After 17 months of delay, the Veterans Administration yesterday agreed to give free medical treatment to military personnel who took part in open-air atomic tests and whose illnesses might have been caused by exposure to radiation.

The policy change, announced at a Senate Veterans' Affairs Committee meeting, was a belated victory for the nation's "atomic veterans"--an estimated 250,000 to 500,000 servicemen who participated in atomic tests in Nevada and the Pacific between 1945 and 1962.

Free medical care brings the veterans one step closer to obtaining the same limited rights that the VA has granted reluctantly to Vietnam veterans exposed to the defoliant Agent Orange.

But VA officials said the agency does not intend to soften its policy of rejecting most "atomic vets' " disability compensation claims VA scientists say they do not believe there is any evidence that those veterans were exposed to enough radiation to suffer health problems.

The agency also said it opposes legislation requiring several VA studies of "atomic veterans," including one comparing the health of veterans exposed to radiation with those who were not exposed. A similar study was ordered in 1979 for veterans exposed to Agent Orange but has not yet been started.

Congress ordered the VA in 1981 to provide free health care to veterans for any ailment that might reasonably be assumed to result from exposure to radiation or to Agent Orange. The VA began providing such care to veterans exposed to Agent Orange, but it limited treatment for "atomic veterans" to those suffering from cancer or thyroid diseases.

Those restrictions irritated Sen. Alan Cranston (D-Calif.), who has repeatedly demanded that the VA change its policy and yesterday criticized it for taking 17 months "to carry out the law as Congress intended."

A parade of witnesses, mostly from the National Association of Atomic Veterans, also denounced the VA for failing to enact standard procedures for deciding when "atomic veterans" should receive disability compensation.

A total of 2,067 "atomic veterans" have filed claims for a variety of disabilities that they blame on exposure to radiation. The VA has approved 29 of them, including 15 on appeal.

VA officials acknowledged under questioning by Sen. Alan K. Simpson (R-Wyo.) that the agency had failed to develop standard criteria for deciding atomic veterans' disability claims.

Cranston yesterday introduced a bill that would force the VA to draft such standards both for exposure to radiation and to dioxin-contaminated herbicides such as Agent Orange. It also would require the VA to "resolve every reasonable doubt in favor of the veterans."

In deciding disability claims, one factor used by the VA is the amount of radiation a soldier received as measured by a film detection badge worn during the test. A Defense Department witness said the badges that show only a few servicemen received "significant" doses of radiation.

But Dr. Edward Martel, who helped plan some of the tests, said the badges were unreliable, in part because they did not measure how much contaminated air a soldier inhaled.

A witness from the national Centers for Disease Control in Atlanta also testified that a "statistically significant" increase in cancers and certain blood diseases had been found in soldiers who witnessed a 1957 blast, even though the Army said the badges showed that none of the servicemen had received significant doses of radiation.