Factory-fabricated and truck-transportable nuclear microreactors rely on solid-state moderators to reduce reactor footprint while maintaining performance under extreme temperature and space-constrained environments. Among candidate materials, sub-stoichiometric yttrium hydride (YHx) is highly attractive due to its high hydrogen number density, thermal stability and mechanical integrity. Given all advantageous properties, the yttrium hydrides are susceptible to hydrogen redistribution and loss under fast-neutron irradiation, compromising moderation efficiency and reactor safety margins. A promising mitigation strategy is to envelop YHx moderators with barrier coatings, such as FeCrAl alloys, that is known to suppress hydrogen permeation. However, the coupled behavior of FeCrAl and irradiated YHx under reactor-relevant thermal conditions remains unexplored.

This project addresses that knowledge gap through a multimodal investigation combining in-situ synchrotron X-ray diffraction and transmission electron microscopy studies. YHx disks, neutron irradiated at the High Flux Isotope Reactor (HFIR) facility at Oak Ridge National Laboratory (ORNL), will be paired with oxidized or non-oxidized FeCrAl plates to make distinct YHx–FeCrAl assemblies. The selected YHx disks have identical stoichiometry and irradiation dose, but differ in irradiation temperature. Synchrotron XRD experiments, under controlled thermal cycling, will track crystallographic evolution, hydrogen desorption kinetics and irradiation-induced damage recovery. On the other hand, complementary TEM characterizations of pre-annealed and post-heating alloy specimens will resolve barrier microstructure and hydrogen-induced features at the YHx–FeCrAl interface. This experimental design systematically isolates the effect of irradiation temperature and barrier oxidation, providing fundamental understanding of the influence of FeCrAl on hydrogen retention in irradiated hydrides.

The anticipated scientific outcome will be the first mechanistic framework for temperature-driven hydrogen transport in irradiated YHx coupled with engineered permeation barriers. If successful, this work will significantly advance the state-of-knowledge on hydride moderators and inform design strategies for microreactor deployment. The novel study is expected to be published in a high-impact journal.

The proposed research advances the U.S. Department of Energy Office of Nuclear Energy (DOE-NE) mission to “enable the safe, secure and economical use of nuclear power in the United States” by addressing a critical knowledge gap in the performance of advanced moderators for factory-fabricated and truck-transportable microreactors. Yttrium hydrides are promising candidates, but the long-term stability of these materials under neutron irradiation and elevated in-reactor temperatures remains insufficiently understood. Existing microreactor demonstrations acknowledge this uncertainty solely through conservative analytical assumptions. This reflects the absence of a mechanistic framework for hydrogen retention, or for evaluating how engineered barriers (such as FeCrAl alloys) might mitigate hydrogen loss.

The proposed in-situ synchrotron diffraction studies, complemented by transmission electron microscopy analyzes, directly addresses this gap by resolving the coupled microstructural and phase evolution of FeCrAl and irradiated yttrium hydrides during controlled heating from 500-950°C. A unique aspect of this study is that such materials compatibility tests were never performed at these temperature ranges, even within the legacy programs. By producing high-resolution first-of-kind datasets that clarify the role of FeCrAl coatings as hydrogen permeation barriers for irradiated hydrides under representative in-reactor conditions, this study supports DOE-NE’s Microreactor Program (MRP) under the “Technology Maturation” topic.

While ongoing MRP activities emphasize system-level demonstrations (e.g., MARVEL) and large-scale thermal-hydraulic tests of candidate moderators, these efforts do not resolve the fundamental mechanisms of hydrogen redistribution in hydrides or the mitigating role of permeation barriers. This project, therefore, complements, rather than duplicates, programmatic work by providing a mechanistic basis needed to strengthen material qualification roadmaps. By closing a high-impact data gap, our study enhances licensing readiness, improves economic competitiveness through reduced design conservatism, and ultimately accelerates deployment of resilient, transportable microreactors- outcomes directly aligned with DOE-NE’s mission on microreactors and fission surface power systems, as well as the emergent NASA’s moon reactor mission.