Modification of RELAP5 computer code for Low-pressure Subcooled boiling Condition

In the low-pressure condition, the RELAP5 Mod3.2 result is under predicted for void fraction profile. Therefore, it was recommended that the use of this code for the analysis research reactor transient should be limited where the amount of boiling and voiding are insignificant. The aim of this paper is modified this code for the low-pressure subcooled boiling condition. In this process, the minimum bubble to slug flow regimes transition limit was increased and the interfacial heat transfer coefficient was reduced. Following this modification, the evaluation of the accuracy of the models used in code for the subcooled boiling process was done by comparison of the calculational data according to original and modified code version with the experimental data in the wide range of regime parameters. There is a significant improvement in the void fraction prediction with the modified version. Two high-pressure experiments were simulated with original and modified versions of code. No significant adverse effect was observed. Sensitivity analyses were performed for transition limit and reducing factor interfacial heat transfer coefficient. Three value was considered for each items separately. The results shown values of 0.2 and 100 are better result for all of the experiments as transition limit and reducing factor, respectively.