PLYMOUTH — Watchdogs are relieved that newly published regulations to reduce greenhouse gas emissions in Massachusetts contain a provision that will prevent Pilgrim Nuclear Power Station from picking up subsidies that might keep it open past its announced 2019 closure date.

The clean energy standard, developed by the state Department of Environmental Protection and Executive Office of Environmental Affairs, qualifies only zero-carbon producers that became operational after Dec. 31, 2010, for clean energy credits. Pilgrim has been operating since 1972.

“This is a happy day,” Pilgrim Watch director Mary Lampert said. “For a period of time, we were worried.”

Entergy Corp., Pilgrim’s owner-operator, plans to shut down Pilgrim on May 31, 2019, because it is a financial loser that has been unable to compete with inexpensive natural gas. Subsidies for producing zero emissions could have changed the plant’s future, activists worried.

Entergy had announced a plan to shut down James Fitzpatrick Nuclear Power Plant in New York but instead sold it to Exelon after the state instituted subsidies for nuclear plants.

Department of Environmental Protection Commissioner Martin Suubert said the date preventing older clean-energy generators from qualifying for credits was aimed at encouraging development of new zero-emissions operations in Massachusetts.

“We wanted to make sure we were increasing the procurement of new clean energy,” Suubert said.

The spokesman for the Nuclear Energy Institute, a policy organization that represents the nuclear industry, said state regulators were making a mistake.

“Nuclear is the largest source of clean energy,” John Keeley said. “If you valued nonemitting technologies, why would you want to eliminate any of them?”

Although the December 2010 cutoff had been in a draft of the regulations published in December, it came up for reconsideration when nuclear plant owners Exelon, Dominium Resources, and NextEra Energy Resources argued older nuclear power plants should qualify.

Groups such as Pilgrim Watch, Cape Downwinders and Jones River Watershed Association submitted their own arguments to the state for keeping Pilgrim out of the clean-energy mix.

“This should be a boost to the public’s morale and encourage the public to get involved,” Lampert said of their success. “If the public had not mobilized on this, the outcome could have been much different.”

Under the state’s new provisions, electricity providers must procure minimum percentages from clean energy generators. In 2018, that minimum must be 16 percent, and it will increase by 2 percent annually until 2050, when electricity providers must procure a minimum of 80 percent of their energy from clean sources.

Keeley predicts gas emissions in Massachusetts will go up when Pilgrim shuts down. “That’s what happened in California, Wisconsin and Vermont,” he said.

One watchdog said lack of tax credits was not why Pilgrim was shutting down. “The issue with Pilgrim isn’t about energy credits, it’s about public safety,” said Diane Turco, president of Cape Downwinders. “It’s such a damaged reactor, there isn’t any way it could be resurrected through any kind of legislation.”

Leslie Sandberg, district director for state Sen. Julian Cyr, D-Truro, recently reported to citizens groups that legislators had been told by ISO New England — the organization that handles electricity distribution — that Pilgrim must operate until May 31, 2019, or pay $150 million in penalties for every 100 hours of missed electricity.

Indications are that the plant will not operate after that date.

“When a resource notifies the ISO that it is permanently retiring, the resource forfeits its interconnection rights,” ISO spokeswoman Marcia Blomberg wrote in an email. “If the resource later decides to reconnect, it would be required to go through the interconnection study process as a new resource, and its application to interconnect to the high-voltage power grid would be studied behind other applications that were ahead of it in the interconnection queue.”

Entergy spokesman Patrick O’Brien stressed there was no intent to change course, writing in an email, “We made a commitment to serve as a capacity resource for the region until May 31, 2019. Our plans remain unchanged — we are focused on safe and reliable operation and will cease operations of Pilgrim Station no later than June 1, 2019.”

— Follow Christine Legere on Twitter: @ChrisLegereCCT.