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

by Adam Higginbotham

of the control rods themselves, an unintended consequence of NIKIET’s desire to “save neutrons” and make the reactor more economical to run. Like all the manual control rods used to manage the reactor during normal operation, the AZ-5 emergency rods contained boron carbide, a neutron poison that gobbles slow neutrons to reduce the chain reaction. But even when fully withdrawn from their water-filled control channels, the tips of the rods were designed to remain at the ready, just inside the active zone of the reactor—where, if they contained boron carbide, they would have a poisoning effect, creating a slight but constant drag on power output. To stop this from happening, the rods were tipped with short lengths of graphite, the neutron moderator that facilitates fission. When a scram shutdown began and the AZ-5 rods began their descent into the control channels, the graphite displaced neutron-absorbing water—with the effect of initially increasing the