Computational Fluid Dynamics codes are increasingly being considered for safety assessmentdemonstrations in many industrial fields as tools to model accidental phenomena and to designmitigation (risk reducing) systems. Thus, they naturally complement experimental programmes whichmay be expensive to run or difficult to set up. However, to trust numerical simulations, the validity ofthe codes must be firmly established, and a certain number of error sources (user effect, modellingerrors, discretization errors, etc) reduced to the minimum. Code validation and establishment of ?bestpractice guidelines? in the application of simulation tools to hydrogen safety assessment are some ofthe objectives pursued by the HYSAFE Network of Excellence. This paper will contribute to thesegoals by describing some of the validation efforts that CEA is making in the areas of release,dispersion, combustion and mitigation, thereby proposing the outline of a validation matrix forhydrogen safety problems.