Abstract
A new model for the occurrence of the net vapor generation was developed to improve the predictive capability of best-estimate thermal hydraulic codes for transient void behavior under fast transient condition such as reactivity initiated accident (RIA). As a conventional model widely used in various codes is based on the experimental data obtained from steady boiling experiments, it did not well represent transient thermal-hydraulic phenomena near the heating surface. By analyzing our results of RIA simulation experiments previously performed at both low (atmospheric) and high (up to 7MPa) pressure, it was clarified that the concept of vapor condensation in the model needed to be improved. A new model for the net vapor generation was developed by using the thickness of thermal boundary layer as a characteristic length of vapor condensation. The new model was introduced into TRAC-BF1 code and was applied to the analyses for the high pressure experiments, confirming that the predictive capability of the modified code was improved.
