WASHINGTON,  Dec, 14 - Military and nuclear energy officials were motivated by fear of lawsuits and unfavorable publicity in their decisions to keep secret many experiments using radiation on humans, Federal investigators have found.

The President's Advisory Committee on Human Radiation Experiments, the panel charged with unearthing the history of all government-sponsored experiments in which radiation was used on humans, has found new documents showing that as early as 1947 public relations and legal considerations, not security concerns, were principal motives in the decision of Federal officials to cover up radiation experiments.

In the past six months, the panel has searched out and logged hundreds of thousand of papers on experiments with humans, beginning in 1945 at the dawn of the atomic age.

Until the panel began its work, the experiments were thought to have been scattered incidents, but the documents show that they were part of a plan that was debated and approved at high levels. It was also previously thought that there were only a handful of such experiments, but the panel has found hundreds, from the deliberate release of radiation onto the air to the injection of people with radioactive plutonium.

The most recent documents unearthed at the Oak Ridge National Laboratory in Tennessee show that officials of the military and the Atomic Energy Commission at first sought to declassify reports of experiments on humans, in accordance with public statements that scientific reports should not be secret.

For example, Dr. Alan Gregg, the chairman of one of the committees that oversaw the human experiments, wrote in May 1947: "A policy of secrecy in science is neither personally courageous nor politically wise. As Lord Acton said, "Power corrupts, and absolute power corrupts absolutely."

But at the same time, C. L. Marshall, a declassification officer with the Atomic Energy Commission, wrote in February 1947 that a scientific paper outlining experiments in which two people were injected with plutonium should not be declassified. "This document appears to be most dangerous since it describes experiments performed on human subjects, including the actual injection of the metal, plutonium, into the body," he wrote.

He noted that there was no statement in the paper about whether the patients experimented on had given their consent, and concluded: "The experimenters and the employing agencies, including the U.S., have been laid open to a devastating lawsuit, which would, through its attendant publicity, have far-reaching results".

It has long been suspected that legal and public relations concerns helped drive the ethical debate over the experiments on humans, and the advisory committee in recent weeks has found many memorandums substantiating that.

On Oct. 8, 1947, J. C. Franklin, the manager of operations of Oak Ridge, wrote to Carroll L. Wilson, general manager of the atomic agency, "There are a large number of papers which do not violate security but do cause considerable concern to the Atomic Energy Commission insurance branch, and may well compromise the public prestige and best interests of the commission." He added: "Papers referring to levels of soil and water contamination surrounding Atomic Energy Commission installations, idle speculation in the future genetic effects of radiation and papers dealing with potential process hazards to employees are definitely prejudicial to the best interests of the Government. Every such release is reflected in an increase in insurance claims, increased difficulty in labor relations and adverse public sentiment."

He ordered that any such documents be edited or kept secret.

The advisory committee investigators said that documents so far found that an official as highly placed as Dr. Shields Warren, head of the medical division in Washington, had classified human experiments because of public relations or legal implications.

The investigators said the classification on that basis may have been illegal.