Experimental investigation of density stratification behavior during outer surface cooling of a containment vessel with the CIGMA facility

•The density stratification behavior was investigated during the outer surface cooling of the containment vessel with the CIGMA facility.•The influence on the density stratification behavior is studied when the initial helium (simulant of hydrogen) concentration in the vessel is changed.•Helium accumulation occurred in the upper region of the vessel by the top wall cooling.•For the higher initial helium concentration condition, the remarkably high concentration helium layer was formed near the top of the vessel.•However, the density stratification was finally collapsed due to gas mixing induced by natural convection. Cooling of an outer wall of a containment vessel (CV) is expected as an effective accident management method to cool down fluid and structure in a CV during a severe accident. On the other hand, there are the following possibilities. Firstly, hydrogen gas may be generated by a water-zirconium reaction during a severe accident, and the non-condensable gas including hydrogen accumulates due to the outer surface cooling and the density stratification is formed. Secondly, the accumulation of the non-condensable gas degrades heat transfer and may inhibit the cooling of the CV. Many kinds of research regarding these subjects have been conducted. However, the systematic investigation is not conducted adequately focusing on the density stratification behavior that occurred by the outer surface cooling, as well as the condition for stratification collapse. Besides, the heat transfer degradation caused by hydrogen accumulation has not been evaluated quantitatively. Thus, we have constructed an experimental facility CIGMA (Containment InteGral effects Measurement Apparatus), and we conducted the experimental research of the containment thermal hydraulic behavior with the CIGMA facility when the outer surface of the vessel is cooled. In this paper, we investigated the density stratification behavior when the top vessel was cooled by the stagnant water and the density stratification was initially built-up with helium (simulant of hydrogen), air and steam gases. Two different helium concentrations were applied as the initial conditions of the density stratification in order to discuss the influence of the helium concentration on the density stratification behavior. The helium gas accumulation in the upper region occurred by the outer surface cooling, and the concentration of the helium increased in the upper region. In particular, the remarkably high concen