[en] Highlights: • Using MEXIICO furnace to perform slow depressurisation. • Characterisation of the behaviour of fuel pellet during the pressure and temperature ramps. • Study of fission gas release by the fuel pellet. • Analytical and CFD study of fission gas entrained by the Argon flow. - Abstract: The MEXIICO project carried out by the CEA and supported by EDF and AREVA has the objective to characterize the behaviour of irradiated fuel pellet issued from nuclear water reactor during power transients. The MEXIICO experimental loop has been recently implemented in the LECA-STAR facility of the CEA Cadarache. It will allow studying the fuel fragmentation of nuclear fuels submitted to various temperature and pressure histories (up to1600 °C and 1600 bar) using the monitoring of released ⁸⁵Kr activity during the experiment. Since the fission gas release is measured thanks to a gamma detector located in the rear cell, while the nuclear fuel sample is located inside the MEXIICO furnace in the hot cell, it is necessary to take into account the residence time of the gas in the loop to accurately correlate fission gas release to the local temperature and pressure conditions of the sample, which are also time dependent. In this paper, we will compare a thermal hydraulic approach mixing both an analytical method and a numerical CFD simulation to experimental test results. This modelling of the MEXIICO loop will support the interpretation of future tests, and will allow, more precisely, to determine the fuel fragmentation thresholds for various stress conditions.