Background: One advantage of liquid fluoride thorium reactor is that it can separate almost all of fission products including its own transuranic products due to the on-line removing. The removing process can be defined as bubble generation-mass transfer-gas separation. Purpose: A gas-liquid separator is a core component in the removing system. In this paper

a kind of gas-liquid separator adopted to remove the fission gases for Thorium Molten Salt Reactor (TMSR) is studied. The air core formation

which is heavily depending on the back pressure

plays a significant role for separation efficiency. Methods: To illustrate the effect of back pressure on the evolution process of the air core

the air core behavior variation with the back pressure under different swirling numbers (S) and Reynolds numbers (Re) were recorded by visualization technique. Results: The flow pattern indicates that the air core evolution can be defined as four stages with respective characteristics. However

the air core can shape under different swirling numbers and Reynolds numbers differ a lot. Conclusion: It is proven that there is a critical back pressure which contributes an economic and high efficient separation. Thus

the critical back pressures were summarized for all the swirling numbers and Reynolds numbers involved

which can provide a guideline for the application of the separator. Abstract