A federal appeals court in Philadelphia blocked the planned restart of the undamaged reactor of the Three Mile Island nuclear power plant yesterday.

TMI Unit 1, although undamaged, was idled in 1979 when the nation's worst civilian nuclear accident forced the shutdown of its twin reactor, Unit 2. Cleanup operations continue on Unit 2, which was severely damaged by a partial meltdown.

In a one-page order by Judge Collins J. Seitz, the 3rd U.S. Circuit Court of Appeals ordered that the Unit 1 reactor remain in cold shutdown pending a hearing June 27 on the Nuclear Regulatory Commission order last week allowing the undamaged reactor to restart.

The order was requested by the state of Pennsylvania, the Union of Concerned Scientists and a central Pennsylvania public interest group, Three Mile Island Alert.

The court order was issued as workers at Three Mile Island had begun turning on generators to warm up the plant and pumps to introduce cooling water into the reactor chamber.

GPU Nuclear Corp., the operator of the TMI plant, said it planned to lower the uranium fuel rods into the reactor vessel Tuesday, beginning the chain reaction to produce electricity. The utility had said it planned to bring the plant to full power by mid-September.

"The warm-up is going smoothly at this point," GPU Nuclear spokesman Douglas Bedell said yesterday before he learned of the court order. "We anticipate having the plant in a hot-shutdown status ready to produce power over the weekend."

Seitz said that his ruling considered "the protracted history and present posture of this matter as well as the pervasive nature of the public interest involved" in the 1979 Three Mile Island incident.

Seitz set June 27 for the first hearing on the case, declaring that an "expedited review of the case was warranted."

"We regret the court's action relating to the stay but we are pleased the court will consider bringing the issue to a close expeditiously," said GPU spokesman Bedell.

The NRC said it will have no comment until the case is heard. The NRC told the court in a 139-page document filed Thursday that the commission had "conducted the most comprehensive NRC judicatory proceeding ever held," concluding that the risk of another accident at the plant is "so low that it cannot amount to irreparable injury for purposes of a stay."

In a statement released by his office at the state capitol in Harrisburg, Gov. Richard L. Thornburgh said the court's action is "encouraging, to say the least, to those of us who believe that all health and safety issues be resolved prior to any Unit 1 restart."

Thornburgh told the NRC last week before its 4-to-1 vote permitting restart that he felt the NRC had not held comprehensive hearings on whether incompetence triggered the 1979 accident and whether the lives of the people of central Pennsylvania had been endangered.

On Thursday, NRC regional administrator Thomas Murley said the NRC had teams of technicians on hand to supervise the restart.

He said: "One expects the operators of the plant might have gotten rusty. There's a substantial amount of machinery here that hasn't been run for more than six years, but we've satisfied ourselves that this plant is ready to go."