Numerical M&S (modeling and simulation) is an important tool for the design and safety analysis of nuclear reactors. The models and solvers used for this purpose are continuously being developed, tested and improved. This project at KTH Nuclear Power Safety studies the thermal-hydraulic phenomena in new generation liquid metal cooled reactor systems. We actively develop and validate in-house and commercial CFD (Computational Fluid Dynamics) models using high-quality experimental data from our separate effect and integral test facilities. CFD is also used in the exhaustive pre-test analysis to design the experiment. Success of such activities require large computational power due to (i) large number of simulations (in e.g. design exploration, sensitivity and uncertainty analysis), (ii) multi-scale 1D/2D/3D flow fields, (iii) long transients, (iv) complex phenomena/models (e.g. transient, turbulent, conjugate heat transfer, multi-phase). Naturally, this leads often to large meshes and small time-steps. Therefore the HPC (high performance computing) is invaluable for us to be at the leading edge in this scientific field.