Understanding Chernobyl: Positive Void Coefficent (or void coefficient of reactivity)
A void coefficient allows operators to estimate how much the reactivity of a nuclear reactor will change as voids (typically steam bubbles) form in the coolant and moderator.
A positive void coefficient means reactivity in a reactor increases as more voids are created. The RBMK design had a dangerously high positive coefficient. Light water was used as coolant and graphite for moderation. Because the light water was boiling, more steam voids were forming. Steam voids don’t absorb neutrons, leaving them to collide with uranium, which then leads to more heat. More heat means more steam voids will form in the reactor and increase reactivity.
1. Light water is boiling inside the reactor.
2. Because the water is boiling, more steam voids are forming and coolant is being lost.
3. Without coolant, the temperature in the reactor increases exponentially, creating a massive amount of heat.
4. The increase of temperature boils more water and more steam voids form.
5. Even more coolant is lost, allowing more neutrons to fission with uranium-235, which pushes reactivity even higher. The reactor is now embarking on a positive feedback loop because of its high positive void coefficient.
6. Rinse and repeat.