When did the worst nuclear accident happen?

The worst nuclear accident in history happened at the Chernobyl Nuclear Power Plant in Chernobyl, Ukraine, on 26 April 1986. An explosion and fire released massive amounts of radioactive particles into the atmosphere. The deadly radiation spread over Western USSR and much of Europe. The effort to contain the contamination cost 18 billion rubles (~18 billions USD.) The accident resulted in 31 deaths at the site of the explosion. Even today, long-terms affects such as deformities and cancer are still an issue.