b'Development ofModeling hydrogen migration effects in metal hydrides multiphysics object orientedenables economical and compact microreactors.simulation environmentT he introduction of nuclear microreactors is projected to open new markets for the nuclear power industry because of their potential cost-based capabilities tocompetitiveness in non-traditional market segments. An obstacle to model hydrogen migrationdeploying many nuclear microreactor concepts is their reliance on high-assay low-enriched uranium. Despite enabling compact system designs and long operational in hydrides-moderatedlifetime, using high-assay low-enriched uranium entails both technical and microreactors regulatory challenges. The higher enrichment leads to additional costs that hinder the economic competitiveness of microreactor designs and increases proliferation risks, thus sparking the need for updated regulations. Reduction of the fuel enrichment or quantity while maintaining the compact design and high operating temperature can be achieved by using metal hydrides. However, the hydrogen contained in the hydrides tends to redistribute and leak from the moderating elements, which leads to reactivity losses and potential failure of the moderating PROJECT NUMBER:elements and consequent reactor shutdown. 21P1056-010FPThis work assessed the current capabilities of the MOOSE to model hydrogen TOTAL APPROVED AMOUNT:redistribution and dissociation in yttrium hydride, which is the main candidate for $248,151 over 3 years moderation of high temperature microreactors. It was found that the redistribution of PRINCIPAL INVESTIGATOR:hydrogen within the hydride leads to a negative reactivity feedback due to hydrogen Stefano Terlizzi migration toward colder axial zones that are usually associated with lower neutron importance. The magnitude of the feedback descends from the magnitude of the CO-INVESTIGATOR:axial temperature gradient that determines the asymptotic hydrogen distribution in Mark DeHart the hydride. To the best of the teams knowledge, this was the first work in which the effect of hydrogen redistribution on microreactor reactivity was evaluated in a coupled multiphysics setting that included neutronics, heat pipe flow, and heat transfer equations at a full-core scale. The dissociation of hydrogen at the hydride surface together with the leakage through the cladding was also modeled. It was shown that results from dissociation models are strongly influenced by the considerable uncertainty in yttrium hydride material properties. To further develop this insight, a rigorous uncertainty quantification study was performed to identify the main sources of uncertainty in yttrium hydride moderated microreactors. It was found that the epistemic uncertainties related to lack of knowledge of material properties dominate over the aleatoric uncertainties, thus implying the need for additional experimental campaigns to characterize hydrogen dissociation dynamics and temperature gradient driven response in yttrium hydride.50'