Hydrogen is not more dangerous than current fossil energy carriers, but it behaves differently. Thereforehydrogen specific analyses and countermeasures will be needed to support the development of safe hydrogentechnologies. A systematic step-by-step procedure for the mechanistic analysis of hydrogen behaviour andmitigation in accidents is presented. The procedure can be subdivided into four main parts: 1.) 3D modellingof the H2-air mixture generation, 2.) hazard evaluation for this mixture based on specifically developedcriteria for flammability, flame acceleration and detonation on-set, 3.) numerical simulation of theappropriate combustion regime using verified 3D-CFD codes, and 4.) consequence analysis based on thecalculated pressure and temperature loads.