| Fuel cladding is the reactor's primary safety barrier, which prevents radioactive fission fragments from releasing into the coolant. The development of advanced nuclear reactors requires fundamental improvements in the properties of cladding materials, which include increased strength at higher temperature and superior corrosion and radiation damage resistance. Many of the above properties are controlled by interfaces. For instance, internal interfaces (e.g. grain boundaries, GB) can act as defect sinks and external interfaces (e.g. surfaces) are the places where the corrosion reactions take place. It is therefore critical to understand the complicated effects of interfaces on these properties to design advanced fuel cladding.;In the thesis two kinds of cladding materials, silicon carbide (SiC) and zirconium (Zr) are studied. For SiC, high-resolution (scanning) transmission electron microscopy and multiscale materials simulations have been combined to investigate the defect-GB interactions. Specifically, the presence of the interstitial starvation mechanism near GB has been demonstrated. As a competing effect to defect sinks, interstitial starvation leads to vacancies built up near GB and reduces the radiation resistance of nanocrystalline SiC. Using electron energy loss spectroscopy, the evolution of GB carbon composition at different irradiation temperature has also been characterized. GB experiences a carbon enrichment at 300 °C and a depletion at 600 °C. Based on rate-theory calculations, we found this non-monatomic trend of segregation was due to the competition between C flux to GB and C diffusion along GB to strong sinks like surfaces. To the best of our knowledge, it is the first time that radiation-induced segregation of constitution elements has been discovered in ceramic materials.;For Zr, density functional theory calculations have been applied to understand the effects of surface strain on the oxidation process. The interplay between surface strain and interactions between oxygen adsorbates determines the relative stability of different binding sites. The binding energy calculations indicate that the compressive strain can provide a thermodynamic driving force for oxygen diffusing into deeper Zr layers, while tensile strain can facilitate the process of binding oxygen to Zr surface. A continuum model for predicting time-dependent hydrogen pickup fractions has also been developed. |
