Experimental analysis of non-condensable gas and steam distribution due to condensation in the CIGMA facility simulating the reactor building

This study experimentally investigates non-condensable gas transport induced by steam condensation using the CIGMA facility, simulating reactor building conditions of Fukushima Daiichi Unit 3 during a severe accident. Steam and helium, used as a hydrogen surrogate, were continuously injected into the CIGMA’s vessel equipped with partition plates representing the hierarchical structure of the reactor building. Parametric experiments were conducted by varying flow path ratio, steam-to-helium mass ratio, and cooling conditions. The results show that steam condensation is the dominant mechanism controlling non-condensable gas accumulation by increasing the relative concentration of helium. The highest helium concentrations generally occur below the injection point rather than at the injection elevation, indicating downward transport followed by condensation-driven accumulation. Shapiro ternary diagram analysis indicates that condensation-driven changes in gas composition lead to prolonged residence within flammability and detonation regions. These findings highlight the critical role of condensation in hydrogen distribution and provide experimental insight relevant to hydrogen risk assessment and mitigation in reactor buildings during severe accidents.