A measure of how reactivity responds when coolant boils or is lost. In light water reactors it is strongly negative: voids reduce moderation, so power falls, giving inherent self-limiting behaviour, and BWR operators even use recirculation flow and boiling to control power. Designs with a positive void coefficient, such as the RBMK, can see power rise as coolant boils, a feedback central to that design's worst accident.
Void coefficient is more negative toward end of cycle, so the pressure transient will feed back harder.
Also heard as
- void reactivity coefficient
- alpha-void
Related terms
Moderator
The material in a reactor core that slows fast fission neutrons so they are more likely to cause further fission.
Reactivity
A measure of how far a reactor is from criticality, positive when power will rise and negative when it will fall.
Scram
The rapid automatic or manual insertion of all control rods to shut a reactor down in seconds.
LOCA
Loss of coolant accident: a break or leak in the reactor coolant system that lets coolant escape faster than it can be replaced.