NUMERICAL SIMULATIONS OF HYDRIDE EMBRITTLEMENT IN ZIRCONIUM ALLOY

Abstract

The material degrading phenomenon of hydride embrittlement has been a critical concern in the field of nuclear engineering, particularly in zirconium alloys commonly used in nuclear reactor components. A lot of incidents happened in the past where the failure of these nuclear reactor components happened due to the hydride embrittlement of the material. The nuclear components, such as fuel cladding and pressure tubes, are made of zirconium alloys, and consist of fuel bundles and coolant water. The hydride embrittlement of the material in these components can cause the leakage of fission products and shutdown of the nuclear reactor. Delayed hydride cracking (DHC) is a localized form of hydride embrittlement. The hydride embrittlement of the material initiates delayed hydride cracking in the nuclear components. The DHC process combines various subprocesses, including hydrogen diffusion, mechanical deformation, hydride precipitation, hydride growth, and fracture. The DHC process is responsible for several failures of the nuclear components that occurred in the past. Thus, it is essential to investigate the hydride embrittlement and DHC, focusing on the zirconium alloy to understand this complex phenomenon for improving the safety of the nuclear reactor components. This thesis work is focused on developing the numerical model for hydride embrittlement and the DHC process to determine the important parameters related to the hydride embrittlement process. In the hydride embrittlement process, the hydrogen present in the material migrates towards the crack tip in the presence of external stresses. Thus, the hydrogen concentration at the crack tip increases and causes the hydride formation at this location. The stress concentration gradient at the crack tip is one of the main driving force of the hydrogen diffusion process in the presence of a crack in the specimen. The extended finite element method (XFEM) can accurately capture the singularity at the crack tip. Thus, an XFEM-based framework is developed for the simulation of hydride embrittlement in the zirconium alloy using MATLAB. To simulate the hydride embrittlement process, the coupling of processes such as, steady-state hydrogen diffusion, elastic-plastic mechanical deformation and hydride precipitation is done. The developed numerical model is then used to study the effect of external stress on the hydride precipitation near the crack tip. The effect of the hydride precipitation on the hydrostatic stress field is also studied. In the hydrogen diffusion process, the hydrogen flux required for the migration of hydrogen depends on both the stress concentration gradient and the hydrogen concentration gradient. The hydrogen concentration gradient is generated in the material with time. Thus, the developed model for hydride embrittlement is further modified for the coupling of transient hydrogen diffusion, mechanical deformation, and hydride precipitation. The developed model is then used to study the effect of the residual stresses present in the zirconium alloy pressure tubes on the hydride embrittlement process. The developed methodology is validated with the analytical and numerical results of hydrogen diffusion available in the literature. Finally, a microstructurally sensitive computational model is developed to simulate DHC in hydride-forming materials that can capture the key features of the DHC process using COMSOL. The presented model is validated by comparing the threshold stress intensity factor (KIH) predictions with the results available in the literature. The hydride zone is modeled as a mixture of zirconium alloy and zirconium hydride, and its fracture is represented by a damage model. It also considers the anisotropy pre-existing in the Zr-2.5Nb pressure tube material at the mesoscale with the help of grain microstructure containing grains and grain boundaries. The effect of microstructural factors like grain size and grain boundary strength is also studied on the DHC in Zr-2.5Nb alloy. All the simulations performed using self-developed MATLAB codes and COMSOL are validated with the numerical solutions, theoretical and experimental results.