Several advanced nuclear reactor concepts have been proposed in the past few years including the Advanced High Temperature Reactor (AHTR), and the Fluoride Salt Cooled High Temperature Reactor (FHR), where thermal neutrons drive the fission reaction in a core that is primarily composed of graphitic/carbon materials. In this project, neutron powder diffraction and transmission measurements will be performed for three types of graphite materials that include nuclear graphite, carbon-carbon composites, and foam graphite. The structure factor and corresponding atomic pair distribution function will be measured along with thermal neutron transmission. The measurements will be correlated to inform the theory and simulations of neutron thermalization in graphite materials. Irradiated and unirradiated samples will be used in this work. Irradiations and measurements will be performed at the North Carolina State University (NCSU) PULSTAR reactor and associated facilities.

This project is of direct relevance to DOE NE programmatic objectives and to the development of the next generation and advanced nuclear power reactors such as the AHTR and FHR. It presents a unique opportunity to perform studies that investigate the fundamental correlation between the microstructure of neutron moderators and the potential impact on neutronic properties. This is due to the fact that neutron thermalization in matter represents a phenomenon at the intersection of materials science (i.e., atomic microstructure) and neutronics (i.e., neutron/nuclear physics). The structure factors and atomic pair distribution functions of irradiated and uirradiated graphite, that will be measured in this work, represent unique data that can help in reconciling various discrepancies that are currently observed in the behavior of graphite based nuclear reactors. Clearly, such results will help in the design of advanced reactors where graphite is a major core material such as the AHTR and the FHR.