Title:
INVESTIGATING THE RESPONSE OF YTTRIUM HYDRIDE MODERATOR DUE TO CHANGES IN STOICHIOMETRY AND TEMPERATURE

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Mehta, Vedant Kiritkumar
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Kotlyar, Dan
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Abstract
Microreactors are designed to be compact, truck-transportable, and self-regulating with power levels rated anywhere between 1 kWe to 10 MWe. Microreactors are envisioned to be utilized for terrestrial as well as space power applications. Originally, microreactors were envisioned to use HEU fuel with fast spectrum core operation, however, this poses regulatory concerns. As such, recent endeavors rely on the application of Low Enriched Uranium (LEU) fuel. In order to maintain a relatively compact reactor-core with LEU fuel, effective neutron moderation is required; and hence LEU mandates the use of moderators. Solid metal hydrides are being considered due to their structural, neutronic, and containment benefits. Out of all the metal hydrides, Yttrium Hydride (YH_(2-x)) is considered as the primary candidate as it provides relatively high hydrogen density combined with high maximum operating temperature. However, hydrogen dissociation and migration at higher temperatures within the YH_(2-x) element raises concerns as it changes the reactor behavior during operation. The diffusion of hydrogen within the YH_(2-x) matrix under a temperature-gradient causes local shifts in the material properties as YH_(2-x) is altered to YH_((2-x)±Δ). As such, stoichiometric and temperature responses of the YH_(2-x) moderator properties are investigated in this dissertation. To create these properties, atomistic simulations, using Density Functional Theory (DFT), are performed. Furthermore, thermal scattering laws (TSLs) are generated using DFT phonon density of states and NJOY2016 for sub-stoichiometric YH_(2-x) to account for shifts in neutron cross sections at thermal energies. The properties generated from atomistic modeling is further validated with the neutron diffraction experiments performed by Los Alamos Neutron Science Center (LANSCE) and available literature. Finally, a coupling capability is developed and implemented using the Monte-Carlo code MCNP along with the Finite Element based code ABAQUS. The coupled framework is realized via Picard iterations, and allows the investigation of neutronics, heat transfer, and hydrogen mass diffusion. This dissertation provides a general framework to model the design space and performance of YH_(2-x) moderated reactors.
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2020-12-02
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