Measurement of the double differential neutron cross section of UO2 and determination of the thermal scattering law as a function of temperature ; Mesure de la section efficace doublement différentielle de l'UO2 et détermination de la loi de diffusion thermique en fonction de la température

The Monte-Carlo neutron transport calculations in nuclear fuels depend on the crystalline structure and atom dynamics of materials. In the case of uranium dioxide (UO2), when neutron energies lie below a few eV, coherent elastic neutron scattering cross section accounts for the crystalline structure of UO2 and thermal scattering laws (TSLs) take into account the atom vibration behaviors of UO2. Between a few eV to 400kBT where kB is the Boltzmann constant and T represents the temperature of material, the TSLs are not available. Other neutron scattering treatments such as the Sampling the Velocities of the Target nucleus (SVT) algorithm are adopted in the Monte-Carlo calculations.In the present work, the theoretical formalism enabling to calculate the TSLs for solid crystalline, liquid and gas materials has been studied. Based on these theoretical equations, the data processing tool CINEL has been developed. CINEL is able to generate the temperature-dependent TSLs in ENDF-6 format with graphic processing unit (GPU) speedup. The generated TSLs can be further processed to be used as neutron scattering library in the Monte-Carlo neutron transport code TRIPOLI-4®. The performances of the Monte-Carlo calculations are illustrated with the analysis of neutron powder diffraction data onUO2 measured up to 1664 K with the D4 and D20 diffractometers of the Institut Laue-Langevin (Grenoble, France). The comparison of the experimental and simulated pair distribution functions confirms the unusual decrease of the U-O atomic distances with increasing temperature when an ideal fluorite structure (Fm3m space group) with harmonic atomic vibrations is assumed over the full temperature range. The flexibility of the CINEL code allowed to explore disorder or anharmonic oxygen vibrations in theFm3m space group, and to investigate other local crystalline symmetries for UO2 at elevated temperatures.Several neutron scattering treatments have also been studied, namely AK (Asymptotic Kernel), SVT (Sampling the Velocity of the Target ...