CIGMA experiments on integral phenomena related to thermal hydraulics in a reactor containment vessel and building during a severe accident

Abe, Satoshi  ; Hamdani, A.  ; Soma, Shu ; Hangai, Ryosuke; Omori, Masashi; Owada, Akihiko; Omiya, Toshihito*; Shibamoto, Yasuteru 

The Fukushima Daiichi accident underscored the urgent need to understand complex thermal-hydraulic phenomena governing containment integrity and gas mixture distribution during a severe accident. In response, the Japan Atomic Energy Agency (JAEA) established the CIGMA (Containment InteGral Measurement Apparatus) facility, a flagship large-scale installation capable of high-temperature, high-pressure experiments with a steam-air-helium gas mixture. This paper presents key findings from a comprehensive experimental campaign with CIGMA. The JT-SJ series demonstrated the effectiveness of external surface cooling in suppressing top head flange overheating. The CC-SP series revealed spray-induced mixing mechanisms that rapidly homogenize flammable stratifications. The CC-PL series identified condensation processes of the gas mixture that are decisive for containment cooling strategies. Finally, the CC-SJ series provided insights into inter-compartment gas transport relevant to the multi-stage explosions in Unit 3 of Fukushima Daiichi Nuclear Power Station. These results establish a high-fidelity experimental database, offering benchmarks for CFD validation and advancing the development of robust hydrogen mitigation and accident management strategies worldwide.