From Postwar: A History of Europe Since 1945, by Tony Judt (Penguin, 2005), pp. 598-599:
Chernobyl was not the Soviet Union’s first environmental disaster. At Cheliabinsk-40, a secret research site near Ekaterinburg in the Ural Mountains, a nuclear waste tank exploded in 1957, severely polluting an area 8 km wide and 100 km long. 76 million cubic metres of radioactive waste poured into the Urals river system, contaminating it for decades. 10,000 people were eventually evacuated and 23 villages bulldozed. The reactor at Cheliabinsk was from the first generation of Soviet atomic constructions and had been built by slave labour in 1948-51.
Other man-made environmental calamities on a comparable scale included the pollution of Lake Baikal; the destruction of the Aral Sea; the dumping in the Arctic Ocean and the Barents Sea of hundreds of thousands of tons of defunct atomic naval vessels and their radioactive contents; and the contamination by sulphur dioxide from nickel production of an area the size of Italy around Norilsk in Siberia. These and other ecological disasters were all the direct result of indifference, bad management and the Soviet ‘slash and burn’ approach to natural resources. They were born of a culture of secrecy. The Cheliabinsk-40 explosion was not officially acknowledged for many decades, even though it occurred within a few kilometers of a large city—the same city where, in 1979, several hundred people died of anthrax leaked from a biological weapons plant in the town centre.
The problems with the USSR’s nuclear reactors were well known to insiders: two separate KGB reports dated 1982 and 1984 warned of ‘shoddy’ equipment (supplied from Yugoslavia) and serious deficiencies in Chernobyl’s reactors 3 and 4 (it was the latter that exploded in 1986). But just as this information had been kept secret (and no action taken) so the Party leadership’s first, instinctive response to the explosion on April 26th  was to keep quiet about it—there were, after all, fourteen Chernobyl-type plants in operation by then all across the country. Moscow’s first acknowledgement that anything untoward had happened came fully four days after the event, and then in a two-sentence official communique.
But Chernobyl could not be kept secret: international anxiety and the Soviets’ own inability to contain the damage forced Gorbachev first to make a public statement two weeks later, acknowledging some but not all of what had taken place, and then to call upon foreign aid and expertise. And just as his fellow citizens were thus made publicly aware for the first time of the scale of official incompetence and indifference to life and health, so Gorbachev was forced to acknowledge the extent of his country’s problems. The bungling, the mendacity and the cynicism of the men responsible both for the disaster and the attempt to cover it up could not be dismissed as a regrettable perversion of Soviet values; they were Soviet values, as the Soviet leader began to appreciate.