Measure of the deviation of a reactorSystem used to initiate, maintain and control a fission chai... from criticalA reactor is critical when due to nuclear fission as many ne... (self-sustaining) condition. Corresponds to the multiplication factorRatio of neutron number in a neutron generation to the neutr... reduced by 1 and is thus equal to zero in the criticalA reactor is critical when due to nuclear fission as many ne... condition. If reactivity is positive, the reactorSystem used to initiate, maintain and control a fission chai... output increases. In the case of negative reactivity the output level decreases.