Non-condensable gas accumulation and distribution due to condensation in the CIGMA facility: Implications for Fukushima Daiichi Unit 3 (1F3)

Abstract

This study was motivated by previous analysis by TEPSYS, which indicated that different temperatures in the 5th and 4th floors of the reactor building (R/B) of Fukushima Daiichi Unit 3 (1F3) could significantly influence the distribution of non-condensable gases during the severe accident in 2011. Understanding this effect is crucial for assessing the risks associated with non-condensable gas accumulation in R/B, particularly considering that the hydrogen explosion in 1F3 may not have been a single-stage event but could have involved a multi-stage explosion. To investigate the distribution and transport on non-condensable gas under varying cooling conditions, an experimental study was conducted using the CIGMA facility, a large-scale test vessel with segmented sections created by partition plates, designed to replicate the structure and conditions of the R/B. Steam and helium, used as non-condensable hydrogen substituted, were continuously injected at the top of the CIGMA vessel for 10,000 s to replicate the leakage of steam and hydrogen through the shield plug. Two cooling conditions were tested: Case 1 with a cooling temperature of 50 °C, and Case 2 with a cooling temperature of 90 °C. The experiments aimed to explore how varying cooling temperatures impact helium accumulation in different regions of the R/B structure. The study found that in both cases, the highest concentration of non-condensable gases may not always be near the injection point but rather in downstream regions. In these regions, steam and non-condensable gases move downward and continue to condense, significantly enhancing the concentration of helium. Specifically, in Case 1, after 10,000 s, the helium concentration reached 65 % in the middle region (representing the 4th floor of R/B) and 45 % in the top region (representing the 5th floor of R/B). Analysis using the Shapiro ternary diagram showed that, under these conditions, the gas mixture in the middle region posed a potential detonation risk. This study provides important insights into gas distribution within a nuclear reactor building during severe accident conditions, which are crucial for developing more effective safety measures and risk mitigation strategies in nuclear reactor designs.