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

by Adam Higginbotham

The zirconium cladding of the fuel assemblies had melted first, reaching a temperature of more than 1,850 degrees Celsius within a half hour of the explosion, and dissolved the uranium dioxide pellets it contained, pooling into a hot metal soup that then absorbed parts of the reactor vessel itself—including stainless steel, serpentinite, graphite, and melted concrete. The radioactive lava, now containing some 135 tonnes of molten uranium, then began eating its way through the lower biological shield of the reactor, a massive steel disk filled with serpentinite gravel, weighing 1,200 tonnes. It burned clean through the shield and its contents, absorbing around a quarter of its total mass, and ran smoothly into the rooms below. The steel cross supporting the entire reactor vessel—Structure S—reached its yield point and buckled; the shield fell out of the bottom of the reactor, and the searing corium began burning a hole in the floor of the subreactor space. The pipes of the steam suppression system, which had proved unable to save Reactor Number Four in its death throes, now provided a convenient path for the lava as it attempted to leave the building entirely, spreading out to the south and east along four separate routes, melting metalwork, flowing through open doorways, slowly filling corridors and rooms, and creeping down through the plumbing, one floor after another, toward the foundations of Unit Four.