The compactness of nuclear microreactors can only be obtained using dense material components for the nuclear fuel, core heat removal components, reflectors, and moderators. Solid moderators contribute largely to the compactness and benefit from incorporating light atomic weight elements. Hydrogen-bearing materials, such as metal hydrides, offer the highest moderation per unit volume and thus are a strong consideration for the moderator of a microreactor. Yttrium hydride is an attractive option for neutron moderators due to a very high atomic density of hydrogen and relative high temperature stability. Recently, crack-free bulk yttrium hydride was successfully produced and thoroughly characterized for its properties, stability, and hydrogen retention up to 600°C. Hydrogen retention governs neutron moderation and prevents the buildup of hydrogen gas. However, yttrium hydride’s phase stability and high temperature hydrogen retention capability under irradiation is largely unknown. Understanding the kinetics of hydrogen desorption and release from yttrium hydride at elevated temperatures under irradiation is a critical gap to determine the safety and efficient operation of hydride-moderated nuclear reactors. We propose to investigate the phase stability and hydrogen retention of yttrium hydride under high temperature proton irradiation. We will investigate the hypothesis that radiation will reduce the onset temperature for hydrogen desorption and that lower energy recoils will generate increasing hydrogen release with irradiation temperature due to preferential displacement of H over Y due to their difference in mass. The proposing team seeks use, through the Nuclear Science User Facilities, of the Michigan Ion Beam Laboratory (MIBL) for ion irradiation and ion beam analysis and the Low Activation Materials Development and Analysis (LAMDA) facility at ORNL for detailed post irradiation examination using thermal desorption spectrometry (TDS) and transmission electron microscopy (TEM). Yttrium hydride will be irradiated with either 1 MeV H+ or 2 MeV H+ ions to 0.2 dpa at temperatures of 300°C and 600°C. Elastic recoil detection analysis at MIBL will measure the hydrogen content. Comparing ERDA spectra before irradiation with both the irradiated and non-irradiated areas after irradiation, the effect of radiation damage on hydrogen release can be separated from high temperature release. The microstructure will be examined with TEM to determine defect structures and possible phase changes. The hydrogen desorption flux will be measured using TDS to obtain apparent activation energies for hydrogen release. Together, the combination of hydrogen content from ERDA, microstructure from TEM, and activation energies from TDS will identify the hydrogen desorption mechanisms and correlate them to the irradiated microstructure. The estimated period of performance for each condition is 30 hours for proton irradiation beam-time with at least 3 ERDA spectra at MIBL, and 16 hours of PIE in LAMDA to produce and characterize TEM lamella and collect TDS. The outcomes of this work will provide quantitative comparisons of hydrogen desorption and hydride phase stability as a function of irradiation temperature and recoil energy. This knowledge is critical to understanding how hydrogen transport in metal hydrides is affected by irradiation and how to mitigate radiation-induced degradation in solid moderator materials.

The Office of Nuclear Energy (DOE-NE) mission is to advance nuclear energy science and technology to meet U.S. energy, environmental, and economic needs. The DOE-NE launched the Microreactor Program in 2020 and Advanced Materials and Manufacturing Technologies Program in 2021 to develop technologies for the deployment of civilian microreactors and advanced reactors, and the Transformation Challenge Reactor (TCR) program to demonstrate a microreactor. Microreactors are expressed as advanced transportable nuclear reactors operating at low power (<20 MWth) but high temperatures (>600°C), as well as plug-and-play and inherently safe designs. One prerequisite of a microreactor is the compactness, such that a truck can transport the reactor under safe conditions with the current road infrastructure. The compactness of these reactors can only be attainable by use of dense material components for the essentials of the nuclear core, such as fuel, core heat removal components, reflectors, and moderators. Among these essentials, the largest contribution to the compactness is offered by use of solid moderators which benefits from light atomic weight elements, such as hydrogen, carbon, and beryllium. Hydrogen-bearing materials, such as metal hydrides, offer the highest moderation per unit volume and thus are a strong consideration for the moderator of a microreactor. Yttrium hydride has been demonstrated to have adequate high temperature properties and higher hydrogen retention and thermal stability compared to zirconium hydrides. However, there are very limited studies of hydrogen release from yttrium hydride under high temperature irradiation and very minimal mechanistic information. This RTE focuses on the generation of data to quantify the hydrogen release and phase stability of yttrium hydride as a function of irradiation temperature and bombarding particle energy, and from this, determine the mechanisms of hydrogen transport. The availability of this data and the resulting scientific understanding of hydrogen transport under irradiation will support ongoing microreactors development activities led by the Microreactor Program, the Advanced Materials and Manufacturing Technologies Program, follow-on from the Transformational Challenge Reactor program, and aid in the adoption of solid moderators in industrial microreactor concepts.