Nearly 17,000 of the 42,000 U.S. servicemen and civilians who took part in two atomic tests at Bikini Atoll in 1946 probably were exposed to dangerously high radiation, Sen. Alan Cranston (D-Calif.) charged yesterday as he released a General Accounting Office report on the tests.

A 1984 Pentagon study asserted that participants in the tests -- one in the air and the other underwater -- were exposed only to low, safe levels of radiation. The Veterans Administration, citing those findings, has refused to pay the disability claims of 2,500 veterans who blame their health problems on the exposure.

Cranston asked President Reagan to order the Pentagon to conduct another review of the tests. He said that when GAO investigators turned over their report, they told him that about 40 percent of the participants, almost 17,000 people, had suffered higher radiation exposure than previously thought.

The 16-month study does not attempt to relate exposure to subsequent illness but concentrates instead on what it portrays as unreliable monitoring, lack of safety equipment and generally careless procedures by the Navy, Army and other federal agencies involved.

According to the report, one of the tests' weakest elements consisted of participants' "film badges" that measured only up to 2.0 rems of radiation. A rem is the effect of exposure to 1 roentgen of gamma radiation; a roentgen is the quantity of radiation that will produce ions carrying one electrostatic unit of electricity.

During the two-week test period, the military arbitrarily chose 60 rems as the safe limit of exposure; cumulative radiation was not measured. The permissible exposure now is 5 rems per year.

Because the badges were unable to register more than 2 rems, scientists who wrote the Pentagon report counted a fully exposed badge as 2 rems. GAO investigators contended that the level of exposure likely was much higher.

The GAO report also found:

*Most test participants wore little, if any, protective clothing.

*Decontamination procedures were not worked out until nearly a week after the second explosion, during which 34,500 Navy and Army personnel aboard more than 200 ships entered the Bikini lagoon, some within hours of the detonation.

*There was no measurement of radiation that could have affected personnel who in the following weeks returned to unmanned "target ships" that had been anchored in the lagoon during the explosion.

At yesterday's news conference, a veteran of the testing declared his certainty that exposure to the radiation resulted in physical ailments.

Fred C. Thompson, 60, of Alexandria, Va., said, "I've got heart disease, diabetes, muscle and joint problems." Thompson, a vice president of the National Association of Radiation Survivors, noted that such problems are recognized by Japanese nuclear-medicine experts as aftereffects of radiation.

Thompson recalled that the ship he served on in 1946 sailed into the target area 2 1/2 hours after the underwater explosion, and that he later boarded target ships and even went ashore without protective clothing. When he experienced swollen hands after cleaning a contaminated radar antenna, he said, he was treated for heat rash.

"If I'd known then what I know now, I'd have jumped ship," he said.