Multiphysics simulations of field-assisted sintering and mesoscale kinetic monte carlo simulations of gradient microstructures

Abstract

In this thesis, Multiphysics finite element analysis was utilized to simulate the thermal-electrical coupled phenomena in the field-assisted sintering heating element when it is used as a compressive creep testing apparatus. The simulations detected a thermal deviation within the sample during the test and quantified it. After that, the heating geometry was optimized to minimize the sample temperature deviation. As this temperature deviation significantly influences the creep test results, these results provide practical geometrical recommendations for the use of SPS creep testing to achieve minimal sample temperature deviation. Using the results of the parametric analysis and control, a sintering heating element design is proposed that provides ultra large temperature gradients within the sintered sample. The thermal gradient and quantified and the heating geometry was then used to fabricate functionally graded stainless steel 304L and 8 mol.% Y$_2$O$_3$-stabilized ZrO$_2$ (8YSZ). The temperature gradients within the samples were 80 and 122 °C/mm for SUS 304L and 8YSZ, respectively. The results show a graded structure in the samples in terms of porosity, grain size, and hardness. Thereafter, microstructural evolution modelling using mesoscale simulations was used to tackle the evolution in the sintering microstructure of functionally graded stainless steel as well as the high burnup structure formation in UO$_2$ nuclear fuel. Mesoscale simulations of functionally graded stainless steel sintering accurately evaluate the microstructural evolution of sintering, quantify the sintering behavior, and predict the effects of several parameters on the functionally graded density profiles. On the other hand, mesoscale simulations of the high burnup structure formation simulated the formation of the high burnup structure in terms of fission gas bubble evolution and grain recrystallization. These results provide an insight on how the high burnup structure is formed in UO$_2$ fuel and how it interacts with the presence of the radial burnup and thermal gradient profiles. Furthermore, as fuel performance codes and finite element analysis based simulations do not consider the microstructural evolution of the high burnup structure in nuclear fuels, such results can provide significant inputs to these codes to allow for the accurate evaluation of the fuel performance as the microstructure in the mesoscale level develops during the reactor operation that enhances the reactor operation, safety, and design.