An 18-month government study has confirmed that twice as many leukemia cases as expected have developed in ex-GIs exposed to low-level radiation while participating in a 1957 nuclear weapons test nicknamed Smoky.

Although the numbers are small -- eight leukemia cases instead of the 3.5 that would be normal for the 3,100-man group -- the initial findings of the inquiry by the Department of Health, Education and Welfare's Center for Disease Control will have a major effect on the controversy that has developed in the past year over the long-term health effects of low-level radiation.

That is because it comes as a White House-ordered interagency task force on radiation exposure is about to make recommendations after an eight-month inquiry of its own.

One of the msot sharply debated issues within the interagency group in determining what federal care and benefits should accrue to ex-GIs and federal employes who claim that exposure to low levels of radiation brought on cancer years later.

Most government agencies and officials have maintained there was a threshold radiation level and that exposure below that level would not cause an increased risk of cancer.

Based on that assumption, the Veterans Administration has turned down almost all claimants who linked their exposure to radiation during 1950s nuclear weapons tests to leukemia or other forms of cancer they subsequently developed.

A year ago, however, the Smoky case was made the focus of public attention through hearings of the House subcommittee on health. At Smoky, GIs maneuvered near ground zero hours after detonation of a 44-kiloton shot -- three times the power of the Hiroshima bomb.

One Smoky veteran with leukemia, Paul Cooper, was awarded service-connected disability after his case was publicized, but the payment was linked to an illness before his discharge, not to radiation at Smokey. Cooper died of leukemia in February last year.

Last August the VA awarded beneifts to another Smoky veteran with leukemia, Donald C. Coe. That came only after Rep. Time Lee Carter (R-Ky.), a medical doctor and ranking minority member or the subcommittee,, pressured the agency on Coe's behalf.

At the time, VA officials maintained the award was not setting a precedent for the hundreds of radiation-caused cancer claims coming to the agency.

Since then, according to VA officials, seven other radiation cases have been turned down by the VA Board of Veterans Appeals and one has been granted, but not with radiation cited as the basis.

Fifty-one other cases are apending.

A VA official involved in veterans benefits said last week his agency was "eagerly awaiting" results of the CDC report. He hoped it could provide some clear indicator that a statistical relationship did or did not exist between the radiation levels at Smoky and subsequent cancer among the veterans.

According to informed sources, the leukemia finding agmong Smokey veterans is considered "close to significant" in establishing a possible relationship. At the same time, however, a review of fhealth records of some 1,500 Smoky veterans turned up 92 cancer cases where 99 normally would be expected.

Leukemia, a cancer of the blood, already has been linked to high-level radiation cases. The number of Leukemia case among Japanese at Hiroshima and Nagasaki went way up within seven years of the bombings and then dropped.Recent studies have shown the leukemia rates in those cities have not gone back to normal after 30 years.

Recent follow-up studies of the Japanese have shown increased incidents of other cancers linked to radiation.

One government medical analyst said the Smoky cancer data seem to be following the Japanese experience but at a longer interval between expossure and disease -- a factor that could be related to the lower radiation levels involved in Smoky.

This analyst said, based on the CDC data, it would be "appropriate" for the government to apy claimants who could prove they had been at Smoky and were now suffering from leukemia or other cancers recognized as readiation-related.

According to one source, of the eight Smoky leukemia cases only Cooper and Coe received VA benefits. The names of the others have not been released because of the privacy act, but all are said to have died.

One Smoky widow was turned down for benefits last year after she failed to prove her husband was at the site. After getting that proof, hse was told his radiation badge showed almost no exposure.

The White House task force has focused on the barriers to granting benefits to claimants in low-level radiation cases. Aside from the medical controversy, the inquiry has found that many records have been destroyed and those that exist may not be accurate.

The problems in the compensation area go far beyond the GIs involed in 1950s nuclear tests.

More than 100 individual cases have been filed in court by residents of Utah and Arizona on behalf of themselves or relatives who developed leukemia, which they allege was caused by fallout from the Nevade nuclear tests.

A controversial federal study of workers at the government's Hanford, Wash., nuclear facitlity in 1977 reported a slightly in creased number of cancer cases over a 30-year period, and similar findings of higher cancer deaths were reached in a private sutdy of employes at the Navy's Portsmouth, N.H., nuclear ship-repair facility.

Hovering over the entire federal compensation question is what the impact could be on the nation's private nuclear power industry if liability is recognized by the government for illnesses that developed long after exposure to low-level radiation.

"This could involve big bucks," one participant on the White Hose task force said in discussing the compensation issue.

Supporting that point is the close touch that private power consultants are keeping with the White House study.

Drafts of the task force report are expected to be circulated for comment on Capitol Hill and among Washington-based interest groups this week.