Handbook on the Material Properties of Yttrium Hydride for High Temperature Moderator Applications

Abstract

Yttrium hydride is an optimal choice for a high-temperature moderator material in advanced thermal neutron spectrum reactors that require small core volumes. A complete database of the thermomechanical properties of yttrium hydride is not yet available, although it is much needed to understand and predict the moderator performance during service in reactors. This milestone report presents the properties of unirradiated bulk yttrium hydride as a function of hydrogen concentration-including density, crystal structure, specific heat capacity, thermal diffusivity, thermal conductivity, hardness, elastic/shear moduli, Poisson's ratio, fracture strength, microstructure, and thermal stability. This information provides a baseline measurement for the subsequent neutron irradiation response study of yttrium hydride. The recommended empirical treatment of the data is suggested. In addition, other properties (i.e., hydrogen retention, thermal hydrogen migration, and irradiation response) requiring investigation are discussed. Also included are the Preliminary post-irradiation examination (PIE) data of yttrium hydride irradiated at 600 and 900°C to 0.1 dpa in the High Flux Isotope Reactor (HFIR). The thermophysical properties have insignificant change following this low dpa irradiation for both irradiation temperatures.

A Modeling and Neutron Diffraction Study of the High Temperature Properties of Sub-Stoichiometric Yttrium Hydride for Novel Moderator Applications

“…To predict the lattice expansion of stoichiometric YH 2 and Y, the quasi-harmonic approximation was utilized using the Phonopy code [27]. Using a stoichiometric δ-YH 2 supercell led to the creation of eleven supercells with distinct volumes using the VASP scaling parameter, adding two additional volumes beyond our previous work [20]. We observed that the addition of these two volumes had no impact on the coefficient of thermal expansion (CTE).…”

“…In this study, we utilized a DFT-based quasi-harmonic approximation approach (QHA) to study yttrium metal (α-Y). We note that two additional volumes were added for yttrium dihydride (δ-YH 2 ) in this study with regard to our previous work [20], and the DFT-QHA analysis was reevaluated. The DFT code Vienna Ab initio Simulation Package (VASP) [22][23][24] was used to carry out the first-principles quantum mechanical simulations.…”

“…However, the thermal expansion effects were not included in that study, thus limiting that study to ambient conditions only. Hu et al [20] also recently published a handbook of yttrium hydride material, which provides data for various moduli at ambient conditions. Mehta et al [21] also studied the stoichiometry and temperature dependent single-phase heat capacity (c v ), and thermal scattering laws (TSLs) applicable for hightemperature nuclear reactor modeling.…”

“…Since yttrium hydride exists in both single-phase (at higher temperatures) and two-phase (at lower temperatures, see Figure 1) fields, for microreactor operation conditions, both the properties for single-phase yttrium metal (α-Y) and yttrium hydride (δ-YH 2−x ) are required to construct a comprehensive set of thermophysical properties. Accordingly, DFT-QHA simulations were performed for α-Y for the first time in this paper, and the δ-YH 2 DFT-QHA thermomechanical results are thoroughly analyzed, as they were excluded from our previous publication [20]. Furthermore, the results reported in our previous work [20] only contained the c p and the thermal strain data for δ-YH 2 but excluded various moduli as a function of temperature.…”

“…Accordingly, DFT-QHA simulations were performed for α-Y for the first time in this paper, and the δ-YH 2 DFT-QHA thermomechanical results are thoroughly analyzed, as they were excluded from our previous publication [20]. Furthermore, the results reported in our previous work [20] only contained the c p and the thermal strain data for δ-YH 2 but excluded various moduli as a function of temperature. Yttrium hydride samples were also investigated experimentally using neutron diffraction experiments and compared with the DFT results in both that paper and this one.…”