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.
Experts in Britain, the United States, France, and many
other countries responded by denying my story and stating
that such a thing was technically impossible. The earliest
and most sharply worded rebuttal to my story came from the
chairman of the United Kingdom Atomic Energy Authority, Sir
John Hill. His interview with the Press Association
published in the London Times on November 8, 1976, was
reprinted in many European and American papers. He declared
in the most high-handed fashion that my story was "rubbish".
-- Zhores Medvedev, Nuclear Disaster in the Urals, p5
(Amazon preview)