Understanding the oxidation of zirconium alloys using atomistic simulation

Abstract

With current trends to increase fuel burn-up in light-water nuclear reactors, minimising fuel cladding tube degradation mechanisms such as corrosion becomes more important. The corrosion of Zr alloys, which make up the cladding, is an electrochemically-driven process between the metal and the oxygen-containing environment, and has been found to be highly dependent on the microstructure and texture of the oxide layer that forms. In this thesis, we used a combination of density functional theory (DFT) and new crystallographic texture analysis applied to experimental crystal orientation data obtained using electron backscatter diffraction (EBSD) and scanning precession electron diffraction in the transmission electron microscope (SPED), to distinguish between the effects of lattice matching between the metal and the oxide crystals and the transformational compressive stress as mechanisms for both the tetragonal and the monoclinic oxide texture formation. Furthermore, we used DFT to examine how the structural and electronic properties of a low energy monoclinic grain boundary, present in high fraction in experimental texture maps, are affected by the presence of selected point defects that are important for the corrosion process. We found that in single-phase Zr alloys with split-basal texture, the crystal orientation of the hcp metal grains determines the crystallographic texture of the nano-grained oxide layer. In typical grains with the {0001}-hcp basal poles tilted by 30 degrees to 50 degrees from the cladding tube surface, the Zr metal transforms firstly to tetragonal and then to monoclinic ZrO2 according to: {101}{10-2}-t || {111}{10-1}-m || {0001}{11-20}-hcp. However, only some of the symmetrically-equivalent monoclinic orientations continue to grow under the influence of the compressive stress. We found that there is a balance between the growth of planes with small areal footprint but high stiffness such as {10-6}, and planes with larger footprint but lower stiffness, such as {10-3}. Our texture analysis allowed for the application of a local correction based on the interfacial orientation, which showed a two-fold sharpening of the measured fibre texture of the monoclinic oxide. We also observed the presence of metal grains with the {0001}-hcp plane parallel to the surface. In these grains, we found lattice matching to have a stronger effect than the compressive stress. Major and minor epitaxial orientation relationships between the hcp metal and the monoclinic oxide were identified as {11-2}-m || {0001}-hcp and {31-2}-m || {0001}-hcp in the EBSD data. The interface-orientation correction revealed the presence of these texture components in another smaller SPED dataset. Therefore, we suggest that the compressive stress forms better aligned, longer columnar oxide grains in the former type of metal grains, which we link with an observation that these grains corrode slower. We found that the Sigma 3 180 degrees (100) [001] in monoclinic ZrO2 has a very low grain boundary energy of 0.06Jm-2 and very bulk-like local atomic environment. As a representation of a more general grain boundary, we performed analysis of a metastable structure of this boundary, which has a grain boundary energy of 0.32Jm-2. We explored the effect of defect concentration on the energetic favourability of oxygen vacancies, tin and niobium substitutional point defects in the two structures. We found that at the expected oxygen vacancy concentrations near the metal-oxide interface, doubly-charged vacancies would segregate to both boundaries. This energetic favourability of V_O^{..} for the metastable GB is expected to significantly increase in the presence of Sn_Zr^{''}. Furthermore, Nb_Zr^{.} was found to favour the metastable GB at some concentrations. On the other hand, our results for the equilibrium structure showed energetic favourability of Nb_Zr^{''} across a wide range of bulk concentrations, but not of Sn_Zr^{''}, which might contribute to making this interface a difficult diffusion path. In addition, defect clusters of V_O^{..} and Sn_Zr^{''} near the boundaries showed much stronger binding energy compared to that in the bulk, and introduced defect states in the band gap of the electronic structure. Therefore, we suggest monoclinic grain boundaries might act as percolation paths for electron transport through the Zr oxide layer. Together, these observations suggest mechanisms for increased oxygen and electron transport due to tin near the metastable grain boundary, and therefore other more general oxide boundaries.

Date of Award	31 Dec 2019
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Joseph Robson (Supervisor) & Christopher Race (Supervisor)