The Nuclear Regulatory Commission yesterday approved an immediate resumption of construction on the controversial Seabrook, N.H., nuclear power plant.

By a 4-to-0 vote the NRC said it accepted the Aug. 4 Environmental Protection Agency reapproval of the proposed cooling system, a decision the NRC said "eliminates the condition which led to the suspension of the Seabrook construction permits."

The NRC had stopped work on Seabrook on July 21, pending the EPA review of the cooling system, which uses twin tunnels to carry ocean water to and from the plant site, to cool the 2,300 megawatt plant's two reactors.

Despite the fact that he voted with the majority, NRC member Peter Bradford attached an unusual added opinion to yesterday' ruling, criticizing federl handling of the project.

Bradford contends in his statement that the NRC is, in essence, violating the National Environmental Policy Act by virtually eliminating alternative sites for the plant. The act requires that NRC do "a detailed statement on alternatives," Bradford wrote.

By allowing construction of Seabrook to begin in July 1976 without adequately studying alternatives stires, the NRC "forclosed precisely the evaluation that the law requires," Bradford contended.

The NRC said yesterday that it has "deferred decision" on whether or not to suspend completely the search for an aleernate site.

Since consideration in any site comparison is cost, it is nearly impossible for any other site to be more cost-efficient than the one at which construction has already begun. "Unless," on NRC source said, "you find a site that just happens to have 10 percent of your foundation completed."

But Bradford's opinion indicates that his concern is too late in this case. He calls on the NRC to guard against similar problems in the future by deferring the issuance of work permits until all environmental issues are settled.

Yesterday's decision drew predictable reactions from both sides of the issue.

"I'm getting increasingly more enraged," said Diann Garand, an 18-year resident of the tiny coastal community of Seabrook. She has been fighting the plant's construction for nearly a decade.

"Dealing with regulatory agencies is like bashing your head against a stone wall," Garand said. "I'm not terribly surprised at the NRC decision because the whole set of hearings is a sham. I don't know what they're waiting for - a major disaster, I guess."

Public Service Company of New Hampshire, the private utility building the plant, has sunk $400 million into construction already. Construction began in July 1976, was stopped from March 1977 until August 1977, and then stopped for a second time from July 21, 1978, until yesterday.

PSC spokesman Gordon McKenney said the company was "obviously pleased" that construction will be allowed to begin again on the $2.3 billion plant. He said PSC will immediately begin reassembling workers to resume construction.

"I can't wait to get back," said William Somerby, one of about 1,800 workers laid off when construction was halted. "Maybe I can pay some back bills now."

Somerby, whose property in Hampton Falls abuts the Seabrook power plant property, said, "I don't see how they can stop it anyway; they've already sunk too much money into it."

The work stoppage has cost $436,000 a day, PSC claims, mostly in interest costs. PSC attorney Thomas Dignan, Jr., predicts no further construction delays caused by the courts or the regulatory agencies.

But Robert Backus, an attorney for two antinuclear groups that put together the Clamshell Alliance of several protesting organizations, disagrees. He has already filed court challenges to the decisions by the NRC and EPA, and he says NRC commissioner Bradford's opinion will be submitted in support of the contention that the government violated the law in allowing construction to begin on the plant.

Clamshell, meanwhile, is considering sending 18 protesters to the Seabrook site today in an act of civil disobedience aimed at further dramatizing the groups opposition to the plant.

The number 18 is significant because 18 protesters were arrested on the site on Aug. 1, 1976, in the first illegal "occupation" of the plant by Clamshell. There have been two demonstrations with thousands of participants since then.