The THYC computer code. A finite volume approach for 3 dimensional two-phase flows in tube bundles. PWR or LMFBR cores or fuel assemblies, PWR steam generators, tubular heat exchangers, are as many components of a nuclear power plant involving single or two-phase flows in tube bundles. The knowledge of the detailed flow patterns (velocity, pressure, temperature, void fraction) on the shell side is necessary to evaluate DNB in reactor cores during incidental transients, singularity effects (grids, wire spacers, support plates, baffles), corrosion on steam generator tube sheet, bypass effects and vibration risks. For that purpose, the Research and Studies Branch of EDF is now developing a new 3D computer code THYC, at first for PWR’s and LFMBR’s core thermalydraulic calculations and in a second place, for more general applications of flows in tube bundles, thanks to a general porous medium approach. This paper describes the physical model used in THYC (4 partial differential equations, closure relationships specific of tube bundle geometries) then numerical solution method (space and time discretization, solution algorithm). Examples of numerical tests and incidental operation transients in a french PWR 900 MW core, are given. Finally, 3 validation calculations will be presented concerning a tube and shell heat exchanger, a LMFBR fuel assembly in natural, mixed and forced convection, and a lower part of a PWR steam generator