First Principles Approach to the Hydrogen Pick-up during Oxidation of Zirconium Alloys by Water

Sammanfattning: Zirconium alloys are widely used in nuclear power plants as cladding material to contain the fission fuel in the reactor cores. A limiting factor for fuel longevity is the corrosion property of the zirconium alloys. The main corrodent in the reactor core is water. Ideally, the oxidation process of zirconium alloys with water should be accompanied by molecular hydrogen release into the surrounding, but a significant amount of hydrogen is absorbed into the alloy. This process is called hydrogen pick-up, and is along with the oxidation rate decisive to the longevity of the fuel. The mechanisms controlling the hydrogen pick-up are to a large extent unknown. In this study, density functional theory, DFT, was used to gain insights into the mechanism for water induced corrosion of zirconium. The steady state process whereby Zr is oxidized by means of oxide ions at the metal/oxide interface comprises the anode process. This implies release of two electrons of the oxide ion. A sink for these electrons is offered by H+ ions, whereby H2 is formed or hydrogen picked up in the metal. These comprise the cathode processes. A detailed mechanism for electro-catalytic hydrogen evolution was explored. The mechanism comprises formation of a TM-H-Zr three-center hydride ion, followed by a hydride-proton recombination step forming H2. The efficiency of the system to utilize the overpotential for hydrogen evolution was found to be decisive for the fraction of hydrogen that is absorbed in the alloy. The ability of the anode process to sustain the cathode process was investigated, both by addressing the anode potential and its variations with respect to oxygen distribution. Diffusion barriers for redistribution of oxygen atoms in the α-Zr matrix were quantified, and the finding was taken to suggest the possible roles of meta-stable oxygen distributions to the change in hydrogen pick-up fraction. In addition, the drive for hydrogen pick-up by co-absorption with oxygen in α-Zr was studied and shown to be beneficial relative to H2 (g) for oxygen concentrations well above the oxygen solubility limit.