Development of the NASCA code for prediction of transient BT and post BT phenomena in BWR rod bundles

This paper describes a progress in the development of a new subchannel analysis code NASCA being aimed at improved capabilities of analyzing transient two-phase flow phenomena, boiling transition (BT) and post BT phenomena in BWR rod bundles. The basic framework of the code has been constructed on the basis of a two-fluid three-field description. Major physical processes of significance are identified and discussed that require us to elaborate on experimental endeavors in making the mechanistic modeling possible. They are: 1) two-phase flow turbulent mixing and void drift; 2) spacer effects on BT; and 3) rewetting phenomena after BT. These physical models are outlined and computational results based on these models are compared to experimental data available. Also comparisons of prediction and experiment are shown with respect to the onset of BT in single channel as well as 4 x 4 bundle geometry. Finally numerical simulation capability is discussed for the rewetting phenomena. (author)