Crystal binding effects on neutron scattering and criticality in U–Mo fuels

Abstract

Using molecular dynamics simulations, we calculate the phonon densities of states of U-10Mo and U-7Mo at different temperatures. The results are then used to calculate the thermal neutron scattering cross sections of both alloys at temperatures that span anticipated reactor operating temperatures and at several 235U enrichment levels. The cross sections are calculated using a modified version of NJOY that is able to calculate coherent elastic cross sections for disordered alloys. The addition of binding effects decreases the cross section by up to a factor of 21 over the free-atom model, resulting in a difference in keff between 3 and 36 pcm for a simple model of a spherical fuel pellet. Based on these results, we conclude that binding effects have a minor impact on criticality, but the qualitative differences between the U–Mo bound-atom and free-atom thermal neutron scattering cross sections may be important in neutron scattering experiments and in the assessment of uncertainties in reactor models and configurations.