KYSHTYM INDUSTRIAL COMPLEX, U.S.S.R. -- What some experts consider the world's worst nuclear accident occurred here on a September evening in 1957, but there was no government mention of it because this place and its 100,000 inhabitants did not officially exist. Ten thousand citizens were hastily evacuated, tons of soil were moved and buried, roughly 70 square miles were declared unfit for human use, and a special research center was created to monitor the health of the nearby residents. But all of the records were sealed in a thorough, highly successful effort to prevent any disclosure that might have interfered with the secret complex's feverish attempt to build a Soviet arsenal of nuclear weapons to match that of the United States. In the first discussions with any foreigners about what happened, Soviet officials who were present here at the time of the accident revealed last week that a grievous set of technical misjudgments and poor work habits caused the explosion of a large vat of wastes from the production of plutonium, a key weapons ingredient. An estimated 2 million curies of radioactive elements were subsequently deposited by a strong wind in a swath of countryside 65 miles long and 5 miles wide. Soviet officials emphasized that the Kyshtym accident released only a small fraction of the total amount of radiation released by the accident at Chernobyl. Plant officials here insisted, moreover, that the accident did not cause any deaths. But dissident Soviet biologist Zhores Medvedev, who has investigated the blast, estimates that hundreds may have died from the radiation effects. He says that the accident disseminated a larger quantity of the long-term radioactive substance Strontium-90 than Chernobyl, prompting him to term the 1957 incident the worst nuclear accident in history. He and others have cited eyewitness accounts by emigres who claim to have seen hospitals filled with heavily bandaged patients whose skin was sloughing off. There is also a widespread conviction among the local citizenry that many subsequent deaths can be attributed to radiation. The evacuation wiped many small villages and towns off the map and forced the Moscow Institute for Bio-Physics to establish a local branch to keep track of all those exposed. The records of the disaster or its aftermath never have been made public, although Medvedev found oblique references to them in a few published scientific studies. The government, in what amounts to official recognition of its inability to suppress history, now is preparing to release the health records and hold the first public hearing on the accident before a special commission of the new Soviet legislature, the Congress of People's Deputies. "We are going to dig into all these matters," said Yevgeny Velikhov, a vice president of the Soviet Academy of Sciences who helped arrange an unprecedented two-day visit to this city by a group of U.S. congressmen and representatives of an environmental group, the Natural Resources Defense Council. The accounts provided here confirmed many of the allegations in Medvedev's 1979 book on the accident. The book attracted wide attention from British and American nuclear scientists and won only stony silence from Moscow. Medvedev, who was exiled in 1973, said in a telephone interview from London on Saturday, "I feel better they finally have recognized that all this secrecy has made them suffer, and that it's better people know about this." He added that "their silence has contributed to many other accidents," including the 1986 explosion and fire at the Soviet civilian nuclear power plant near Chernobyl, "because it created the false impression that nuclear power is completely safe." Evgeny I. Mikerin, a director of the main department of manufacturing and technology for the state Committee for the Utilization of Atomic Energy, surprised the American visitors when he said that he "would gladly invite Medvedev here now and supply him with the most information he would like." The first step in this direction came last month when a senior official of the Soviet Ministry of Medium Machine Building -- the agency responsible for nuclear weapons manufacturing -- told reporters in nearby Chelyabinsk that there had, in fact, been an accident 32 years ago. An account by Tass, the official Soviet news agency, indicated that the official, Boris Nikipelov, described the accident in general terms without naming Kyshtym. Mikerin provided a much more complete account on a bus ride with the American group en route to Kyshtym from a Chelyabinsk military airport 65 miles to the southwest, and his account later was confirmed by other officials. At the time of the accident, Mikerin was a manager of a plant on the grounds of the complex that produced plutonium by reprocessing used nuclear reactor fuel. He said the radioactive wastes from its operations were dumped into a series of stainless steel and concrete tanks located slightly more than a mile away from the plant, an operation similar to the steel "tank farm" of wastes from U.S. bomb production at Hanford, Wash. To keep the wastes from becoming explosive due to a natural chemical reaction, he said, they were cooled by a coil of water tubing along the interior wall of each tank. The designers of the tanks did not provide a mechanism for repairing the tubes in the event they failed, he added. Sometime in 1956, the tubing in one of the tanks began to leak and was then shut off. Mikerin said that faulty calculations by scientists at the complex indicated that, despite the failure of the cooling tubing, the wastes were stable and not highly radioactive. As a result, more than a year lapsed with little or no effort to devise a means of repair. During this period, the wastes began to dry, Mikerin said, presumably from chemically induced heat, and highly explosive nitrate salts and acetate collected at the surface. By chance, Mikerin said, "a control device in the tank produced a spark," which detonated the salts, and the resulting explosion obliterated the tank and all that it contained. Several officials here cited the 1957 fall season's cool weather, which caused most windows to be shut, as a saving grace. Nevertheless, workers here ended up not only excavating and burying topsoil, but also replacing the roofs of some buildings and washing exterior walls with a mixture of water and sand. He said the "agricultural district," which is largely populated by Tatars heavily dependent on the local land and water, bore the brunt of the disaster. Although the reprocessing plant was "the most contaminated of all our facilities," work there was so important it was halted for only a month, he said. Asked how the accident affected him, Alexei E. Spirin, director of Kyshtym's first plutonium reactor, said he has noticed some "abnormally big plants" in the area during hunting expeditions. He added that he stopped eating wild berries and mushrooms for roughly a decade after the accident. The disaster was first publicly mentioned in a vague, 1958 Copenhagen newspaper account, which alleged it was a factor in the Soviet decision to suspend nuclear tests unilaterally in March of that year. A report by the Los Alamos National Laboratory in the United States alleged that the Soviet military eventually established an unusually realistic training range at the site for chemical-biological-radiological troops, but this could not be confirmed. Mikerin said the accident forced the center's managers to begin removing the waste storage tanks, and arrange for the wastes to be shipped to a special plant in the central Asian city of Krasnoyarsk, where they are mixed with glass for long-term storage.