First-Principles Evaluation of Thermal Neutron Scattering and Cross Sections in Beryllium Oxide

Abstract

This work addresses the long-standing uncertainty in the thermal neutron scattering description of beryllium oxide (BeO), a key moderator and reflector material in advanced reactors. Conventional evaluations rely on semi-empirical phonon models that misrepresent the phonon gap and high-energy optical modes, which can bias the calculated cross sections and transmission. Here, density functional–theory phonon spectra with two exchange-correlation functionals are used to generate self-consistent thermal scattering laws, neutron cross sections, and transmission through realistic BeO slabs.The resulting elastic and inelastic cross sections remain compatible with standard evaluations and measurements, yet reveal sizable model-dependent differences at low temperature and in the vicinity of the phonon gap, where inelastic cross sections can differ by more than a factor of 2. At reactor-relevant temperatures, the ab initio–based data reproduce the measured total cross section and the reference evaluation within a few tens of percent, and predict the thermal energy transmission through 15-cm BeO slabs between roughly 60% and 70%.These results show that ab initio lattice dynamics provide a robust route to update the BeO scattering data when experimental phonon information is limited, and they establish a direct quantitative link between microscopic phonon properties and macroscopic moderation and shielding performance in nuclear systems.