Experimental study on gas mixing and transport induced by external cooling on large-scale facility

Highlights: • The distribution of helium under different injection characteristics is analyzed. • The pressure and temperature transients under external cooling in containment is evaluated. • The natural circulation caused by condensation and the resultant influence on hydrogen behavior is discussed. Migration and distribution of gas mixtures of hydrogen generated in severe accidents are related to thermal hydraulic behavior of the containment, which is a key issue for safety analysis. A series of experiments are conducted on a large-scale test facility, to study the effects of external cooling on stratification and mixing behavior of gas mixtures. Different concentration distributions are formed by changing injection conditions, including injection momentum, initial pressure and composition of gas mixtures. Results show that the increase of injection momentum or initial pressure will forming a more uniform distribution of helium, while the increase of hydrogen fraction of injection will promote stratification. During external cooling period, hydrogen concentration increases caused by condensation, and hydrogen stratification is weakened by natural circulation. The mixing behavior caused by natural circulation will be influenced by initial concentration distribution. The experimental data can be helpful for codes assessment.