The accurate and efficient simulation of turbulent thermohydraulic flows in complex geometries is a key requirement for the design and topological optimization of nuclear safety devices. Conventional body-fitted meshing approaches often lead to high computational costs and limited flexibility when moving boundaries or iterative mesh change are involved. To address these challenges, we propose an Immersed Boundary Method (IBM) based on the Penalized Direct Forcing approach[1], specifically tailored for turbulent nuclear safety applications. A major contribution of this work is the development of immersed wall functions for both velocity and temperature fields, extending the conventional wall-law framework to an immersed setting. These wall functions are further coupled with additional closures for turbulence quantities (k and ω), enabling the consistent integration of the RANS k-ω SST turbulence model within the IBM framework. This innovation allows the method to capture near-wall behavior without resolving the boundary layer, thereby reducing grid requirements while maintaining accuracy in heat transfer and turbulence predictions. The methodology has been implemented in the open-source platform TRUST/TrioCFD[2] of the CEA. Several validation cases are presented, ranging from canonical turbulent channel and pipe flows to more representative thermohydraulic configurations relevant to nuclear safety. Comparisons against body-fitted simulations with use of wall law demonstrate the robustness of the approach and highlight the computational savings achieved by combining immersed boundary techniques with wall-function modeling. Beyond static configurations, the method is also suited to problems involving moving or deformable boundaries, opening perspectives for the optimization of innovative passive safety devices, and for fluid-structure interaction.