Assessment of relap5-3d for application on in-pool passive power removal systems

This research investigates the capability of RELAP5-3D© to reproduce relevant thermal-hydraulic phenomena which characterize the operation of an in-pool passive power removal system. The activity has been carried out in the framework of a validity benchmark proposed within the OECD/NEA/CSNI/WGAMA “Thermal-hydraulics of passive safety systems in water-cooled reactors”, involving the PERSEO (in-Pool Energy Removal System for Emergency Operation) facility. The experimental system offers useful data for the assessment of computational tools. It reproduces an innovative passive decay heat removal (DHR) system, based on an Isolation Condenser (IC), operating in natural circulation, submerged in a pool. The Department of Astronautical, Electrical and Energy Engineering (DIAEE) of “Sapienza” University of Rome has developed a thermal-hydraulic model of PERSEO, using RELAP5-3D© v.4.3.4. The capability of the code to evaluate heat transfer coefficients (HTCs) in pool boiling and laminar wavy condensation modes, has been recognized as a relevant weakness. Indeed, under the operative conditions of the PERSEO facility, the correlations implemented in the code to evaluate HTC, are forced to work out of the validity range. For this reason, two multiplicative factors, derived on the basis of different correlations available in the literature, have been evaluated and included in the model to enhance the HTC. The comparison between computational results and experimental data has shown a satisfactory agreement, confirming the necessity to improve the HTC evaluation to overcome the criticalities highlighted by the code.