Chernobyl disaster (1986): Explosion at 01:23 · 399

A test, a design and a late admission Chernobyl is the public-record type case for a Level 7 nuclear accident: dated official deaths, revised international reports, and a treaty wave that still governs notification and safety culture. Operators intended to see whether a spinning turbine could power emergency pumps during a station blackout until diesel generators started. The test had been postponed and was run on a night shift with the reactor in an unstable low-power state. Control-rod and coolant conditions violated the plant’s own operating limits.

When the emergency-protection (AZ-5) command was given, the RBMK’s graphite-tipped rods briefly added reactivity at the core bottom. Power rose in a fraction of a second. Steam explosions and a graphite fire followed. This page does not reconstruct a procedure that could be copied; the public record is the sequence as later reconstructed by INSAG and Soviet investigators.

Firefighters and plant staff entered the ruined block in the first hours. UNSCEAR’s 2008 annex describes 134 confirmed acute radiation syndrome cases among them, 28 fatal within four months, plus the two blast deaths. There were no ARS cases in the general public. Pripyat’s roughly 49,000 residents were told to take a three-day leave on 27 April.