Ab Initio Modelling of Zirconium Hydrides

Abstract

Zirconium alloys are very suitable as fuel claddings and structural components in nuclear reactors due to their low cross section for ab- sorption of thermal neutrons and high resistance to corrosion and irra- diation damage. Zirconium alloys are however susceptible to hydrogen embrittlement. Precipitation of zirconium hydrides is attributed to the embrittlement effect in zirconium alloys. In this project ab-initio simulations of Zr and the three different zirconium hydrides -ZrH, -ZrH1:5 and -ZrH2 are performed to calculate structural, elastic and thermal properties of these different compounds. The goal with the project was to calculate the equilibrium structures of the H-Zr system using ab-initio methods. The results from the calculations of thermal and mechanical properties were found to be in good agreement with previous results and experiments where these were available. Calcula- tions of the phonon band structure for the different hydrides show that the band structure of the hydrides has two characteristic bands, one corresponding to vibrations in the zirconium sublattice and one cor- responding to vibrations in the hydrogen sublattice. The calculated heat capacity of the different compounds was found to be described very well by the Einstein model for the heat capacity of a solid. By calculating the free energy of formation for the different compounds it was found that at high hydrogen concentrations -hydride is the equi- librium structure. At intermediate hydrogen levels the equilibrium structure can't be determined.