During severe accident sequences hydrogen is produced, and together with steam it is released into the containment. For some accident sequences in BWRs this flow is injected into a water pool; in this case most fission products are contained in the water pool. If the steam/hydrogen flow is released into a dry containment the hydrogen is distributed in the containment and its distribution is influenced by hot and cold structures, condensation, sedimentation and the heat input by fission product decay. The knowledge about the hydrogen distribution is important for the assessment of whether hydrogen accumulates somewhere reaching concentrations with the potential for detonation processes.

Hydrogen detonation can be avoided either by pre-inertisation (as for all BWR containments), by post-inertisation or dilution or by reducing the amount of hydrogen using igniters or catalytic recombiners. It is evident that the mixing of different gas flows as well as the heat addition by recombiners and igniters create complex three-dimensional velocity and concentration fields in the containment volume; the effectiveness of recombiners and igniters are influenced by these fields.