Method for operating a reactor of a nuclear plant

A method of operating a reactor (1) of a nuclear plant in which the reactor (1) comprises a reactor vessel (6) enclosing a core having a plurality of fuel elements (7) and a number of control rods (8),

• wherein each fuel element (7) includes a plurality of elongated fuel rods (9), which each has an upper end (9′) and a lower end (9″) and includes a cladding (10) and nuclear fuel in the form of fuel pellets (11) enclosed in an inner space (12) formed by the cladding,
• wherein the fuel pellets (11) are arranged in the inner space to leave a free volume in the inner space, wherein the free volume comprises an upper plenum (12′), containing no nuclear fuel and provided in the proximity of the upper end of the fuel rod, a lower plenum (12″), containing no nuclear fuel and provided in the proximity of the lower end of the fuel rod, and a pellet-cladding gap between the fuel pellets (11) and the cladding (10),
• wherein a reactor coolant, during operation of the reactor, is re-circulated as a coolant flow through the core in contact with the fuel rods (9) and is added to the reactor via a feed-water conduit (4) as feed-water having a normal feed-water temperature providing a sub-cooling of the reactor coolant, and
• wherein each of the control rods (8) is displaceable a control rod distance to be inserted into and extracted from a respective position between respective fuel elements in the core, the method including the following steps of operation:
• operating the reactor at a normal power and a normal sub-cooling during a normal state,
• monitoring the reactor for detecting a defect on the cladding of any of the fuel rods,
• upon detecting a defective fuel rod having said defect on the cladding thereof, changing the operation of the reactor to a particular state that causes an increase of the free volume at least in the defective fuel rod in which the defect is detected,
• operating the reactor at the particular state during a limited time period, and
• operating, after said time period, the reactor at substantially the normal state.