Public hearings on whether to re-open the undamaged Three Mile Island 1 reactor will take at least a year, further jeopardizing the parent company's already shaky financial status, the Nuclear Regulatory Commission learned yesterday.

A spokesman for General Public Utilities, which owns both Three Mile Island 1 and 2 in Pennyslvania, said the company had been counting on the unit unaffected by the March accident coming back on line by Jan. 1. The delay could be "a significant financial problem" if short-term credits are withdrawn as a result, the spokesman said.

Three Mile Island 1 was closed with several other plants for changes in the wake of the March 28 breakdwon of Three Mile Island 2. Cleanup of unit 2 will not begin for several years and it is not known when, if ever, it will resume operation.

The commissioners decided last week to rule personally on whether unit 1 could reopen and to do it only after a full public hearing. Other units that were closed, including Rancho Seco in California and Davis Besse in Toledo, Ohio, will be allowed to operated while hearings on their status continue.

Kenneth McKee of GPU estimated that the untility is spending $22 million to replace loct nuclear power with more expensive oil-fired power for every month the two untis are out of action. Cost involving unit 1 alone are about $13 million, he said.

The NRC staff said the year's proceedings will break down as follows: 20 calender days for filing petitions to participate in the hearings, 15 days for the commission to answer the petitions, 25 days for the petitions to be amended and for issues of the hearing to be refined, 15 days for the petitions to be examined and for a special pre-hearing conference, and five days to set the agenda. That is 80 days.

Then there are 60 days for recovery of information, during which a staff safety evaluation comes out, and on the 60th day there is the main pre-hearing conference that sets dates and priorities. There are five more days to the issurance of the pre-hearing conference order, 20 days to file testimony and 15 days to the start of the hearing. We are now at Day 180.

Sixty days later the hearings end. Parties filed proposed findings by 40 days later, replies are filed in 10 days and a decision is reached by the Atomic Safety and Licensing Board in another 10 days. That is 335 days. Then the NRC itself has four months to review the decision and issue its order.

"This is the shortest feasible schedule," said Guy Cunningham, assistant chief counsel for the hearings. "In practice most schedules include extensions of time."

The NRC also voted to ask for public comment on its intention to rquire more stringent state and local emergency planning as a condition of issuing future operation licenses to nulcear reactors. General Counsel Leonard Bickwit said that procedure could take us as little as six months.