Patent Number: 
Section: claims

1. 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); anddetermining 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), andchanges in the power distribution pattern (PDP). 2. A method according to claim 1, wherein the correlations comprise local changes in the power distribution pattern (PDP) followed by a release of fission gases. 3. A method according to claim 1, wherein the correlations comprise local changes in the power distribution pattern (PDP) followed by an increase in an on-going release of fission gases. 4. A method according to claim 1, wherein the calculated local power levels (LPC) are used to establish the power distribution pattern (PDP). 5. A method according to claim 4, wherein the calculated local power levels (LPC) are calculated by simulation models, the simulation models using core input signals comprising power affecting factors. 6. A method according to claim 5, wherein the power affecting factors comprise process parameters including the given total reactor power, the flow of the coolant and the temperature of the coolant at least in one position of the reactor. 7. A method according to claim 1, wherein the method further comprises the steps of:recording local power differences (LPD) between the sensed local power levels (LPS) and the calculated local power levels (LPC);combining the release of fission gases and the recorded local power differences (LPD); andobserving correlations between changes in the release of fission gases and changes in the recorded local power differences (LPD) in order to determine a position of the defect on the cladding of any of the fuel rods. 8. A method according to claim 1, wherein the sensors (S) regularly measure at least one of a local neutron flux and a local gamma flux.