Threat of Combustible Mixture Formation in Small Rooms of an NPP’s Unit Containment during a Severe Accident

— Protection of the containment integrity is one of the key objectives pursued in managing a severe accident at an NPP. Possible hydrogen inflammation in the containment can be regarded as the most dangerous threat for the containment integrity. Inert state of a steam–air–hydrogen mixture can be secured by maintaining a high steam concentration. With a steam concentration above 55%, the mixture is in an non-flammable state whatever the concentrations of its other components. The containment reinforced concrete walls lined with metal sheets behave as heat sinks with a very high thermal inertia. Heat removal through walls may have a significant influence on the mixture composition in the containment or in its individual part (especially if the rooms have comparatively small volumes) if part of steam from the mixture undergoes long-term condensation, as a result of which its concentration in the mixture decreases. The article presents an investigation, by means of detailed modeling, of the heat transfer processes through the walls of containment rooms with a view to reveal factors that are of importance in considering hydrogen challenge. It is found that the wall surface area to the room volume ratio is a factor having an effect on the formation of dangerous hydrogen-containing mixture compositions. It is shown that the wall heat absorption capacity decreases only slightly in the course of an accident. In rooms with a small volume, there may be a real risk that the medium in them may quite quickly lose its inertness as a result of heat removal through the walls. The possibility of a detonation mixture to form is not even excluded. In the case of a severe accident, there will be a risk of dangerous hydrogen-containing mixture compositions formation in small rooms within the containment irrespectively of the strategies implemented for the containment’s main rooms unless special measures and means are taken, e.g., application of accident management strategies or installation of hydrogen recombiners.