A new open-source multi-physics solver for nuclear reactor analysis, named GeN-Foam (Generalized Nuclear Foam), has been developed by the Laboratory for Reactor Physics and System Behavior at the PSI and EPFL. The solver features a general applicability to pin- or plate-fuel, or homogeneous nuclear reactors, with a high flexibility in terms of geometrical detail and physical complexity of the problem to investigate. A "bottom-up" approach has been adopted where the most advanced numerical methodologies are used ab initio as basis for the creation of a new code. This is opposed to past activities that adopted a "top-down" approach aimed at an improved use and interaction of available legacy codes.