The Soviets also covered up Kyshtym in 1957, a cover up that lasted for three decades. Many officials in the West were complicit in that cover up, adopting extraordinary skepticism of the available evidence to discredit those that either knew or believed a major nuclear incident had occurred.
One of the victims of that cover up in the West was Zhores A. Medvedev. He wrote a book (Nuclear Disaster in the Urals) in response to critics of his claims of a major nuclear disaster. The book is a truly fascinating survey of open sources of academic work published by Soviet researchers that reveals the incident indirectly; the papers (typically studies of the effect of contamination of various organisms; fish, plants, worms, deer, etc.) could only have been written if a large area of land had been heavily contaminated, and this subtlety was lost on the state censors, who were themselves not aware that Kyshtym had happened.
Medvedev was eventually vindicated when the Soviets fessed up in the late '80s.