US Researchers still stumped over nature and scope of Soviet nuclear accident in the 1950s

The exact nature of a nuclear ''accident'' that has been called the Soviet Union's Three Mile Island remains a mystery, and probably will stay so for some time despite the controversy it has created in the West.

This is the conclusion of an exhaustive search of Soviet scientific literature on the topic, completed by Frank L. Parker of Vanderbilt University for the United States Department of Energy.

The existence of a large radioactive area in the Ural Mountains near Sverdlovsk was first brought to the West's attention in 1976 by Soviet dissident Zhores Medvedev, although a Viennese newspaper reported rumors of such an area in 1959. The Soviet mathematician wrote of a catastrophic accident there in 1957 or 1958 - ascribing it to a nuclear chain reaction in carelessly disposed radioactive wastes - merely to illustrate a point about dissident scientists in the USSR.

But his assertion in 1976 touched off an immediate outcry, particularly from English nuclear scientists. Sir John Hill, head of the British Atomic Energy Authority at the time, branded the story as false. And the question was immediately embroiled in the struggle between pro- and anti-nuclear energy partisans in the West. To back his case, Mr. Medvedev subsequently wrote several articles and a book detailing a number of thinly veiled references in Soviet literature to this large contaminated area. US scientists, investigating these references, found fairly clear evidence that such an area exists. But the controversy has continued.

In recent years a group of scientists from Los Alamos Scientific Laboratory in New Mexico have written articles criticizing Medvedev's description of the size of the incident and his assertion that it resulted from a radioactive waste disposal accident.

Instead, they have suggested that far fewer than the tens of thousands of people Medvedev claimed were affected and that the contamination could have resulted from fallout from distant weapons tests or an accumulation from decades of sloppy radioactive waste-handling methods. The technical basis for these suggestions has been sharply attacked in turn.

Based on his review of the scanty evidence available, Dr. Parker argues a ''best supposition'' is that the area has been afflicted by repeated releases of radioactive material and an explosion in a nuclear fuel reprocessing plant nearby. But he stresses that ''the more information we obtain, the less convinced we are about what occurred, and the more likely it seems that it may not have been a single incident.''

In US Central Intelligence Agency documents on the topic, released to Ralph Nader's Critical Mass Group under the Freedom of Information Act, the dates and causes of the accident vary so widely that the only conclusions possible are an approximate location and time, and the fact that a large area was contaminated, Parker reports.

Parker also interviewed a number of Soviet emigres. A mathematician living in nearby Chelyabinsk reported that in 1956 there was a great glow in the sky to the north that he was at a loss to explain. It was never repeated, he said.

A Russian construction engineer who worked in a nuclear complex in the area reported that the main river there was repeatedly contaminated by routine mishaps and as many as 10,000 people had been evacuated from its banks prior to 1957. Shortly after he left the area, friends who remained told him that there had been a serious accident at a fuel reprocessing plant where he worked.

Parker's search of Soviet scientific literature drew a blank, probably because work in military medicine and the results of nuclear accidents both appear to be classified.

Consequently, he concludes that the real story will not be known until the Soviets choose to make such information available to the West, or some knowledgeable witnesses to the accident or accidents emigrate.

These conclusions are summarized in a Battelle Technical Report entitled ''Search of the Russian Scientific Literature for the Descriptions of the Medical Consequences of the Kyshtym 'Accident.' ''