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

by Adam Higginbotham

While conducting tests before the two reactors could be brought into normal operation, the start-up teams of nuclear engineers in Ignalina and Chernobyl noticed a small but disturbing glitch. When they used the AZ-5 scram button to shut down the reactor, the control rods began their descent into the core, but instead of completing a smooth shutdown, the rods initially had the opposite effect: for a brief moment, reactor power rose instead of falling. The specialists discovered that the severity of this “positive scram” effect depended on the conditions inside the reactor at the moment the shutdown began—particularly on the ORM, the measurement indicating how many of the 211 control rods were withdrawn from the core. If more than 30 of these rods remained inserted when the scram began, the AZ-5 mechanism worked as intended, and the reactor would shut down quickly and safely. But when the total number of inserted rods fell below 30, the behavior of the reactor at shutdown became increasingly unpredictable, and the AZ-5 system struggled to do its job. With only 15 rods in, the technicians found that initial dampening of fission inside the reactor was marginal; it took six seconds before reactivity began to fall. And under some circumstances—7 rods or fewer—pressing the AZ-5 button might not shut down the reactor at all, but instead trigger a runaway chain reaction. If this happened, the increase in reactor power following an AZ-5 trip might be so great that it would no longer...