b'Informative design of highInformative first principals based computational materials temperature metal hydridescience elucidates the nature of the hydrogen retention in moderators in microreactors metal hydrides as impacted by the alloying elements.M icro nuclear reactor (microreactor) technology focuses on the development of passively safe, compact, and transportable reactors that operate at low power (20MW) and high temperatures (500C). One key design factor for the transportability is using solid neutron moderators to reduce the size and mass of the reactor. Because hydrogen nucleus is an effective moderator, materials with high hydrogen number density at microreactor-relevant temperatures PROJECT NUMBER:are considered as solid moderators, such as metal hydrides. Major technological 21A1050-020FP challenges with metal hydride moderators are posed as the redistribution of TOTAL APPROVED AMOUNT:hydrogen in the metal hydride under a temperature gradient and its potential loss $1,443,700 over 3 years from the moderator, including the cladding, over time. If hydrogens redistribution or loss is beyond the design envelope, the criticality condition of the reactor can be lost. PRINCIPAL INVESTIGATOR:Both challenges are governed by the mobility of hydrogen in which the quantum Mahmut Nedim Cinbiz effects are prominent. Thus, it is imperative to understand hydrogen retention CO-INVESTIGATORS: and diffusion mechanisms in the condensed matter via electronic structure based Chase Taylor, INLatomistic modeling. Jianguo Yu, INL This project investigated the transport characteristics of hydrogen in condensed Miaomiao Jin, INLmatter using cutting edge electronic structure based multiscale computations and Daniel Labrier, Idaho State Universityphenomena-specific experiments to develop advanced metal hydride moderators. Michael Short, Massachusetts InstituteMultiscale computations included density functional theory, ab initio molecular of Technologydynamics, molecular dynamics with density functional theory accuracy machine COLLABORATORS: learned interatomic potentials, and kinetic Monte Carlo techniques. Results of the Los Alamos National Laboratory multiscale computation were compared with the physical properties.North Carolina State University High accuracy multiscale computations proposed the root cause of high hydrogen Oak Ridge National Laboratoryretention as the biased charge transfer from yttrium or zirconium to hydrogen. University of Michigan The biased charge transfer to hydrogen originated from significant overlapping of specific electronic orbitals and localized electron densities at deep energy levels. The biased charge transfer was also responsible for the strong chemical interactions like high hydrogen binding energy and consequent large energy barrier for hydrogen diffusion. Based on our findings, we concluded that the charge transfer was one of the alloying metrics for binary and ternary alloys, and the careful examination of the available literature also supported the use of this metric. Thus, we performed series of simulations on binary and ternary alloy hydrides and verified the alloying metric. The project also established a strong collaboration with Los Alamos National Laboratory, Oak Ridge National Laboratory, and Brookhaven National Laboratory. 26'