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

by Adam Higginbotham

unit’s five-level rapid power reduction system, known in Russian as AZ-5. Pushing this button would drive a special bank of twenty-four neutron-absorbing boron carbide control rods—as well as every one of the remaining 187 manual or automatic control rods that remained withdrawn at the time—simultaneously into the core, quenching the chain reaction throughout the reactor. Yet the AZ-5 mechanism was not designed to bring about an abrupt emergency stop. Dollezhal and the technicians of NIKIET believed that suddenly cutting off the electricity generated by the reactor would be disruptive to the operation of the Soviet grid. And they thought that such an immediate shutdown would be necessary only in the extremely unlikely event of a total loss of external power to the plant. So they designed the AZ-5 system to only gradually reduce the reactor’s power to zero. Rather than dedicated emergency motors, the system was driven by the same electric servos that moved the manual reactor control rods, used by the operators to manage reactor power during normal operation. Starting from their fully withdrawn position above the reactor, it would take between eighteen and twenty-one seconds for the AZ-5 rods to descend completely into the core; the designers hoped that the rods’ slow speed would be compensated for by their great number. But eighteen seconds is a long time in neutron physics—and an eternity in a nuclear reactor with a high positive void coefficient.