With the BACCHUS-3D/SP computer program, the steady-state and transient thermal-hydraulic behavior of single-phase coolant flow in a reactor bundle geometry and the thermodynamics of the pins can be described in a three-dimensional geometrical representation that relies on the porous-medium concept. The geometrical representation of the bundle, the mathematical modeling of the physical coolant behavior, and the numerical treatment of the governing equations with the implicit continuous-fluid Eulerian technique and details of their numerical solution are described. Experiments in heated and unheated 19-pin bundles with sodium and water as a coolant are used to check the physical models for the turbulent exchange of momentum and enthalpy between adjacent control volumes in the bundle. Further code validation has been made with the computation of experiments performed in 7- and 37-pin bundles and in a 60-deg sector of the SNR Mk la 169-pin bundle. The comparison between computed and experimental data offers insight into the interpretation of these experiments and allows an assessment of the advantages and shortcomings of the porous medium approach.