Criticality and reactivity of a reactor

In a reactor or an atomic bomb, the criticality factor k is, for each fission, the average number of secondary neutrons causing in turn a fission. The value of k determines whether the chain reaction become explosive (k greater than 1) or not (k less than 1). Criticality should remain equal to 1 in a reactor. Criticality excursions above this value are tolerated only if they are brief and do not exceed a few per thousand. The difference (k-1) to the unit value is called reactivity.