Cryogenics is used to cool superconducting Radio Frequency cavities in particle accelerators such as the Large Hadron Collider (LHC) at CERN, and in the design of its possible successor, the Future Circular Collider (FCC). Liquid Helium is used as cooling fluid, which poses additional hazards for workers inside such environments. Oxygen deficiency hazard (ODH), as well as exposure to extremely low temperatures, are most critical due to an accidental release of the cold gas from the cryomodules into the surrounding space [1]. Reliable numerical tools are required to predict the behaviour of the release, mixing, stratification and dispersion under realistic ventilation conditions [2]. This work presents a numerical framework for simulating helium release scenarios in the FCC tunnel, with emphasis on the governing physical models, numerical methods, and boundary-condition strategies relevant for safety analyses. The simulations are performed using the open-source CFD framework OpenFOAM, using a model which resolves transient compressible flow, buoyancy and heat transfer, as well as transport and mixing of chemical species. Helium–air mixtures are treated using ideal-gas thermodynamics, in combination with the JANAF and Sutherland models for temperature-dependent coefficients. Turbulence effects are modeled using the RANS k–ω SST model, with wall functions to allow coarse meshing in the large domain. The numerical setup is validated using a reference helium release experiment conducted in the LHC tunnel [1], with a controlled helium release rate and realistic ventilation conditions. Results are quantified in terms of temperature and oxygen concentration, as well as helium cloud behaviour over time. These quantities define key parameters for the safety and safe evacuation of the personnel. From the benchmark case results, usage of coarser or even 2D meshes can be explored to find adequate conservative safety parameters. Based on these findings, safety guidelines and design implications are formulated for the FCC accelerator tunnel design.