In the application of the RETRAN code to the analysis of very small LOCA one of main concerns is placed on use of the bubble rise model in the upper plenum, because the bubble rise model may cause a numerical divergence problem and coefficients used to describe it are based on experimental results of large LOCA. In order to solve this problem, a method, which enables us to predict the mixture level in the upper plenum without use of the bubble rise model, was proposed. For this method the local void distribution in the core and upper plenum was derived by using a simplified slip model. It was shown that results predicted from the derived equation are in excellent agreement with experimental data. Additionally it was found that local void in the upper plenum has a uniform distribution unlike a linear distribution in large LOCA. Communication between the upper plenum and uppr head was investigated. By introducing the concept of Taylor instability. it was proved that counter-current flow between them is possible.