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Notes and Reflections on Books and Media

by Hannah Leitheiser

Void Coefficient

#nuclear #voidcoefficient #chernobyl

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

Adam Higginbotham




- Adam Higginbotham, Midnight in Chernobyl: The Untold Story of the World's Greatest Nuclear Disaster (2019)

I've got to go to sleep very soon, but before I do, I'll try to explain something important to those trying to understand the recent HBO show on Chernobyl: void coefficient.

To start, a void in this reactor type refers to steam -- not really a void, but seemingly close enough for nuclear engineers. When a reactor is critical, that means the neutrons from one fission are making their way to another nucleus to continue the chain of fission quickly enough to bring the reactor up to its designed power.

Boron rods absorb neutrons, reducing power. The graphite rods in a Chernobyl-type reactor slow down the neutrons, which makes them more likely to cause fission. This is confusing, but in subatomic particles speed relates to energy, so perhaps it would be better to say the graphite reduces neutron energy to make them more compatible with fission.

Water can both slow down neutrons and absorb them. There are two kinds of water: heavy and light. Heavy water has extra neutrons and is less likely to absorb neutrons, but you have to extract the heavy water from naturally occurring water, which costs more. The Chernobyl reactor used light water.

Because the Chernobyl reactor already had graphite for slowing down neutrons, the most important contribution of water was neutron absorption. Since steam contains fewer water molecules per volume, when the reactor became hot enough to form steam, fewer neutrons were absorbed, meaning the reaction would begin to run faster -- a positive feedback loop. This is what is meant by a positive void coefficient.