Chernobyl disaster (1986): The 01:23 excursion · 271

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.

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.