H in α-Zr and in zirconium hydrides: solubility, effect on dimensional changes, and the role of defects.

Structural, thermodynamic and elastic properties of the hydrogen?zirconium system including all major hydrides are studied from first principles. Interstitial hydrogen atoms occupy preferentially tetrahedral sites. The calculations show that a single vacancy in ?-Zr can trap up to nine hydrogen atoms. Self-interstitial Zr atoms attract hydrogen to a lesser extent. Accumulation of hydrogen atoms near self-interstitials may become a nucleation site for hydrides. By including the temperature-dependent terms of the free energy based on ab initio calculations, hydrogen adsorption isotherms are computed and shown to be in good agreement with experimental data. The solubility of hydrogen decreases in Zr under compressive strain. The volume dependence on hydrogen concentration is similar for hydrogen in solution and in hydrides. The bulk modulus increases with hydrogen concentration from 96 to 132?GPa.