Journal of Physics: Condensed Matter J. Phys.: Condens. Matter 27 (2015) 025402 (12pp) doi:10.1088/0953-8984/27/2/025402 H in α-Zr and in zirconium hydrides: solubility, effect on dimensional changes, and the role of defects M Christensen 1 , W Wolf 1 , C Freeman 1 , E Wimmer 1 , R B Adamson 2 , L Hallstadius 3 , P E Cantonwine 4 and E V Mader 5 1 Materials Design, Inc., 6 First National Place, Angle Fire, NM 87710, USA 2 Zircology Plus, 36848 Montecito Dr, Fremont, CA 94536, USA 3 Westinghouse Electric Sweden AB, SE-721 63 V¨ asterås, Sweden 4 Global Nuclear Fuel–Americas, PO Box 780, M/C F12, Wilmington, NC 28402, USA 5 Electric Power Research Institute (EPRI), 3420 Hillview Ave, Palo Alto, CA 94303, USA E-mail: mchristensen@materialsdesign.com Received 8 September 2014, revised 19 November 2014 Accepted for publication 26 November 2014 Published 15 December 2014 Abstract 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 α-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. Keywords: zirconium, hydrogen, vacancies, interstitials, hydrogen solubility, absorption isotherms, ab initio (Some figures may appear in colour only in the online journal) 1. Introduction The behavior of hydrogen in metals continues to be a topic of fundamental interest and vital importance in many technological applications. For example, even small quantities of hydrogen can have serious detrimental effects on mechanical properties of metal alloys due to hydrogen- induced embrittlement. The possible detrimental impact of hydrogen is of particular concern in nuclear power plants, where zirconium alloys are used for fuel cladding and structural components. The oxidation of these zirconium alloys by decomposition of water leads to the release of hydrogen, which can diffuse into the metal, dissolve and lead to the precipitation of hydride phases causing a volume increase and embrittlement. These processes are coupled with structural and mechanical changes caused by irradiation with high- energy neutrons and other particles. Particularly intriguing is the experimental evidence[1–5] that zirconium samples pre-loaded with hydrogen show a different behavior under irradiation compared with pure zirconium. For these reasons, the zirconium–hydrogen system has been studied extensively for many decades and a comprehensive review is provided by Strasser et al [6]. While initially most of these studies were purely experimental, more recently, a growing number of computational investigations have been performed using both empirical potentials and ab initio methods thus offering a deeper understanding and more comprehensive materials property data. To this end, embedded atom interatomic potentials have been developed by Mendelev and Ackland [7] for the study of phase transitions and the calculation of basic materials properties including elastic coefficients, point defect energies, stacking fault 0953-8984/15/025402+12$33.00 1 © 2015 IOP Publishing Ltd Printed in the UK