RELAP5-3D is the latest in the RELAP5 code series developed at the Idaho National Laboratory (INL) for the analysis of transients and accidents in water–cooled nuclear power plants and related systems as well as the analysis of advanced reactor designs. RELAP5-3D isometric image of the LOFT input model generated with RGUI. The RELAP5–3D code is an outgrowth of the one-dimensional RELAP5/MOD3 code developed at the INL. The most prominent attribute that distinguishes RELAP5–3D from its predecessors is the fully integrated, multi-dimensional thermal-hydraulic and kinetic modeling capability. RELAP5–3D is available from the INL through membership in the International RELAP5 Users Group (IRUG).