4,” since it resulted in an emergency shutdown.

When Pilgrim workers discovered the valve was not closing fast enough to meet standards Tuesday, they closed one of the four steam lines and limited reactor output to 75 percent to prevent a pressure buildup. On Thursday, they were working toward full shutdown.

Sheehan said it was “too soon to tell if wear and tear is the cause of the slow closure time.”

“Entergy is evaluating the cause of the valve slowness and will have to develop a plan to fix the problem,” Sheehan said. “We will be closely following the company’s troubleshooting and repair efforts.”

Mary Lampert, director of Pilgrim Watch, said she could save the NRC the trouble of waiting for a cause to be found.

“Let me help them out,” she wrote in an email. ”The cause is a continuation of Entergy's record of poor maintenance — prioritizing their bottom line over public safety — and NRC allowing them to do so.”

After investigating last summer’s valve problem, Entergy concluded a faulty strut supporting the gas pressure line had allowed the line to drop onto the main steam line. After years of vibration, the gas pressure line ruptured and failed to open the valve.

“The August 2015 problem was, according to Entergy’s report, caused by an action pre-2001 that remained ‘hidden’ for over a decade until it caused the failure,” wrote David Lochbaum, director of the Nuclear Safety Project for the Union of Concerned Scientists, in an email.

The company checked the gas pressure lines on the other seven valves for signs of wear after the rupture.

“Entergy confined its fixes to only the specific thing that failed that time,” Lochbaum wrote. “It’s a good example of the difference between doing it right and doing it Entergy’s way.”

Lochbaum said one need only look at the NRC’s performance standards matrix, which ranks nuclear plants nationwide, to see Entergy is a poor performer in general.

“Only 11 reactors in the entire country are not in Column 1, where the NRC expects them to be,” Lochbaum wrote. “Poor performance has moved 11 reactors out of Column 1, and Entergy owns five of the 11 reactors, including all three of the worst performers.”

Pilgrim, along with Arkansas I and II, all Entergy-owned, are the only reactors in the NRC’s lowest performance column.