Ab-initio calculations of the hydrogen–uranium system: Surface phenomena, absorption, transport and trapping

Abstract

Density functional theory calculations have recently been validated for a-uranium (a-V) metal and, in this work, applied to the initial steps of uranium hydriding: surface phenomena, absorption into the bulk, bulk transport (diffusion) and trapping at defect sites. We investigate the surface chemistry of hydrogen (H) on the (001) surface of a-V, and the influence of high-coordinate geometries on H2 adsorption and dissociation on the metal surface. In the adsorbed state H already has a partially ionic character. Although H adsorbs exothermically to the (001) surface, with respect to the reference state of H 2 , we find that it is endothermic to absorb H into the bulk, with off-center octahedral absorption (Le. a square-pyramidal coordination ofH in the lattice) having the lowest absorption energy of 0.39 eV relative to molecular H2. H absorption in interstitial sites is calculated to cause a local softening of the bulk modulus. The diffusion barrier for H in unstrained a-V is calculated to be 0.6 eV. The energy of H absorption in a site adjacent to the chemical impurities C, S, Si was lowered by an amount proportional to the atomic radii of the impurity atom, and the resulting lattice strain Si > S > C. Thus, impurities may promote hydriding by providing surfaces or pre-strained zones for H uptake.

The Hydrogen Corrosion of Uranium: Identification of Underlying Causes and Proposed Mitigation Strategies

“…51 In contrast, quantum chemistry calculations by Balasubramanian (LLNL) on idealized (i.e., non-optimized geometry) uranium clusters with a substitutional surface impurity show enhanced dissociation barriers for molecular hydrogen at a C impurity (+82%) and reduced barriers at a Si impurity (-45%). 53 Aside from the fundamental differences in computational approach, the LLNL and LANL results for C are not necessarily contradictory as they technically address different phenomena: molecular hydrogen may not favorably dissociate into atomic hydrogen at a surface carbon site, and atomic hydrogen, once created by whatever mechanism, may energetically favor binding at a carbon site within the bulk.…”

“…51 In comparison, an Arrhenius fit to the experimental solubility data of Powell for hydrogen in uranium 29 gives an estimated activation barrier of 0.29 eV, 41 while several other experimental studies report values in the range 0.26-0.35 eV. 9,11,52 This strain amplitude is within the elastic regime; a uniaxial static tensile strain of about 1% was measured by Garlea and co-workers (Y-12) for cast depleted uranium at its tensile yield strength of ~275 MPa, 2 giving an estimated hydrostatic strain value and elastic limit of 3%.…”

“…To investigate uranium hydride corrosion initiation, Taylor and Lillard (LANL) modeled the interaction of atomic hydrogen (H) with a uranium surface using density functional theory (DFT). 51 In accordance with the best practices for ab initio modeling of material surfaces, these authors effectively created a semiinfinite -uranium (001) surface using a 96-atom, three-layer substrate slab terminating in a vacuum gap and subjected to three-dimensional periodic boundary conditions. The resulting supercell, incorporating H at various high symmetry sites, was energetically and structurally optimized via iterative minimization algorithms.…”

“…4, predict that atomic hydrogen should favor dissolution in tensile strain fields, such as those that occur near or at defect structures in uranium. 51 A follow-up study by these authors also found that only in the vicinity of defects where the metal can readily expand -into boundaries, vacancies, and voids -can the local interstitially dissolved hydrogen concentration approach that required to form uranium hydride, whereas the global hydrogen solubility in the defect-free bulk lattice is negligible (Section 1.2). 56 Similar experimental observations and theoretical predictions have been reported for other hydride-forming metals.…”

The effects of metal surface geometry on the formation of uranium hydride

“…For example, recent density functional theory modelling studies by Taylor and Lillard [29] have investigated the interatomic penetration of hydrogen into uranium surfaces. Their study suggested that lattice diffusion of hydrogen into a defectfree metal surface is energetically unfavourable and diffusion was significantly lower than experimentally determined values.…”