Radiation-Induced Structural Degradation in Glass to Metal Interface: from Physics to Practical Applications

Various diagnostics in tokamaks like ITER require optical (non-metallic) windows for which the joining to the vessel (non-metal to metal bonding) is not straightforward: possible mismatch in thermal expansion coefficient, good mechanical strength and leak tightness required even at high neutron flux and high temperature. The investigation and the understanding of these types of joints under nuclear conditions is the topic of this PhD. Besides the optical windows in ITER, the study of glass-metal joints is also useful for the fixation of fiber optic sensors in MTRs and for sensor developments like the one developed for ultrasonic visualization in a non-transparent liquid metal coolant, like in the future reactor MYRRHA. Various experiments (helium leak tightness test, thermal cycling tests and push-out tests) were performed on both non-irradiated and neutron irradiated samples. The analyses of all experiments were focused on the microstructural behavior of the glass-metal interface. Finite element modeling of the joints was also done for a better understanding of their behavior. Mainly the effects of temperature and pressure on the model were investigated. According to this study, brazing and diffusion bonding seem to be the optimum techniques to fabricate optical windows as they can provide strong, hermetic joints.