Evaluation of thermal neutron scattering law of nuclear-grade isotropic graphite

Graphite is a candidate of moderator in innovative nuclear reactors such as high-temperature gas-cooled reactors and molten salt reactors. Scattering of thermal neutrons by a moderator material has a significant impact on the reactor core design. To contribute to the development of the innovative nuclear reactors, thermal neutron scattering law data for nuclear-grade graphite were evaluated. Inelastic scattering component due to lattice vibration was evaluated based on phonon density of states computed with first-principles lattice dynamics simulations. The simulations were performed for ideal crystalline graphite. The evaluated inelastic scattering component well reproduced the experimental double-differential cross sections of scattered neutrons from nuclear-grade isotropic graphite measured at Materials and Life science experimental Facility (MLF) of J-PARC. Coherent elastic scattering component due to crystal structure was evaluated based on the neutron scattering and transmission experiments performed at MLF of J-PARC. Regarding comparison with the neutron transmission experiment, it was found that the quantification of small-angle neutron scattering due to structures larger than crystal, such as pores in graphite, is important.