Abstract: A method determines a volumetric power distribution of a core of a nuclear reactor using a set of detectors for measuring a neutronic flow provided outside the reactor vessel, and a set of probes for measuring the temperature of the coolant at the outlet of the fuel assemblies. The method comprises the step of determining a first volumetric power distribution using a neutronic calculation code instantaneously solving the diffusion equation and updating the isotopic balance of the core during the fuel depletion based on values of the core operation parameters, and the step of determining a new volumetric power distribution by adjusting the first volumetric power distribution using the measures provided by the sensors for measuring the neutronic flow and provided outside the reactor vessel and by the temperature measuring probes.