USSR tried to cover up information on the disaster. They said to soviet people: don't worry, we have this under control and refused to cancel May 1st parade in Kiev. People in affected areas found out about the dangers from western media first, only later the state admitted what had happened. The state later (months, years after the disaster) tried to suppress what Legasov had to say and refused to give him the medal.

Pretty bad record for the USSR government, and pretty well depicted in the series.