Abstract: In a process for running a nuclear reactor cooled with light water, by displacing control rods and modifying the physical and/or chemical state of the coolant, the value of the running parameters of the reactor are continuously measured and recorded and the parameters describing the state of the reactor core are calculated for each substantial modification of the running parameters. The parameters describing the state of the core are measured at defined intervals of time and the calculated parameters are replaced by the measure parameters. The modified parameters resulting from predetermined sequences of action taken on the control rods or the coolant, and the action which must be taken on the running parameters in order to comply with a given program, are calculated by way of predictions. Action is taken on the control rods and the coolant in accordance with the results of the calculation.