Harrisburg, Pa. — The US Nuclear Regulatory Commission has opened hearings on the proposed restart of the undamaged Three Mile Island nuclear reactor. The proceedings, lasting 4 to 6 months, will determine whether Metropolitan Edison Company can safely operate a nuclear plant. The undamaged Unit No. 1 reactor, shut down for refueling in January 1979, was set to restart when the nation's worst commercial nuclear accident damaged its twin Unit No. 2 reactor on March 28, 1979. On Aug. 9, the commission prohibited the restart of Unit No. 1, pending hearings to find out whether the $380 million reactor, which first went on line in 1974, could be operated safely in view of the accident. Company officials say the reopening is crucial to the financial viability to the company, which faces a possible $1 billion bill to clean up Unit No. 2.