Void Coefficient
A number which is used to determine the behavior of a nuclear reactor as the water within the core boils off, forming a steam void. This water acts as both a neutron moderator, increasing thermal neutron count and fission rate, as well as reactor cooling and absorbing neutrons.
A reactor whose void coefficient is greater than zero will increase the rate of fission as the water in the reactor boils to steam; these designs are rarer today and are more dangerous, as the fission can more easily cause a runaway chain reaction in the core. These reactors are said to have a "positive void coefficient".
Most modern nuclear reactors have "negative void coefficients," meaning that as the water boils, fission slows down.
A reactor whose void coefficient is greater than zero will increase the rate of fission as the water in the reactor boils to steam; these designs are rarer today and are more dangerous, as the fission can more easily cause a runaway chain reaction in the core. These reactors are said to have a "positive void coefficient".
Most modern nuclear reactors have "negative void coefficients," meaning that as the water boils, fission slows down.
The RBMK-1000's dangerously high positive void coefficient was a major contributing factor to the Chernobyl Disaster.
Void Coefficient by John Reactor April 18, 2026
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