Phonons and thermal neutron scattering in [gamma]-U and U-Mo alloys

Abstract

Elimination of high-enrichment uranium fuels in research and test reactors around the world is one of the many efforts that supports the goal of minimizing proliferation risks. These efforts have pushed for the development of low-enrichment, metallic uranium fuels such as U-Mo alloys. The development of these fuels requires accurate prediction of their neutron transport properties, which in turn may be affected by the distribution of scattered neutrons from bound nuclei at thermal energies. In order to estimate such a distribution in a reactor, thermal neutron scattering cross sections are needed. Calculations of these cross sections require knowledge of the phonon scattering properties of the medium. Due to the difficulty in measuring these properties, thermal neutron scattering data have not been measured for uranium, molybdenum, and their alloys, and phonon effects have consequently been omitted from neutron transport models. In this dissertation, thermal neutron scattering in [gamma]-U, Mo, and U-Mo alloys is studied. The fundamental quantities of interest, the cross sections, were calculated using the NJOY software. NJOY was modified to include the calculation of the coherent elastic cross sections for non-stoichiometric alloys. Such modifications allowed us to generate thermal scattering cross sections for U-10Mo and U-7Mo at different temperatures and enrichment levels. The phonon density of states, which is the main input needed to calculate the cross sections, was generated through molecular dynamics simulations. We find that the incorporation of phonon effects decreases the cross sections compared to free-atom models. The effects that this decrease has on neutron transport were studied using simple reactor models. We conclude that binding effects have a minor impact on criticality, but they might be important in the assessment of uncertainties in reactor configurations and models as well as in neutron scattering experiments.