III—12. The generation and combustion of large volumes of hydrogen and carbon monoxide are severe accident phenomena that can threaten the integrity of the containment. The major cause of the generation of hydrogen is the oxidation of zirconium metal and, to a lesser extent, the interaction of steel or any other metallic component with steam when the metal reaches tempera­tures well above normal operating temperatures.

III—13. In addition, ex-vessel hydrogen generation needs to be considered. Such hydrogen is produced mainly as a result of the reactions of ex-vessel metallic core debris with steam, and in the long term by molten core-concrete interactions (para. III-17) and by the extended radiolysis of sump water.

III-14. Molten core-concrete interactions may also produce carbon monoxide, which is also combustible under certain conditions.

III-15. Under severe accident conditions, significant hydrogen concentrations could be reached locally in a short time (of the order of some minutes to an hour, depending on the containment design, the scenario and the location) and globally in a longer period of time.

III-16. When the ignition limit is exceeded, combustion of hydrogen is possible and can take different forms, depending on the concentrations, the atmospheric conditions in the containment and the geometry: diffusion flames (which are mainly responsible for thermal loads), slow deflagrations (which are mainly responsible for quasi-static pressure loads), fast deflagrations (for which dynamic effects become important) and detonations (for which the velocity of the flame front exceeds the speed of sound in the unburnt gas, giving rise to extremely severe dynamic effects). Depending on the mode of combustion, the integrity of the containment may be threatened by stresses beyond the structural design limits.