SUBJECT: Ph.D. Dissertation Defense
BY: Vedant Mehta
TIME: Wednesday, November 18, 2020, 3:00 p.m.
TITLE: Investigating the Response of Yttrium Hydride Moderator Due to Changes in Stoichiometry and Temperature
COMMITTEE: Dr. Dan Kotlyar, Chair (NRE)
Dr. Bojan Petrovic (NRE)
Dr. Steven Biegalski (NRE)
Dr. Christopher Perfetti (UNM-NRE)
Dr. Michael Cooper (LANL-MST)


Join the meeting on a computer or mobile phone: Phone Dial-in +1.408.419.1715 (United States(San Jose)) +1.408.915.6290 (United States(San Jose)) Meeting ID: 584 053 742 Microreactors are designed to be compact, truck-transportable, and self-regulating with power levels rated in-between 1 kWe to 10 MWe. For civilian applications, Low Enriched Uranium (LEU) fuel requires the use of moderators. Solid metal hydrides are being considered due to their structural, neutronic, and containment benefits. Out of all metal hydrides, Yttrium Hydride (YHx) is considered as the primary candidate as it provides relatively high hydrogen density combined with high maximum operating temperature. Hydrogen dissociation and migration at higher temperatures within the YHx element raises concern as it changes the reactor behavior during operation. The diffusion of hydrogen within the matrix under a temperature-gradient causes local shifts in the material properties as YHx is altered. As such, stoichiometric and temperature response of the moderator properties of YHx is investigated in this thesis. To create these properties, atomistic simulations using Density Functional Theory (DFT) is utilized. Furthermore, thermal scattering laws (TSLs) are generated using DFT phonon density of states and NJOY2016 data for sub-stoichiometric YHx 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 at 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 the neutronics, heat transfer, and hydrogen mass diffusion. This thesis provides a general framework to model the design space and performance of YHx moderated reactors.