Pennsylvania Gov. Richard L. Thornburgh said yesterday that lack of credible information was his biggest problem in deciding how to react to the nuclear accident at Three Mile Island, and he recommended a series of steps to prevent such a situation from occurring again.

In the two days immediately after the March 28 accident, "We were plagued by a variety of sources of information whose credibility was eroding," Thornburgh told the presidential commission investigating TMI.

The commission, which opened three days of public hearings here yesterday, is focusing on Thornburg's decision not to evacuate the area around the troubled power plant after the accident.

The governor insisted that there was "no reluctance on my part to act," and that an evacuation "would have been defensible." But a nuclear accident, unlike a natural disaster, such as a flood, does not have clearly definable limits, he said, and an attempt to evacuate a five-mile radius around the plant could have started people fleeing from 10, 20 or even 100 miles around.

That consideration, coupled with the fact that radiation readings around the plant were not alarmingly high, led to a decision against a general evacuation.

Thornburgh sought to emphasize, however, that all his early decisions had to be made in an atmosphere of confusion and rumor.

On Friday, March 30, his office received a telephone call from Harold Collins of the Nuclear Regulatory Commission urging an evacuation.

"I had never heard of Collins then, and to this day have never met him," Thornburgh said, adding that by then his "reliable sources of information had just about dried up."

Looking back on the confusion, Thornburgh offered three recommendations to the commission:

That the NRC tighten its licensing process to require improved training for plant operators and management. He noted a "thinness of technological capability" on the part of TMI's operator, Metropolitan Edison Co.

That the NRC provide either an on-site resident inspector for each plant or a quicker response from Washington to eliminate the sort of "interim of uncertainty" he faced in the first few days of the accident.

That "mechanics of communication," primarily telephones, at atomic plants be improved.

Concerning the future of nuclear power, Thornburgh said that it is "no longer a field that can be left to the experts," and that when plant sites are being chosen, a much broader constituency should be consulted than has been in the past.

He also said states should have to inspect plants within their borders, and should develop the capability of gathering expert information during a crisis.