The technology

In most Western reactors, the coolant doubles as the moderating agent. This was not the case in the RBMK reactor used in Chernobyl. As the reactor failed, the moderating activity of the graphite was supposed to remain at a constant level, but instead, the steam formed by the coolant further increased reactivity.

In Western reactors, the level of moderation decreases as the coolant produces steam, ie there are fewer neutrons. Correspondingly, reactivity is reduced instead of increased. Today, a reactor is only approved if the steam-bubble coefficient remains negative at all times. Although the RBMK reactors had addressed this problem to some degree by increasing uranium enrichment and adding further control rods, this did not fully remedy the flawed design.

Unlike most modern reactors, Chernobyl’s reactor unit 4 was not embedded in a massive containment building. Ultimately, this is the main reason why so much radioactive material could escape into the atmosphere. The graphite fire that ignited after the roof was blown off lasted for almost 14 days, propelling vast amounts of fission products into the air.

Nuclear power plants of the same design as Chernobyl are still in operation in a number of eastern European countries.