Abstract: It consists of determining by an iterative calculation the neutron flux .phi.(j,g) at each point j of the core in the form of a sum of two components, the first component being associated with a predetermined state of the core defined by predetermined interaction probabilities of the neutrons in the core and the second component being linked with the real state of the core, with said predetermined state and with said first component. The power given off P(j) and the number of emitted neutrons S(j) at each point of the core are then deduced from the neutron flux. The power given off P(j) and the neutron flux .phi.(j,g) are used for controlling the core regulating means, such as the control rods or the boron concentration.