The performance of the RELAP5 thermal-hydraulic system code was analyzed in predicting very fast transient condensation processes in horizontal pipes. The code significantly underpredicted the heat transfer from the primary to the secondary side in case of rapid wall condensation process in the so called Inverse Edwards Pipe Experiment, where the condensation pipe was immerged in a cool water pool, and hot steam injection was performed into a pipe, which was closed on one side. The RELAP5 condensation model for horizontal pipes was modified in order to take into account a stratified flow pattern, and the effect of the local void fraction. The modified RELAP5 code was compared to the original code through the calculation of the Inverse Edwards Pipe Experiment. An improved prediction of the heat transfer process was achieved, considering the temperature, pressure and void fraction distribution along the horizontal pipe during the transient condensation process. (C) 2014 Elsevier B.V. All rights reserved.

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