MARS code improvements for condensation heat transfer in vertical tube with presence of non-condensable gas

Multi-dimensional Analysis of Reactor Safety (MARS) code is a thermal-hydraulic systems analysis code developed by Korea Atomic Energy Research Institute (KAERI) based on RELAP5/MOD3 and COBRA-TF codes. As a verification of modeling capability of MARS code for condensation, thermal hydraulic experiments have been collected and simulated by using MARS. Total five experiments of condensation in vertical tube have been simulated (POSTECH, KAIST, NASA, MIT, UCB). As results, two things have been found; 1) MARS code may use a wrong correlation in small-diameter cases, 2) MARS code overestimates Heat Transfer Coefficient (HTC) in vertical tube. As improvements, new criterion to determine the HTC correlation considering the flow condition is suggested. In addition, new method for heat transfer coefficient for laminar flow is developed. The calculation results of modified MARS predict the experiments better. (author)