The 1986 meltdown of reactor number 4 at the Chernobyl Nuclear Power Plant was the greatest nuclear disaster the world has yet seen. Less well known is the Kyshtym disaster in 1957, which resulted in a massive release of radioactive material in the Eastern Ural region of the Soviet Union. This was a catastrophic underground explosion at a nuclear storage facility near the Mayak power plant in the Eastern Ural region of the USSR. Information about the disaster was tightly restricted by Soviet authorities, with predictably bad consequences.
Zhores Medvedev was one of the first qualified scientists to provide information and hypotheses about the Kyshtym disaster. His book Nuclear Disaster in the Urals was written while he was in exile in Great Britain and appeared in 1980. It is fascinating to learn that his reasoning is based on his study of ecological, biological, and environmental research done by Soviet scientists between 1957 and 1980. Medvedev was able to piece together the extent of contamination and the general nature of the cause of the event from basic information about radioactive contamination in lakes and streams in the region included incidentally in scientific reports from the period.
It is very interesting to find that scientists in the United States were surprisingly skeptical about Medvedev's assertions. W. Stratton et al published a review analysis in Science in 1979 (link) that found Medvedev's reasoning unpersuasive.
A steam explosion of one tank is not inconceivable but is most improbable, because the heat generation rate from a given amount of fission products is known precisely and is predictable. Means to dissipate this heat would be a part of the design and could be made highly reliable. (423)They offer an alternative hypothesis about any possible radioactive contamination in the Kyshtym region -- the handful of multimegaton nuclear weapons tests conducted by the USSR in the Novaya Zemlya area.
We suggest that the observed data can be satisfied by postulating localized fallout (perhaps with precipitation) from explosion of a large nuclear weapon, or even from more than one explosion, because we have no limits on the length of time that fallout continued. (425)And they consider weather patterns during the relevant time period to argue that these tests could have been the source of radiation contamination identified by Medvedev. Novaya Zemlya is over 1000 miles north of Kyshtym (20 degrees of latitude). So the fallout from the nuclear tests may be a possible alternative hypothesis, but it is farfetched. They conclude:
We can only conclude that, though a radiation release incident may well be supported by the available evidence, the magnitude of the incident may have been grossly exaggerated, the source chosen uncritically, and the dispersal mechanism ignored. Even so we find it hard to believe that an area of this magnitude could become contaminated and the event not discussed in detail or by more than one individual for more than 20 years. (425)The heart of their skepticism depends on an entirely indefensible assumption: that Soviet science, engineering, and management were entirely capable of designing and implementing a safe system for nuclear waste storage. They were perhaps right about the scientific and engineering capabilities of the Soviet system; but the management systems in place were woefully inadequate. Their account rested on an assumption of straightforward application of engineering knowledge to the problem; but they failed to take into account the defects of organization and oversight that were rampant within Soviet industrial systems. And in the end the core of Medvedev's claims have been validated.
Another official report was compiled by Los Alamos scientists, released in 1982, that concluded unambiguously that Medvedev was mistaken, and that the widespread ecological devastation in the region resulted from small and gradual processes of contamination rather than a massive explosion of waste materials (link). Here is the conclusion put forward by the study's authors:
What then did happen at Kyshtym? A disastrous nuclear accident that killed hundreds, injured thousands, and contaminated thousands of square miles of land? Or, a series of relatively minor incidents, embellished by rumor, and severely compounded by a history of sloppy practices associated with the complex? The latter seems more highly probable.So Medvedev is dismissed.
After the collapse of the USSR voluminous records about the Kyshtym disaster became available from secret Soviet files, and those records make it plain that US scientists badly misjudged the nature of the Kyshtym disaster. Medvedev was much closer to the truth than were Stratton and his colleagues or the authors of the Los Alamos report.
