Improvement of the MELCOR condensation heat transfer model for the thermal-hydraulic analysis of a PWR containment

During a hypothetical loss-of-coolant accident or a severe accident in a light water nuclear reactor (LWR), condensation heat transfer directly affects the pressure behavior and hydrogen distribution in the containment. This work aims at the improvement of the condensation heat transfer model in the MELCOR code, which is a fully-integrated computer code that models the progression of severe accidents in a LWR. It is known that MELCOR generally under-predicts the condensation heat transfer. For the improvement of MELCOR, we have assessed the MELCOR condensation model first. Other two models developed by Liao and Dehbi were also assessed for a comparative study. The assessment range is limited to the thermal-hydraulic conditions inside the containment building during a hypothetical design basis accident or a severe accident. We selected six condensation experiments of COPAIN, CONAN, Park, Anderson, Dehbi, and Kang and, then, assessed the three models against these data. The results of calculations showed that the MELCOR condensation model generally under-predicts the condensation heat transfer by about 45% and, however, it is the most precise and consistent in comparison with the other two models. Based on these results, the MELCOR model was selected as a base for model improvement. Then, four modifications were suggested: The effect of condensing surface curvature on condensation was considered into the MELCOR model first. The effective diffusion coefficient is applied for steam-air-helium tests. A multiplier to enhance condensation was obtained from the best fit of the calculated-to-measured plot. At last, to reflect the effect of superheated steam, a degradation factor deduced from the COPAIN experiment was adopted. The results of the modified MELCOR model showed very good agreements with most of the experimental data within ±30% error.