Chernobyl disaster (1983): Unit 4 online · 377

Pripyat / Ukrainian SSR, Soviet Union On 26 April 1986 an explosion at reactor 4 of the Chernobyl plant in the Ukrainian SSR became the worst nuclear accident on the public record and rewrote international nuclear law. At 01:23 on 26 April 1986, unit 4 of the Chernobyl Nuclear Power Plant exploded during a planned turbine-rundown test. The reactor was an RBMK-1000, graphite-moderated and light-water-cooled, a Soviet design with a positive void coefficient and a slow emergency-protection system. Two workers died in the immediate blast; UNSCEAR later recorded 28 further deaths from acute radiation syndrome among plant and emergency staff within four months.

Pripyat, the purpose-built atomgrad next to the plant, was evacuated on 27 April. The Soviet state confirmed an accident only after Swedish monitors detected the plume on 28 April. IAEA’s INSAG-1 (1986) first stressed operator error; INSAG-7 (1992) shifted the main cause to design and a weak safety culture. The 1987 Chernobyl trial convicted plant managers. The New Safe Confinement was slid over the 1986 sarcophagus on 29 November 2016.

The engineering case is closed: a test plus RBMK design features produced a prompt power excursion. What remains contested is how to count later cancers and how to value the exclusion zone. English-language memory now runs through the 2005 Chernobyl Forum, UNSCEAR annexes and, after 2019, a television miniseries that is not a primary source.