Pennsylvania Gov. Richard L. Thornburgh urged the Nuclear Regulatory Commission yesterday not to restart the Three Mile Island nuclear power plant's undamaged reactor until the NRC can assure that an accident like the one that closed the plant six years ago will never happen again.

"I find your declaration that 'no further hearings are warranted' to be ill-timed, ill-conceived and inexplicable," Thornburgh, a Republican, told the five NRC commissioners, who have scheduled a vote next week on whether to restart the the plant's undamaged Unit 1 reactor. "You must settle all the issues relating to the health and safety of the people before you even think about cranking up that reactor again."

For the past year, the NRC has held dozens of hearings on the request of General Public Utilities Corp., the plant owner, to lift orders keeping Unit 1 closed. In February, it voted to close off the hearing record with the session yesterday.

Thornburgh was joined by Pennsylvania Sen. John Heinz (R) and Arlen Specter (R), who spoke before an overflow crowd that interrupted them frequently with applause.

Heinz told the commission, "We urge you to continue holding public hearings on the safety issues and on the issues of management integrity and competence. This is a serious public issue that the public has a right to deal with in public hearings. The failure to do so creates the appearance of a cover-up. What is this commission trying to avoid? What is it afraid it will find?"

Specter said: "Continue these hearings. They might take five days, they might take 10 days, they might take 17 days, but you must resolve these issues to the satisfaction of the people of Pennsylvania. If there is one thing that is sacrosanct in this system, it is due process of the law."

But Rep. Don Ritter (R-Pa.), whose district is one of three that would receive electricity from the plant, said, "The record must speak for itself. That record shows that this is one of the most adjudicated public hearings ever held and that record shows TMI Unit No. 1 can be operated safely." Ritter's remarks also drew cheers in the packed NRC hearing room.

But Reps. Bob Edgar (D-Pa.) and George W. Gekas (R-Pa.), whose constituents also would be served by the reactor, supported further hearings. Gekas said: "I can't perceive how we can proceed with the restart of Unit 1 before we clean up Unit 2 -- and I mean clean up Unit 2."

Gekas contended that hearings have focused on technical problems, which he said did not cause the accident at TMI. "The fundamental problems are people-related," he said, "and to find out what caused these fundamental problems you have to continue the hearings."

Thornburgh contended that the NRC has never dealt with the question of the management integrity of Metropolitan Edison Co., which operates the plant. Officials of Metropolitan Edison were convicted of falsifying data on the plant in reports filed with the NRC.

"I find it disturbing, to say the least, that you would entertain a restart vote without a full and open hearing of your own," Thornburgh said.

Leaders of public-interest groups opposing the restart of the plant vowed to sue the commission if it votes to allow the reactor to be restarted.

"You can be assured we will challenge you in the courts," Norman Aamodt of the Susquehanna Valley Alliance told the commission, "and you can be assured that the delay imposed by that challenge will be longer than any delay you could expect by resuming the hearings."

"You have insulted the governor of Pennsylvania and have turned your backs on the public you've been chosen to protect," said Vera Kucinski, chairman of Three Mile Island Alert. "A vote for restart is a clear vote against the people of central Pennsylvania."

Specter suggested that it might be time to change the NRC into a regulatory agency with a "single, strong commissioner." He said he is thinking seriously of introducing legislation to accomplish that. "The delay in this case," he said, "is a strong argument for a single administrator or at least a change in its present makeup."