What happened at the plutonium-producing facility near Ozersk in 1957?

The event you are referring to is known as the Kyshtym disaster. On September 29, 1957, a buried tank containing liquid nuclear waste at the Mayak industrial complex—located near the modern-day city of Ozersk on the eastern slopes of the Ural Mountains—suffered a catastrophic explosion.1

The disaster was caused by a failed cooling system that allowed the tank's contents to overheat due to radioactive decay, eventually reaching approximately 660 °F (350 °C). The resulting nonnuclear explosion, equivalent to at least 70 tons of TNT, breached the tank and released a massive plume of radioactive fallout into the atmosphere.1

The Soviet government kept the event secret until 1989. The disaster contaminated roughly 9,000 square miles of land, forced the evacuation of over 10,000 people, and resulted in widespread radiation sickness and long-term health crises for the local population. It is classified as a Level 6 accident on the International Nuclear and Radiological Event Scale.1