Void coefficient

In nuclear engineering, the void coefficient (more properly called "void coefficient of reactivity") is a number that can be used to estimate how much the reactivity of a nuclear reactor changes as voids (typically steam bubbles) form in the reactor moderator or coolant. Reactivity, in the nuclear engineering sense (not to be confused with chemical reactivity), measures the degree of change in neutron multiplication in a reactor core. Reactivity is directly related to the tendency of the reactor core to change power level: if reactivity is positive, the core power tends to increase; if it is negative, the core power tends to decrease; if it is zero, the core power tends to remain stable. The reactivity of the core may be adjusted by the reactor control system in order to obtain a desired power level change (or to keep the same power level). It can be compared to the reaction of an automobile as conditions around it change (for instance, wind intensity and direction or road slope), and therefore the corresponding counter-measure that the driver applies to maintain road speed or execute a desired manoeuvre.