In a reactor or an atomic bomb, the criticality factor k is, the average number of secondary neutrons able to trigger secondary fissions. The value of k determines whether the chain reaction takes an explosive turn (k greater than 1) or not (k less than 1). Criticality should be kept equal to 1 in a reactor. Criticality excursions over this value are permissible if they are brief and do not exceed a few per thousand. The difference k-1 is known as reactivity.