Scientists at Los Alamos National Laboratory have worked out a new solution to the two-decades-old mystery of the Soviet Union's "Kyshtym nuclear disaster" in which many square miles of central Russia were devastated by nuclear radiation and left blackened and devoid of people, trees, and plants.

The most widely accepted explanation of the event was put forward in the 1970s in books and articles by Soviet exile Zhores Medvedev, who said that in 1958 carelessly stored nuclear wastes accidentally fissioned and exploded like a small nuclear bomb, contaminating scores of miles around Sverdlovsk and Chelyabinsk. Apparently hundreds were killed, the Russians had to burn villages to the ground, and the vegetation of the area laid waste.

The new explanation disputes the idea of a nuclear explosion, and attributes the disaster instead to extremely careless disposal of radioactive water and smoke in the area around a nuclear plant. It says there possibly was an ordinary chemical explosion as well that helped spread radioactive debris over the Russian countryside.

The incident is important in at least two ways, according to specialists in nuclear issues: first, to help understand how such a massive nuclear-waste disaster might have occurred and to help understand the process of radioactive waste disposal; and, second, as an opportunity to measure the effect of radiation on plants and animals.

The new explanation, produced by chemist Diane M. Soran and physicist Dan Stillman at Los Alamos after a two-year study of both public and classified data, accepts much of the detail of Medvedev's description of the incident. Soran and Stillman, however, say the incident centered around the plutonium production plant at Kyshtym and was not a nuclear explosion.

The report--a sanitized 28-page version of a classified report that was three times as long--identified three sources of radiation that escaped into the landscape:

* Water used to cool the plutonium production reactor was pumped through the reactor core, picking up radiation, then pumped out into a cooling pond and from there directly into the chief river of the region, the Techa.

* Gas generated in the process of dissolving fuel elements to remove their plutonium apparently was sent directly out a smokestack while still laden with radioactive particles and other pollutants. An "acid rain" containing nitric acid and radioactive iodine probably was pumped out into the air of the region steadily for a decade, killing vegetation for miles around the plant.

* Liquid radioactive waste from the plant was not put into double-lined and sealed steel tanks as is now done in the United States, but was simply poured out into a dry lake bed. From there it could have seeped into the Techa, into ground water, and later as dried residue could have been blown by the strong winds of the area over nearby villages.

The report also said that the storage of waste in open lake beds could have led to a dangerously flammable chemical combination--ammonium nitrate and hexone. If a chemical explosion occurred it would have spewed radioactive particles of dirt and clay for miles.

Eventually the destruction reached a point that the Soviets evacuated the area, and burned the contaminated huts of the peasants who had lived there to prevent people from returning home.

The area became so contaminated that after the evacuation Soviets used the region to train troops in dealing with biological and nuclear war.

The report said a nuclear detonation was not necessary to explain the Kyshtym disaster. "The Soviets managed to contaminate the Techa River Valley without any help from such a catastrophe . . . created a contaminated area near Kyshtym through carelessness and blatant disregard for their people . . . . "