A simulation model of ultrasonic wave changes due to irradiation-induced microstructural evolution of thick 304 stainless steel blocks

It is desired to develop an ultrasonic inspection technique capable of non-destructively measuring various forms of irradiation-induced degradation in thick structural components of nuclear reactors. Of particular interest is development of techniques that can elucidate the spatial distribution of microstructural components that increase or decrease component volume or that change mechanical or physical properties, all arising from spatial variations in neutron flux-spectra, irradiation temperature and applied stresses. Ultrasonic testing is one of the widely used non-destructive testing techniques for material characterization. Ultrasonic waves, currently used as a non-destructive technique to detect cracks, change as a result of interacting with microstructural components and these changes can be employed to measure irradiation-induced changes in microstructure. In order to evaluate the depth distributions of irradiation-induced microstructural changes, we develop below a numerical simulation model of ultrasonic wave changes caused by microstructural changes in austenitic stainless steel based on the theory of ultrasonic wave propagation. In this simulation we focus primarily on simplified distributions of void swelling which were experimentally found to dominate the microstructures of thick stainless steel blocks removed from the EBR-II reflector. The simulation produced results that were consistent with the experimental results, providing confidence that this technique can indeed be used to interrogate reactor internal components to measure not only the bulk-averaged swelling but also the depth distribution of swelling in thick components. This study revealed that the technique developed in this study has the potential to be exploited as a new inspection technique for detecting irradiation-induced microstructural changes, especially for void swelling.