In nuclear power plants, the concrete biological shield (CBS) is designed to be very close of the reactor vessel. It is expected to absorb radiation and acts as a load-bearing structure. It is thus exposed during the lifetime of the plant to high level of radiations that can have consequences on the long term. These radiations may result especially in a decrease of the material and structural mechanical properties. Given its key role, it is thus necessary to develop tools and models, to predict the behaviors of such structures at the macroscopic scale.
Based on the results obtained at a lower scale - mesoscopic simulations, from which a better understanding of the irradiation effect can be achieved and experimental results which are expected to feed the simulation (material properties especially), it is thus proposed to develop a macroscopic methodology to be applied to the concrete biological shield. This approach will include different phenomena, among which radiation-induced volumetric expansion, induced creep, thermal defromations and Mechanical loading.
These physical phenomena will be developed within the frame of continuum damage mechanics to evaluate the mechanical degradation at the macroscopic scale in terms of displacements and damage especially. The main challenges of the numerical developments will be the proposition of adapted evolution laws, and particularly the coupling between microstructural damage and damage at the structural level due to the stresses applied on the structure.

Reinforcing steel plays a major role in the behavior of reinforced concrete structures. Nevertheless, significant conservatisms may sometimes be imposed by design codes, raising questions about the feasibility of construction or the viability of the structure (economic, environmental, etc.). It is within this context that the doctoral research takes place. Building on recent developments, the work aims to propose an innovative design approach relying on the use of nonlinear finite element calculations, combined with topological optimization algorithms (defining reinforcement directions and bar cross-sections) and evolutionary optimization algorithms (determining the placement of bars with fixed cross-sections).
The method should, through an iterative process, yield solutions that meet an optimal design configuration. Considering the multiple, potentially conflicting objectives to minimize (such as cost, feasibility, strength, and carbon footprint), the approach will guide the configuration of input parameters based on an analysis of the relevant output results.
Applying the method to complex, practice-based case studies (for example, beam-column junctions) will demonstrate its relevance compared with more conventional design methods. By the end of the thesis, the doctoral candidate will have developed advanced skills in the use and development of state-of-the-art tools, ranging from nonlinear finite element simulation to modern optimization techniques based on artificial intelligence.

This PhD project is part of research on fluid–structure interactions (FSI) in complex media, particularly fluid mixtures involving multiple phases (liquid/liquid or liquid/gas) and/or suspended particles. The objective is to develop a thorough, multi-scale understanding of the coupled mechanisms between deformable structures (such as droplets, interfaces, or flexible walls) and the flows of complex mixtures, by combining theoretical modelling, advanced numerical simulations, and comparison with experimental data.

The numerical simulation of the mechanical behavior of structures subjected to dynamic loads is a major challenge in the design and safety assessment of industrial systems. In the nuclear industry, this issue is particularly critical for the analysis of severe accident scenarios in Pressurized Water Reactors (PWRs) such as the Loss of Coolant Accident (LOCA), during which the rapid depressurization of the primary circuit can lead to pipe rupture. Developing physically representative models and robust, efficient numerical methods to simulate such phenomena with high fidelity remains an active area of research.

Among the existing non-local approaches, phase-field methods have emerged as a interesting framework for simulating crack initiation and propagation. However, most current studies are limited to quasi-static or low-rate dynamic problems, where wave propagation effects can be neglected. In contrast, high-rate dynamic regimes - relevant to accidental loads - require explicit time integration schemes for the mechanical equations, which are sensitive to the stability condition. The classical elliptic formulation of the damage evolution equation is therefore not ideally suited to this context. To address these limitations, recent works have proposed and assessed hyperbolic phase-field formulations, which are naturally more compatible with explicit dynamics and allow better control of crack propagation kinetics.

The objective of this PhD thesis is to advance this emerging modeling strategy through three main research directions:
- Extend the theoretical framework of the hyperbolic phase-field formulation for damage within the context of generalized standard materials, which is suitable for ductile fracture;
- Propose solutions to the negative impact of damage evolution on the critical time step;
- Rely on an dynamic fracture experimental test campaign to calibrate simulations, with a focus on the identification of damage-related parameters

This research is to be conducted in collaboration between CEA Paris-Saclay, ONERA Lille, and Sorbonne Université, with CEA as the main host institution.