By a 4-to-1 vote that drew cries of "murderers" from the audience, the Nuclear Regulatory Commission moved yesterday to permit the restart of the undamaged half of the Three Mile Island nuclear power plant, shut down more than six years ago after the worst commercial nuclear accident in history.

"This decision has not been an easy one for me, but the facts as currently known do not raise a significant safety issue," NRC Chairman Nunzio J. Palladino said in voting with the majority. "We now have reasonable assurance that TMI can and will be operated safely."

The action, which will not take effect immediately, rescinds two 1979 orders shutting down reactor No. 1, which was being refueled on March 19, 1979, when its twin reactor at the Middletown, Pa., plant suffered a partial meltdown of its uranium fuel core.

Yesterday's vote to restart Unit 1 came despite protests from Pennsylvania Gov. Richard L. Thornburgh (R) and Sens. John Heinz (R-Pa.) and Arlen Specter (R-Pa.). Palladino said the restart order would be delayed to allow "aggrieved parties" to seek relief in U.S. court. At least three lawsuits seeking to prevent the order from taking effect are expected to be filed.

More than 200 people marched on the plant yesterday to protest the NRC vote; more than 80 demonstrators were arrested.

NRC Commissioner James K. Asselstine cast the vote against the restart, declaring to frequent applause that the commission had "become satisfied with Band-Aid, short-term fixes to this question. This vote does not promote public confidence and only hardens public opposition to the restart of Three Mile Island."

While voting with the majority, Commissioner Frederick Bernthal said he was against the restart. "This commission should have gone the extra mile and reopened hearings on this whole restart issue," he said. "This vote has set the stage for endless wrangling on what is true and untrue, what is right and wrong and what is known and unknown. I suspect the only winners in this will be the lawyers, whose future in this case is now assured."

The vote was no surprise to the overflow crowd in the NRC hearing room here but the meeting was emotional nonetheless.

Asselstine and Bernthal were frequently applauded as they spoke against restart. Shouts of "Right on!" were heard as the two commissioners made a point, and when Palladino announced that restart had been approved, the crowd became angry.

"Murderers! You are all murderers!" one person shouted. Said another: "You are murdering our children."

The Reagan administration supported restarting the undamaged No. 1 reactor as a step toward returning the "nuclear option" to the electric power industry. Last week Energy Secretary John S. Herrington called TMI a national security issue, telling reporters: "We must get Three Mile Island restarted."

The industry has long conceded that the partial meltdown at TMI six years ago eroded public support for nuclear power. The accident brought new construction and licensing in the United States virtually to a halt, and delays brought on by more than 150 days of hearings sent construction costs soaring for previously approved plants. Also, new licensing regulations forced utilities to pay higher interest rates on bank loans for plants under construction.

Though nobody was injured in the TMI accident, signs of what occurred there are still visible in the Pennsylvania countryside southeast of Harrisburg. The dome of the crippled No. 2 reactor was removed only a few months ago, and the "plenum" that held the damaged fuel rods in place was detached last month.

Cleanup of the reactor is expected to cost $1 billion and take 10 years, perhaps longer. The expense is to be shared by the nuclear power industry, a special fund started by Thornburgh, and the Energy Department, which said it will finance its share as a "research and development exercise."

The extent of the damage to the No. 2 reactor has yet to be determined, but closer and closer examination has turned up more and more disturbing findings.

A longstanding estimate that the exposed fuel suffered temperatures of 3,500 degrees Fahrenheit when it lost its cooling water has twice been revised upward. The second and latest calculation was 5,100 degrees, meaning that at least a partial meltdown occurred.

"This commission should have waited for an expeditious cleanup of what is still the worst accident in nuclear history before taking a restart vote," Commissioner Bernthal said yesterday. "Leaving the cleanup to an uncertain future in the hands of an uncertain utility General Public Utilities Inc., owner of TMI is a neglect of elementary public policy decision-making," he said.