Method of and an apparatus for monitoring the operation of a nuclear reactor

A method of monitoring the operation of a reactor of a nuclear plant, in which the reactor comprises a core having a plurality of fuel assemblies, wherein each fuel assembly includes a plurality of fuel rods, wherein each fuel rod comprises nuclear fuel and a cladding, the nuclear fuel being enclosed by the cladding, the plant also comprising forcing means arranged to force a coolant in a flow through the reactor and the core, conveying means arranged to convey an off-gas stream from the coolant, a core simulator, and sensors (S) at different positions in the core, wherein the method comprises the steps of:

• operating the reactor during a normal fuel operation cycle at a given total reactor power, during which fission gases are produced in the fuel rods;
• continuously measuring during the normal fuel operation cycle a radioactivity level in the off-gas stream for sensing a 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;
• calculating calculated local power levels (LPC) at different positions in the core by means of the core simulator;
• regularly establishing based on the calculated local power levels (LPC) an instantaneous power distribution (PDI) in the core during the normal fuel operation cycle by means of the core simulator;
• establishing a power distribution pattern (PDP) based on the instantaneous power distributions (PDI) over time during the normal fuel operation cycle by means of the core simulator;
• sensing sensed local power levels (LPS) by means of the sensors (S);
• comparing the sensed local power levels (LPS) to the calculated local power levels (LPC);
• recalculating and correcting the instantaneous power distribution pattern (PDI) to establish a corrected power distribution pattern (PDP) if the sensed local power levels (LPS) and the calculated local power levels (LPC) do not correspond;
• combining the sensed release of fission gases and at least one of the established and the corrected power distribution pattern (PDP);
• and
• determining a position of the defect on the cladding of any of the fuel rods by correlating:
• changes in the sensed release of fission gases and changes in the sensed local power levels (LPS),
• differences between the sensed local power levels (LPS) and the calculated local power levels (LPC), and
• changes in the power distribution pattern (PDP).