Construction of the controversial Seabrook, N.H., nuclear power plant could be resumed as early as next week as a result of a decision yesterday by the Environmental Protection Agency.

EPA Administrator Douglas Costle yesterday reaffirmed the agency's earlier decision that the plant's planned open ocean cooling system complies with the Clean Water Act.

The Nuclear Regulatory Commission halted construction of the $2.3 billion facility - New Hampshire's first atomic power plant - on July 21 to give the EPA time to evaluate the cooling system and to give the NRC time to look at possible alternative sites for the plant.

NRC spokesman Carl Stoiber said yesterday that the commission will now give the parties involved until noon Wednesday to submit their comments on the EPA determination. Shortly after that, Stoiber said, the NRC will decide whether to allow construction of the plant to resume.

Only hours after yesterday's EPA announcement, both Public Service of New Hampshire - the utility building the plant - and the NRC staff filed their comments. Both called on the commission to allow construction on the twin-reactor, 2,300-megawatt unit to resume.

The EPA decision came as a shock to environmental groups that have fought the power plant in the courts and through regulatory boards for eight years. Spokesmen for the groups say they will meet soon to decide whether to again appeal the EPA decision.

The Clamshell Alliance, a coalition of the antinuclear power groups that has held two mass demonstrations at the plant site, attacked the EPA decision, saying it was "a serious, even tragic, violation of the responsibilities given the EPA by the American people. [It was] made with unprecedented haste in a politically charged atmosphere." Alliance sources said the group is now considering further civil disobedience protests.

"If construction is allowed to resume, citizens around the nation will be forced to conclude we have no meaningful recourse through the regulatory agencies," said spokeswoman Sharon Tracy.

Environmental groups claim the cooling system in question which will discharge water heated to 39 degrees above that of the ocean, is detrimental to the prolific marine life in the estuary off the tiny coastal community of Seabrook. Costle disagreed, saying the system, which utilizes 25-mile-long tunnels to carry ocean water to and from the plant, "would not have a significant effect" on the "population of fish, shellfish and wildlife in and on the receiving waters."

Pro-nuclear forces were jubilant.

"I'm on cloud nine," said New Hampshire Gov. Meldrim Thomson, an advocate of the Seabrook plant who went to Washington this week to voice his support for the project. "All our sky is bright and sunny and every cloud in New Hampshire has a silver lining now . . ."

"We're delighted: we're very, very happy," said Gordon McKenney of PSNH, the utility that has invested more than $400 million in the project. Company officials say the delays have cost PSNH about $15 million a month. He said appeals by environmentalists were nothing but "legal harassment."

Delays have also meant layoffs of about 1,800 workers at the site. A suit filed by the New Hampshire Building and Construction Trades Council aimed at heading off the work stopage order failed in a federal court in Washington.

But another unresolved issue is the NRC search for alternative sites. Hearings on possible new sites for the plant are scheduled for October.

"The alternate site issue is very difficult one" said attorney Robert Backus, representing the environmentalists. "[It] is not going to easily be ignored. The Public Service Co. has a Burger King approach - they want to do it their way."

He noted that the NRC must also consider environmentalists' claims that the Seabrook site is subject to earthquakes. "It's not clear to me that the commission is going to stop looking for alternative sites," he said.

But an editorial by William Loeb, publisher of the powerful statewide daily newspaper, the Manchester Union Leader, said:

"Any further delays by the commission would be further evidence of malice and a determination by the Carter administration to stop the Seabrook plant."