Patent Number: 
Section: claims

1. A method for analyzing at least one fuel rod comprising a stack of nuclear fuel, a rod comprising packed zones completely filled with fuel and intermediate zones partially full of fuel, the method being implemented by an analysis system comprising a gamma ray spectrometry device having a sensor and a spectrometry processing unit, the method comprising the following steps:measuring gamma ray radiation emitted by the nuclear fuel with the sensor of the gamma ray spectrometry device to acquire spectrometry measurements of said at least one fuel rod;acquiring a count profile with the spectrometry processing unit associated with a non-migrating isotope in said at least one fuel rod, a count profile being derived from the spectrometry measurements taken along the at least one rod for said non-migrating isotope with the sensor of the gamma ray spectrometry device;determining with the spectrometry processing unit a set of at least one indicator K_i to quantify a reduction in material at an intermediate zone of index i, the said indicator being deduced from the count profile; anddetecting a change in geometry of said at least one fuel rod with the spectrometry processing unit by comparing the set of at least one indicator K_i against a set of at least one reference value RK indicative of the initial geometry of the nuclear fuel stack. 2. The analysis method according to claim 1, comprising a step of estimating the location of the intermediate zones by analyzing the count profile with the spectrometry processing unit. 3. The analysis method according to claim 1, in which the detecting the change in geometry is detected in at least one intermediate zone and deduced by statistical analysis of the indicators of K_i, a change in geometry being flagged when at least one value K_i is incompatible with a theoretical measurement spread that could be expected with no change in geometry. 4. The analysis method according to claim 1, in which the detecting the change in geometry is detected for at least one intermediate zone by comparing K_i with a predefined reference value RK, a change in geometry being detected when the ratio between K_i and RK exceeds a previously-chosen comparison threshold k1 or lies below another previously chosen comparison threshold k2. 5. The method according to claim 4, in which the reference value RK is determined statistically by the mean of the K_i values observed experimentally over a plurality of rods of the same type. 6. The method according to claim 4, in which the reference value RK is determined by theoretical calculation for a given pellet geometry. 7. The method according to claim 1, in which the rod is a rod or a section of rod of PWR type made up of a plurality of pellets, an intermediate zone corresponding to an inter-pellet zone. 8. The method according to claim 1, in which a non-migrating isotope of Ru-136 type is used to determine the count profile. 9. A system for analyzing at least one nuclear fuel rod comprising the gamma-ray spectrometry device and the spectrometry processing unit implementing the method according to claim 1. 10. A computer program stored on a non-transitory computer readable storage medium containing instructions for executing the method according to claim 1 when the program is executed by a processor of the analysis system.