Since the neutron flux depends on the spatial position within the reactor, the core volume is divided into evolution cells (cell dimensions are typically Az « 5 cm and Ar « 5 cm). The evolution of each type of nucleus within the cell takes into account nucleus disappearance by neutron-induced reac­tions and by natural decay, and nucleus production by neutron reactions on a parent nucleus and by natural decay of a parent. The description of these four processes constitutes a system of Bateman’s coupled equations whose solution is a vector that contains the number of nuclei of each type.