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

by Adam Higginbotham

By inserting electromechanical rods containing neutron-absorbing elements—such as boron or cadmium, which act like atomic sponges, soaking up and trapping delayed neutrons, preventing them from triggering further fission—the growth of the chain reaction can be controlled incrementally. With the rods inserted all the way into the reactor, the core remains in a subcritical state; as they are withdrawn, fission increases slowly until the reactor becomes critical—and can then be maintained in that state and adjusted as necessary. Withdrawing the control rods farther, or in greater numbers, increases reactivity and thus the amount of heat and power generated, while inserting them farther has the opposite effect. But controlling the reactor using only this fraction of less than 1 percent of all neutrons in every fission makes the control process acutely sensitive: if the rods are withdrawn too quickly, too far, in too large a number—or any of the myriad safety systems fail—the reactor may be overwhelmed by the fission of prompt neutrons and become “prompt supercritical.” The result is a reactor runaway, a catastrophic scenario accidentally triggering a similar process to the one designed into the heart of an atomic bomb, creating an uncontrollable surge of power that increases until the reactor core either melts down—or explodes.