The invention concerns a method of monitoring the operation of a reactor of a nuclear plant. The reactor is operated at a given total reactor power during a normal fuel operation cycle. The radioactivity level in the off-gas stream is continuously measured to detect a possible release of fission gases from the fuel rods as a consequence of a fuel leakage due to a defect on the cladding of any of the fuel rods in any of the fuel assemblies. An instantaneous power distribution is regularly established in the core and a power distribution pattern over time is established based on the instantaneous power distributions. The release of fission gases and the established power distribution pattern are then combined and correlations between changes in the release of fission gases and in the power distribution pattern are observed in order to determine a position of the defect.