Influence of coupled factors on premixing and fragmentation of mild thermal interaction

Highlights: • The physics of mild thermal interaction during nuclear meltdown is clarified. • Fraction of agglomerated debris and fragments size distribution are in line with those found in DEFOR-A. • A bubble dynamics phenomenon, named of premix expansion is featured based on Rayleigh's equation. • Restricting premixing zone can reduce chances of steam explosion. After core melt jets drop into the water pool, they break up into several parts and premix with water, and subsequently fine-fragmentation may take place, leading to steam explosions. Besides, mild thermal interaction can also occur in fuel–coolant interactions (FCIs) during core melt accidents in light water reactors. However, this phenomenon is not fully understood owing to the various coupled mechanisms involved. Therefore, we conduct an experimental study to understand the influence of the underlying factors and thermal interaction mechanisms on the premixing and fragmentation processes of FCIs within a confined space. Based on the morphology method, the coupled effects of melt mass, water level, and melt superheating temperature are investigated, quantified in terms of jet breakup length, solidification time, and fragments size distribution. Based on the corresponding pressure and temperature histories, two types of influencing factors are studied in detail: the effect of melt properties coupled with the water level; and the effect of melt superheating temperature coupled with the water level. Finally, a new bubble dynamic phenomenon called the premix expansion is featured based on Rayleigh's equation. These experimental data on mild thermal interaction are expected to be of significance in the development of models and validation of codes.