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Molecular Dynamics Study of Zirconium and Zirconium Hydride

Date

2013-11-14

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Degree Level

Masters

Abstract

Molecular dynamics (MD) simulations were used in order to investigate structure and mechanical properties of zirconium and zirconium hydride. Calculation of temperature dependent failure of zirconium, diffusion of hydrogen in zirconium, properties of interfaces in zirconium and zirconium hydride and effect of hydrogen on crack nucleation and propagation were in good agreement with available experimental data. These are the first computer simulations where large-scale atomic/molecular massively parallel simulator (LAMMPS) code was used with the Embedded Atom Method (EAM) and Modified Embedded Atom Method (MEAM) to study structure and mechanical properties of zirconium hydrogen system (Zr-H) and zirconium hydride (ZrH2). Verification of methods was done in order to establish the best potential for zirconium and zirconium hydride. EAM and MEAM potentials successfully predicted lattice parameters, mechanical properties and variation of lattice parameters with temperature for α-Zr. MEAM potential was used to predict correctly the face centered structure for ZrH2 and also its mechanical properties. Temperature dependent stress-strain curves were calculated in order to predict yielding point for α-Zr. Results indicate early yielding and failure with increase of temperature in zirconium on application of tensile and compressive strains. Anisotropic stress variation with temperature in α-Zr was calculated. Hydrogen ingress through diffusion of hydrogen in zirconium is a mechanism responsible for formation of hydrides. Temperature-dependent hydrogen diffusion and activation energy for diffusion was calculated and the agreement with experiments was satisfactory. Anisotropy of diffusion of hydrogen is observed for Zr crystal. Hydrogen diffusion was also modeled under tensile and compressive strain and a possible formation of hydrides in the direction perpendicular to applied strain was observed. The effect of strain on orientation of hydride was investigated. Hydride {111} oriented crystal was strained along [1 1 ̅ 0] and [111] direction. Energy as a function of strain is calculated along both directions [111] and [1 1 ̅ 0] and it was found that energy of the system increase with increase in strain along [1 1 ̅ 0] and decrease with increase of strain along [111] direction. Calculated stress and strain curves indicate lower stresses along [111] direction and this causing the hydride to reorient in a direction perpendicular to applied strain. Structure of the interface (0 0 0 1) α-Zr // {1 1 1} δ-ZrH2 is modeled in order to investigate the crack initiation at this interface. Interfacial cracking of hydride under stress is observed. This observation is in good agreement with available experimental studies. Cracks are seen to nucleate earlier at higher temperature. Cracks and voids are common defects in zirconium fuel cladding. A crack is modeled along (0 0 0 1) plane of zirconium with hydrogen. In the presence of hydrogen cracks nucleate in zirconium causing fracture. This observation is in good agreement with previous experimental studies. Bonds surrounding atoms and stress concentration analysis were performed using OVITO and VMD software’s respectively. Weaker bonds and higher stress concentrations are observed in the presence of hydrogen in zirconium. The presented results clearly demonstrate that MD simulation can be used to predict structure and processes that are important for understanding failure in Zr based nuclear materials.

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Keywords

Molecular Dynamics, Zirconium, Zirconium hydride, EAM, MEAM

Citation

Degree

Master of Science (M.Sc.)

Department

Mechanical Engineering

Program

Mechanical Engineering

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