Procedure and tools for deterministic analysis and control of hydrogen behavior in severe accidents

A systematic step-by-step procedure for the determmistic analysis of hydrogen behaviour and mitigation in severe accidents is presented. The procedure can be subdivided into four main parts: (1) modeling of the H2-air-steam mixture generation in the containment, (2) prediction of the risk potential of this mixture with respect to flammability, flame acceleration and detonation on-set, using newly developed criteria, (3) numerical simulation of the appropriate combustion regime with verified 3D-computational fluid dynamics (CFD) tools, and (4) consequence analysis based on the calculated pressure and temperature loads. The methodology was used to examine the relative merits of different mitigation schemes in studies for a future plant design. Two options for hydrogen control were investigated (a) recombiners only, (b) recombiners with few igniters. Option (a) still allows accumulation of considerable hydrogen masses in the containment, which could lead to significant dynamic pressure loads to the outer containment shell. Option (b) lead in the investigated plant and accident scenario to early ignition of standing flames and negligeable pressure development.