Recent incidents of pipe cracking in nuclear power plants in France, Belgium, and Slovenia have highlighted the deficiencies in the monitoring and predictive capabilities of preventive maintenance programs within the nuclear industry. These shortcomings impede the timely detection of key aging mechanisms in structural components, such as stress corrosion cracking, thermal fatigue, and their combination. In these processes, material damage often initiates at grain boundaries, leading to the formation of intergranular microcracks. Over time, these microcracks can expand along the grain boundaries and coalesce into larger macroscopic cracks, potentially compromising the structural integrity of the loaded component. Therefore, developing accurate models to predict a component’s susceptibility to early-stage microcracking as well as damage progression in final-stage macrocracking is of utmost importance to minimize the costs associated with unplanned plant outages and component replacements.
The primary goal of PhD work is to develop an advanced computational tool designed to estimate a lifespan of a primary-water components (e.g., a pipe) in nuclear power plants. The innovative aspect of the approach resides in dynamically decoupling of the intrinsic multi-scale nature of the damage progression into two interconnected modeling stages. These stages, running in parallel, each address specific length and time scales.
The tool will enable fast and precise calculations of macroscopic crack growth in a large-scale component model by capturing the relevant microscale and mesoscale phenomena occurring in front of the crack tip. A newly developed local damage aggregate model will be key to this process. This model will be dynamically updated ahead of the crack tip to simulate relatively slow, diffusion-driven growth of metallic oxides on grain boundaries, which will be additionally populated with preexisting brittle complexes, such as chromium carbides or ferrites, depending on the component’s heat treatment during manufacturing. This approach will provide detailed distributions of grain boundary damage ahead of the crack tip, considering factors like time, crack-tip distance, local plasticity, and material microstructure. A significant portion of fully damaged grain boundaries within the aggregate model will signal a macroscopic crack growth in the parent macroscale component model.
The inherent damage heterogeneity of the proposed aggregate model, resulting from the variability of microstructure as well as uneven oxidation and defect precipitation kinetics, will enable probabilistic life-time predictions of the component model. These predictions will reflect the experimental observations and in-service behavior of larger components with a heterogeneous microstructure, a direct consequence of the fabrication process.
The Reactor Engineering Division has a long-standing research tradition in computational structural mechanics, structural integrity, and ageing of nuclear materials. Our research team specializes in developing multiscale computational simulation tools for polycrystalline metallic alloys. We focus on explicit microstructural modeling and simulation of ageing effects to enhance understanding of local damage mechanisms, such as microcrack initiation and propagation, which are exacerbated by corrosion and neutron irradiation. We employ (strain-gradient) crystal plasticity and phase field theory within the finite-element and FFT-based homogenization techniques. The following three important results have been achieved in the last five years (2020 – 2025):
1. A micromechanical analysis of intergranular stress corrosion cracking of an irradiated austenitic stainless steel was performed in collaboration with CEA, France to assess local cracking conditions. The statistical analysis showed that cracking occurs preferentially for grain boundary normals aligned with the mechanical loading axis, but also for low values of slip transmission. Micromechanical simulations based on reconstructed 3D microstructure rationalized the correlation obtained experimentally into a single stress-based criterion. The results were published in a highly renowned journal Acta Materialia.
2. A simple analytical model of intergranular normal stresses was proposed for a general elastic polycrystalline material with arbitrary shaped and randomly oriented grains under uniform loading. The model provides algebraic expressions for the local grain boundary normal stress and the corresponding uncertainties, as a function of the grain boundary type, its inclination with respect to the direction of external loading and material elasticity parameters. Such knowledge is a necessary prerequisite in any local damage modelling approach, for example, to predict the probability for intergranular stress corrosion cracking or fatigue crack initiation in structural materials. The results were published in a renowned journal European Journal of Mechanics, A Solids.
3. My first PhD student joined our research group in 2022. His work resulted in a new model for predicting the localization of plastic deformation in polycrystalline aggregates under mechanical loading. The model is particularly suitable for materials that exhibit strain softening, such as metals exposed to neutron irradiation. These materials can develop localized deformation bands at the level of crystalline grains, leading to stress concentration at grain boundaries and eventually to crack formation. The numerical implementation of the model was carried out in an in-house code using the FFT homogenization. The method effectively mitigates numerical instabilities associated with strain localization and enables the regularization of the resulting bands. Understanding the formation of localization bands in irradiated materials is crucial for predicting the lifespan of reactor components. The results of this work were published in a highly renowned journal International Journal of Plasticity.