Gamma spectrometry is an efficient tool for measurement of radionuclides activity, but it can be used in a more qualitative way to determine the diameter of gamma radioactive materials. This application has been developed in CENG at Siloe research reactor, particularly for the determination of nuclear fuel swelling when irradiated during normal or incidental conditions. Gamma spectrometry takes place on the measurement of the sample activity profile at the main energy of a fission product, this fission product is chosen for its good cohesion with the fuel. The profile is compared to a simulation on the basis of a calculation in straight line attenuation of gamma radiation; the adjustment of the sample diameter in the calculation is made in order to obtain an agreement between measured and computed profiles. Several parameters can disturb the measurement (gamma emitters distribution in fuel section, radiation scattering...) and make the adjustment of a simulation pattern more difficult. Then it becomes necessary to qualify the method with an irradiated sample calibrated in diameter. (author). 6 figs., 5 refs.