A method of operating a nuclear reactor is disclosed. The reactor (1) encloses a core having a plurality of fuel rods (9). Each fuel rod (9) includes a cladding and fuel pellets of a nuclear fuel. The fuel pellets are arranged in an inner space of the cladding leaving a free volume comprising an upper plenum, a lower plenum and a pellet-cladding gap. The reactor is operated at a normal power and a normal inlet sub-cooling during a normal state. The reactor is monitored for detecting a defect on the cladding of any of the fuel rods. The operation of the reactor is changed to a particular state after detecting such a defect. The particular state permits an increase of the free volume in the defect fuel rod. The reactor is operated at the particular state during a limited time period, after which the reactor is operated at the normal state.