Modélisation multi-échelle de la fissuration d'un élément en béton armé pour l'évaluation des débits de fuite en situation d'accident grave

For reactors in the 1300 and 1450 MWe series, the reactor building consists of a double concrete-walled structure. For these reactors, severe accident scenarios (SA) consider an internal pressure of 5 bars (absolute) and a temperature of 150°C, the internal atmosphere then being composed of an air-steam mixture and aerosols (fine particles in suspension). Such conditions induce cracking in the first wall of the containment through which leaks are possible. It is, therefore, necessary to have robust and reliable tools to estimate these leaks.

The objective of the thesis is first to obtain a realistic three-dimensional cracking pattern using a continuous/discrete two-step simulation approach of a reinforced concrete mock-up. Secondly, the results provided by the model are used as input data for fluid mechanics simulations to assess leakage rates in severe containment accident conditions. This is done in parallel with the calibration of the model with the experimental results obtained in the framework of the COBRA program on the MACUMBA facility operated by IRSN. This program consists of cracking two mock-ups of Representative Structural Volumes (RSV) of a containment wall, then applying different pressure, temperature, and hygrometry gradients to either side of them and evaluating the leakage ratios and aerosol retention.

The modeling stages are (1) computation of a continuous damage field with the finite element method (FEM); (2) re-analysis of the highly damaged area with a discrete element method (DEM) to compute physical crack features (Oliver-Leblond et al., 2013); and (3) simulation of the leakage flow rate through the previously computed cracks by a CFD software.

Matalah estimated the average crack opening per element via a post-processing based on fracture energy regularization (Matallah et al., 2010). However, this method does not allow for a complete description of the crack geometry (path, opening, roughness length, etc.). On the other hand, the discrete element method has shown its ability to represent concrete cracking characteristics explicitly (Oliver- Leblond, 2019) and will be used in this thesis.

At the end of the thesis, the development of numerical tools for the estimation of the 3D patterns of complex crack systems in a containment wall's RSV (see Figure 1 for current results using the MATLAB post- processing tool). These cracking patterns, which incorporate various parameters directly from discrete element calculations, will serve as input data for fluid mechanics flow calculations