What happened
The IAEA's own 1991 and 2005 retrospective reports concluded the accident stemmed from a combination of a flawed RBMK reactor design (with known safety weaknesses the Soviet nuclear establishment had not disclosed even to plant operators) and a rushed, poorly supervised safety test. But the disaster's second failure was political: the instinct to protect the state's image over public safety, delaying evacuation and public acknowledgment for days while thousands of civilians — including at the 1 May Kyiv parade — remained unwarned and unprotected.
Gorbachev later wrote that Chernobyl, more than any single event, convinced him that the old Soviet system of secrecy could not survive, and directly accelerated his glasnost reforms. The UN and WHO estimate the eventual death toll from radiation-linked cancers in the thousands, though the exact figure remains disputed between Soviet-era Chernobyl Forum estimates and higher figures from Greenpeace and some independent scientists.