In the context of thermalhydraulics, Computational Fluid Dynamics (CFD) codes are widely used for design and safety analysis. CFD codes solve the Navier-Stokes equations in three dimensions. They mostly rely on the Reynolds-averaged formulation of the Navier-Stokes equations. This approach allows for a detailed representation of the flow while requiring a limited numbers of hypotheses (turbulence models, law of the wall). A fine spatial discretisation is needed in order to achieve good prediction capabilities. This implies a large number of control volumes. The computational resources necessary to carry out a calculation at the industrial scale, such as a two-phase flow transient on the entire primary side of a nuclear reactor, are often prohibitive by present-day standards.

In order to cut the computational cost, a coarser spatial discretisation can be retained. Depending on the case of interest, the best practise guidelines of the RANS approach might not all be respected. Further hypotheses need to be added in order to maintain the quality of the model’s predictions. Such models may include pressure drops, heat transfer correlations or mixing terms. This approach is often referred to as a porous media approach.
Regardless of the method, the system of interest is often restricted to an open-loop model, which requires boundary conditions for the equation system to be solved.
Multi-scale coupling methods aim at using each approach where it best suited. The rationale is to reduce the computational burden while capturing the relevant physical phenomena.
Multi-scale coupling can be either one-way or two-way. In a one-way coupling, boundary conditions obtained from a first calculation are used as boundary conditions for another calculation. There is no feedback from the second calculation on the first one. In a two-way coupling, the coupled codes exchange data in the form of boundary conditions, usually at each time step. There is feedback between the two codes. Two-way is the method that is selected in the following.
The boundary conditions used in the standard approach are developed for cases were only macroscopic data are available, flow rate and temperature at the inlet, pressure at the outlet. In the context of a multi-scale coupling, data that are more detailed can be available such as velocity and pressure fields. This thesis work aims at developing boundary conditions, which can take benefit of all the available data in order to make the coupling as seamless as possible.
As an example, in case of two code instances, each one solving a portion of a physical domain relying on the same discretisation and modelling options, the results obtained from these two instances should be identical to that of a single code instance relying on the same discretisation and modelling options solving the entire domain.

The containment building is the third safety barrier in nuclear power plants. Its role is to protect the environment in the event of a hypothetical accident by limiting releases to the environment. Its function is therefore closely linked to its tightness, which it must maintain throughout its operating life. Traditionally, the estimation of the leakage rate is based on a good knowledge of the hydric state and potential mechanical disorders, associated with transfer laws (such as permeability) in a chained (thermo-)hygro-mechanical simulation approach. While the mechanical behaviour of the structure is now generally well known, using advanced simulation tools, progress is still needed to improve the understanding and quantification of flows. This is particularly the case in the presence of heterogeneities (cracks, honeycombs, reinforcement, cables, etc.), all of which can locally disrupt permeability. This is the context of the proposed thesis topic. The aim is to improve the understanding and representation of flows through a reinforced concrete structure using an approach that combines experimental tests and modelling. An initial analysis will be used to define an optimised experimental design based on several configurations (leak paths, type of flow, temperature, saturation, etc.), which will then be implemented during the thesis. The results will be analysed in order to characterise empirically the influence of the leakage path on the macroscopic laws classically used (Darcy's law). A more refined simulation approach will then be developed, based on the finite element method. The aim will be to reproduce the experimental results and extend them to the behaviour of containment vessels, thereby improving the modelling tools currently available.

In certain accidental situations, it is important to assess the consequences of severe thermal loading on the mechanical behaviour of concrete structures, particularly with regard to potential cracking. This is particularly the case in the study of corium-concrete interaction. As part of the assessment of the consequences of a hypothetical severe accident, a core meltdown may be considered. The molten mixture, known as corium, then spreads into the reactor and comes into contact with the concrete. Various phenomena can occur, leading to partial ablation of the material. Given the stakes involved in terms of environmental protection, it is essential to have modelling tools that can represent the mechanisms involved. The aim of this thesis is to develop a comprehensive simulation methodology to represent the mechanical consequences of corium-concrete interaction, including local-scale modelling to represent the ablation of the cementitious material. Particular attention will be paid to the concrete cracking model (development of a model adapted to severe thermal loading, concrete ablation criteria) and to the thermal-mechanical-flow chaining of tools for representing the penetration of corium into cracks. This work will be carried out in collaboration between CEA SACLAY (which has the first tools for simulating thermomechanical behaviour) and CEA Cadarache (which has numerical and experimental expertise in corium-concrete interaction).