Thermal Neutron Scattering Measurements of YHx for the Transformational Challenge Reactor

The Transformational Challenge Reactor (TCR) targets application of advances in manufacturing, materials, and computational sciences to accelerate development of new nuclear reactor technologies as well as their adoption by the U.S. industry [1]. The TCR leverages advanced manufacturing capabilities at Oak Ridge National Laboratory (ORNL) to facilitate an agile reactor design with the ability to take advantage of advanced materials and data science-informed certification schemes. Since TCR is gas-cooled a thermal reactor, the choice of neutron moderating material is of crucial importance. To achieve a highly efficient and compact core while minimizing the amount of high assay low enriched uranium required [2], a hydrogen-bearing moderator that is stable to high temperatures is required. This initially led to the selection of metal hydrides (i.e., ZrHx and YHx) for the TCR demonstration core. ZrHx has been extensively studied and used in previous reactor designs since the 1950s due to its low absorption cross section and general availability [3]. However, during the same period, the unavailability of high-purity yttrium as an industrial metal barred it from being considered as another candidate although it exhibits advantageous thermal stability compared to ZrHx [4, 5]. Today yttrium metal of high purity is available as an industrial metal and therefore, YHx was chosen as the moderator material for the TCR. However, a complete thermal scattering library is currently lacking and is needed for an accurate representation of TCR core neutronics properties. This paper details the first step towards the evaluation of a YHx thermal scattering library through the conduct of thermal neutron scattering experiments for various hydrogen atomic concentrations x = 1.62, 1.74, 1.85, 1.90 at 5 Kelvin and room temperature at the Spallation Neutron Source (SNS) at ORNL. This information supplies the necessary data for TCR’s YHx moderator while providing basic information that is necessary for design and deployment of any other reactor taking advantage of this high performance moderator.