A scientific panel yesterday said that despite a five-year, $265 million research effort, it is still unable to say whether the potential radioactivity from nuclear power plant accidents is much less than previously thought.

The long-awaited report from an American Physical Society group shocked government and industry officials who had anticipated that the research would provide a basis for weakening federal nuclear safety regulations.

Harvard University physicist Richard Wilson said his panel "declined to agree" with an assertion last last year by the American Nuclear Society and other industry groups that the maximum amount of radioactivity that could be released is only one-tenth to one one-thousandth the amount.

"Some on the review committee felt we should explicitly disagree with it," Wilson told the Nuclear Regulatory Commission during a two-hour briefing. "No one wanted to support the conclusions of the ANS."

Industry and government officials were clearly dismayed after the meeting.

"I don't know what it all means yet," said Carl Goldstein of the Committee for Energy Awareness, an industry organization devoted to promoting nuclear power and coal-generated electricity. "They certainly hedged."

NRC Commissioner Fred Bernthal said he was "troubled that here we are in 1985 with significant additional research that needs to be done. This issue should have been addressed 10 years ago."

Industry and NRC officials had looked to the American Physical Society "peer review" and acceptance of the ANS conclusions as a point from which to begin rewriting regulations to reduce the evacuation zones around nuclear plants from a 10-mile radius to two miles.

The research also was anticipated to provide the justification for changing policies that prevent future plants from being located near population centers and asking Congress to forgo increasing the required insurance coverage for accidents, as the NRC has recommended.

The research was spawned by the March 1979 accident at the Three Mile Island plant, the worst in the industry's history, though no one was killed and observations that much less radioactive iodine was released than expected.