A scientific report based on Soviet-era documents that were released after the fall of the Soviet Union appeared in the Journal of Radiological Protection in 2017 (A V Akleyev et al 2017; link). Here is their brief description of the accident:
Starting in the earliest period of Mayak PA activities, large amounts of liquid high-level radioactive waste from the radiochemical facility were placed into long-term controlled storage in metal tanks installed in concrete vaults. Each full tank contained 70–80 tons of radioactive wastes, mainly in the form of nitrate compounds. The tanks were water-cooled and equipped with temperature and liquid-level measurement devices. In September 1957, as a result of a failure of the temperature-control system of tank #14, cooling-water delivery became insufficient and radioactive decay caused an increase in temperature followed by complete evaporation of the water, and the nitrate salt deposits were heated to 330 °C–350 °C. The thermal explosion of tank #14 occurred on 29 September 1957 at 4:20 pm local time. At the time of the explosion the activity of the wastes contained in the tank was about 740 PBq [5, 6]. About 90% of the total activity settled in the immediate vicinity of the explosion site (within distances less than 5 km), primarily in the form of coarse particles. The explosion gave rise to a radioactive plume which dispersed into the atmosphere. About 2 × 106 Ci (74PBq) was dispersed by the wind (north-northeast direction with wind velocity of 5–10 m s−1) and caused the radioactive trace along the path of the plume . Table 1 presents the latest estimates of radionuclide composition of the release used for reconstruction of doses in the EURT area. The mixture corresponded to uranium fission products formed in a nuclear reactor after a decay time of about 1 year, with depletion in 137Cs due to a special treatment of the radioactive waste involving the extraction of 137Cs . (R20-21)Here is the region of radiation contamination (EURT) that Akleyev et al identify:
A somewhat surprising finding of the Akleyev study is that the exposed population did not show dramatically worse health outcomes and mortality relative to unexposed populations. For example, "Leukemia mortality rates over a 30-year period after the accident did not differ from those in the group of unexposed people" (R30). Their epidemiological study for cancers overall likewise indicates only a small effect of accidental radiation exposure on cancer incidence:
The attributable risk (AR) of solid cancer incidence in the EURTC, which gives the proportion of excess cancer cases out of the sum of excess and baseline cases, calculated according to the linear model, made up 1.9% over the whole follow-up period. Therefore, only 27 cancer cases out of 1426 could be associated with accidental radiation exposure of the EURT population. AR is highest in the highest dose groups (250–500 mGy and >500 mGy) and exceeds 17%.So why did the explosion occur? James Mahaffey examines the case in detail in Atomic Accidents: A History of Nuclear Meltdowns and Disasters: From the Ozark Mountains to Fukushima. Here is his account:
In the crash program to produce fissile bomb material, a great deal of plutonium was wasted in the crude separation process. Production officials decided that instead of being dumped irretrievably into the river, the plutonium that had failed to precipitate out, remaining in the extraction solution, should be saved for future processing. A big underground tank farm was built in 1953 to hold processed fission waste. Round steel tanks were installed in banks of 20, sitting on one large concrete slab poured at the bottom of an excavation, 27 feet deep. Each bank was equipped with a heat exchanger, removing the heat buildup from fission-product decay using water pipes wrapped around the tanks. The tanks were then buried under a backfill of dirt. The tanks began immediately to fill with various waste solutions from the extraction plant, with no particular distinction among the vessels. The tanks contained all the undesirable fission products, including cobalt-60, strontium-90, and cesium-137, along with unseparated plutonium and uranium, with both acetate and nitrate solutions pumped into the same volume. One tank could hold probably 100 tons of waste product.
In 1956, a cooling-water pipe broke leading to one of the tanks. It would be a lot of work to dig up the tank, find the leak, and replace the pipe, so instead of going to all that trouble, the engineers in charge just turned off the water and forgot about it.
A year passed. Not having any coolant flow and being insulated from the harsh Siberian winter by the fill dirt, the tank retained heat from the fission-product decay. Temperature inside reached 660 ° Fahrenheit, hot enough to melt lead and cast bullets. Under this condition, the nitrate solutions degraded into ammonium nitrate, or fertilizer, mixed with acetates. The water all boiled away, and what was left was enough solidified ANFO explosive to blow up Sterling Hall several times, being heated to the detonation point and laced with dangerous nuclides. 
Sometime before 11: 00 P.M. on Sunday, September 29, 1957, the bomb went off, throwing a column of black smoke and debris reaching a kilometer into the sky, accented with larger fragments burning orange-red. The 160-ton concrete lid on the tank tumbled upward into the night like a badly thrown discus, and the ground thump was felt many miles away. Residents of Chelyabinsk rushed outside and looked at the lighted display to the northwest, as 20 million curies of radioactive dust spread out over everything sticking above ground. The high-level wind that night was blowing northeast, and a radioactive plume dusted the Earth in a tight line, about 300 kilometers long. This accident had not been a runaway explosion in an overworked Soviet production reactor. It was the world’s first “dirty bomb,” a powerful chemical explosive spreading radioactive nuclides having unusually high body burdens and guaranteed to cause havoc in the biosphere. The accidentally derived explosive in the tank was the equivalent of up to 100 tons of TNT, and there were probably 70 to 80 tons of radioactive waste thrown skyward. (KL 5295)So what were the primary organizational and social causes of this disaster? One is the haste created in nuclear design and construction created by Stalin's insistence on moving forward the Soviet nuclear weapons program as rapidly as possible. As is evident in the Chernobyl case as well, the political pressures on engineers and managers that followed from these political priorities often led to disastrous decisions and actions. A second is the institutionalized system of secrecy that surrounded industry generally, the military specifically, and the nuclear industry most especially. A third is the casual attitude taken by Soviet officials towards the health and wellbeing of the population. And a final cause highlighted by Mahaffey's account is the low level of attention given at the plant level to safety and maintenance of highly risky facilities. Stratton et al based their analysis on the fact that the heat-generating characteristics of nuclear waste were well understood and that effective means existed for controlling those risks. That may be, but what they failed to anticipate is that these risks would be fundamentally disregarded on the ground and in the supervisory system above the Kyshtym reactor complex.
(It is interesting to note that Mahaffey himself underestimates the amount of information that is now available about the effects of the disaster. He writes that "studies of the effects of this disaster are extremely difficult, as records do not exist, and previous residents are hard to track down" (kl 5330). But the Akleyev study mentioned above provides extensive health details about the affected population made possible as a result of data collected during Soviet times and concealed.)