Midnight in Chernobyl: The Untold Story of the World's Greatest Nuclear Disaster

by Adam Higginbotham

Before completing the change, he was supposed to choose a level at which the computer would maintain reactor power in the new operating mode. But, somehow, he skipped this step. The reactor proved as unforgiving as ever. Bereft of fresh instructions, the computer defaulted to the last set point it had been given: near zero. Now Toptunov watched in dismay as the glowing gray figures on the reactimeter display began to tumble: 500 . . . 400 . . . 300 . . . 200 . . . 100 megawatts. The reactor was slipping away from him. A series of alarms sounded: “Failure in measuring circuits.” “Emergency power increase rate protection on.” “Water flow decrease.” Akimov saw what was happening. “Maintain power! Maintain power!” he shouted. But Toptunov could not stop the numbers from falling. Within two minutes, the power output of Unit Four had plunged to 30 megawatts—less than 1 percent of its thermal capacity. By 12:30 a.m., the reactimeter display was almost at zero. Yet for at least four more minutes Toptunov took no action. While he waited, neutron-scavenging xenon 135 gas began to build in the core, overwhelming what little reactivity remained. The reactor was being poisoned, plunging into what the operators called a “xenon well.” At this point, with the reactor’s power stalled at its minimum, and more xenon accumulating all the time, nuclear safety procedures made the operators’ course quite clear: they should have aborted the test and shut down the reactor immediately. But they did...