To evaluate nuclear power plant reactor building containment, on-site pressurization tests up to 4 bars are performed to measure leakage rates, influencing French Nuclear Safety Authority's decision to authorize plant operation. Given the concrete's aging nature in the context of lifespan expansion, there is a need for a tool to estimate leakage rates over time, considering potential accidental loadings like coolant loss or seismic events. Experimental studies like MAEVA [1] or VERCORS [2] have assessed the leakage ratio at the structural scale through two main mechanisms: concrete porosity and cracks. These results are crucial for developing simulation tools within quasi-brittle material mechanics framework (concrete) and porous/fractured medium transport (diffusion and flow). Numerous investigations have demonstrated that various aspects of crack geometry (e.g., opening, roughness, tortuosity) influence leakage rates within the specimen scale [3]. However, precisely characterizing the three-dimensional geometry of concrete cracks is a significant challenge. A new method has been created that merges Finite Element Analysis, beam-particle modeling, and Computational Fluid Dynamics (CFD) to accurately predict concrete crack geometries and air leakage rates. A first finite element model calculation is performed in the model's first phase, implying macroscopic continuous finite element analysis, not discussed in this article, to handle the high processing needs of discrete simulations. Then, as described in this work, a refined discrete beam-particle model computation is performed and utilized as input for a CFD model, as presented in this contribution. By accurately predicting leakage ratios for diverse crack patterns and aligning with experimental data, the model significantly improves our comprehension of air leakage in nuclear reactor containment vessels, strengthening safety evaluations and regulatory processes. REFERENCES [1] Granger, L. et al.: Containment Evaluation under Severe Accidents (CESA): synthesis of the predictive calculations and analysis of the first experimental results obtained on the Civaux mock-up. Nucl. Eng. Des., 209 (1), 2001, p. 155–163. [2] Charpin, L. et al.: Ageing and air leakage assessment of a nuclear reactor containment mock-up: VERCORS 2nd benchmark. Nucl. Eng. Des., 377, 2021, p. 111136. [3] Akhavan, A. et al.: Quantifying the effects of crack width, tortuosity, and roughness on water permeability of cracked mortars.