Improvement of the subcooled boiling model in MARS for low-pressure conditions

The void behavior in subcooled boiling has a great effect on flow and heat transfer characteristics and, thus, it is important to predict well the subcooled boiling phenomena in nuclear reactors. During the last several decades, many experiments were conducted to observe the subcooled boiling phenomena, such as a bubble generation, a bubble departure, and a bubble condensation. Some models were developed for application in thermal-hydraulic computer codes, e.g., RELAP5, TRAC, CATHARE and MARS. Some of these efforts have been particularly focused on the subcooled boiling at low pressure over the past 20 years to analyze the safety of research reactors operating near atmospheric pressure and to investigate the long-term core cooling during a LOCA of advanced light water reactors. To solve these problems, some modifications are suggested and the results are discussed. In this work, we have assessed the subcooled boiling model of the MARS code, mainly focused on lowpressure conditions. From the results of the assessment, the following deficiencies of the MARS subcooled boiling model were found. It cannot consider the effects of inlet subcooling temperature and liquid velocity on axial development of void profile. It considers the effect of equivalent diameter incorrectly. To solve these problems, we have modified the net vapor generation point model and the wall evaporation model of the MARS code. The results of the modified model clearly showed better agreement with experiment data than the original MARS model for low-pressure conditions.