The severe reactor accident at Fukushima Daiichi has pronounced the need to understand the flow and transport processes inside the containment and connected buildings in more detail to design and evaluate efficient safety measures. Since several years CFD models, mostly based on commercial CFD solvers, are developed to provide highly resolved insight and thus support in assessing the effectiveness of safety measures and possible combustion loads that may challenge containment integrity. In the recent years, the open source CFD package OpenFOAM® is attracting academic and industrial users due to the increased choice of available models, numerical methods and in particular due to access to its complete source code. Nevertheless, in the nuclear safety field, development and validation work was limited to specific issues or distinct physical phenomena but not oriented towards full-scale containment analysis. This paper summarizes the design and implementation of a new tailored solver ‘containmentFOAM’, which is developed for full-scale simulation of accidental containment flows, mixing processes, pressurization and assessment of passive safety systems. The validation strategy is discussed and first application oriented validation results are presented with regard to gas mixing in buoyant turbulent flows, wall bounded flows including condensation, aerosol transport, deposition and gas radiation heat transfer. © 2019 American Nuclear Society. All rights reserved.