Under heavy pressure from opponents of nuclear power, the Nuclear Regulatory Commission yesterday postponed a decision on reopening an undamaged reactor at the Three Mile Island power plant until hearings are completed on the plant operators' competence and integrity.

The five-member commission put aside a staff recommendation that would have set the reopening process in motion. More than 100 residents of the area around the plant, which is 10 miles southeast of Harrisburg, packed the commission meeting room.

The residents, who have been fighting the reopening for almost six years, declared the delay a victory. "I think they commissioners were prepared to vote for a reopening, but they got scared off by the political heat," said Joanne Dorshow, a lawyer for the Government Accountability Project, which has been working with residents. "They wanted to slip it by the people."

However, the claimed victory was expected to be short-lived.

Philip R. Clark, president of General Public Utilities Nuclear Corp., which operates the plant, said in a statement that he was pleased with the NRC's "apparent determination to press" for a decision on reopening the reactor and that he looked forward to "a decision at the earliest possible time."

TMI's Unit 1 reactor was closed for repairs March 28, 1979, the day the core of its twin Unit 2 reactor, 200 yards away, overheated, the worst accident in the history of commercial nuclear power.

The plant has remained shut under NRC order ever since, despite repeated requests by GPU to reopen the undamaged unit.

An NRC atomic safety and licensing board has been holding hearings since November on the company's integrity and competency to operate a nuclear plant.

Reportedly under consideration yesterday was a recommendation from the NRC general counsel's office to proceed with a vote on restarting the Unit 1 reactor before the hearings are completed. The commission refused to make the recommendation public.

The board has completed the first phase of hearings -- a look at whether company President Herman Dieckamp lied to Congress and the NRC about the accident. The second phase, on whether the company has an adequate training program to operate TMI safely, is nearly complete.

Hearings are pending on the company's integrity and whether it falsified key safety records prior to the 1979 accident. Metropolitan Edison, the GPU subsidiary that operated TMI at the time of the accident, pleaded guilty last year to using inadequate safety tests at the plant and fudging the results.

Commissioner James K. Asselstine objected yesterday to any short-circuiting of the hearing process and expressed bewilderment about why the NRC staff now seems to be looking favorably on restarting the reactor.

"The staff tells us this organization General Public Utilities Nuclear Corp. is okay largely because they're different people" from the ones operating TMI at the time of the accident, Asselstine said. He said hearings should examine whether the company fostered an environment that allowed safety violations.

Meanwhile, an action by the judge conducting the hearings has called the process into question. In a letter last month to U.S. District Court Judge Sylvia Rambo, administrative law judge Ivan Smith asked for leniency in sentencing James R. Floyd, a former TMI supervisor convicted of cheating on NRC operator exams.

"I have always felt that Mr. Floyd's deception was an impulsive act and that it was not motivated by personal ambition," Smith wrote. "Deception in the future is very unlikely."

Accusing Smith of "at least the appearance of impropriety and bias," the Commonwealth of Pennsylvania last week asked him to disqualify himself.