Assessment of the RELAP5/MOD3.3 code for condensation in the presence of air using experimental data and theoretical model

This study has been performed to assess the condensation module of RELAP5/MOD3.3 code, which is still widely used in the nuclear industry, for in-tube condensation in the presence on noncondensable gas under forced convection conditions. The experimental works, conducted at the Massachusetts Institute of Technology (MIT), the University of California-Berkeley (UCB), and the Middle East Technical University (METU), have been utilized in order to realize the assessment process in the wide range of parameters. To investigate the relationship between mixture Reynolds number and interface temperature, the theoretical model based on the energy balance at the interface has been developed and the results have been compared with the RELAP5/MOD3.3 findings. The entrance, interfacial waviness, suction and interfacial shear stress effects have been considered in the modeling to obtain accurate estimation of the heat transfer coefficient, particularly at the entrance region. The comparisons show that the proposed model predicts the heat transfer coefficient reasonably well with a maximum mean deviation of 17.3% for the simulated cases. On the other hand, RELAP5/Mod3.3 cannot evaluate the relationship between the mixture Reynolds number and the interface noncondensable gas concentration and predicts the heat transfer coefficients with the mean deviations around 150%, 85% and 50% for the METU, the UCB and the MIT databases, respectively. The findings reveal that the RELAP5/MOD3.3's capability to simulate the condensation with noncondensable gas phenomenon drastically decreases with increasing mixture Reynolds number.