PVST, a tool to assess the Power to Volume scaling distortions associated to code simulations

•General overview of the Power to Volume Scaling method.•Scaling distortions associated to RELAP5 equations, models and correlations.•RELAP5 tool for scaling integral test facility input decks to NPP size. System codes along with necessary nodalizations are valuable tools for thermal hydraulic safety analysis. In order to assess the safety of a particular power plant, in addition to the validation and verification of the code, the nodalization of the system needs to be qualified. Since most existing experimental data come from tests performed on scaled-down facilities, any qualification process must therefore address scale considerations. Along these lines, the Group of Thermal Hydraulic Studies at Technical University of Catalonia (GET) developed SCUP, a scaling-up methodology for qualifying full-scale nodalizations through a systematic procedure based on the extrapolation of post-test simulations of Integral Test Facilities (ITF) experiments. For that purpose, GET created the “Power-to-Volume-Scaling Tool” (PVST), a RELAP5mod3 add-on software to easily generate scaled-up input decks following the Power to Volume (PtoV) methodology. The present paper describes the main features of this software as well as the scaling distortions that can be expected from applying PtoV to the RELAP5mod3 equations. In this sense, a detailled description of the PtoV methodology is introduced together with the particular criteria applied in the design of the main components of an ITF. In addition, an exhaustive study of the RELAP5mod3 equations is carried out to identify which are dimensionally affected by the PtoV scaling factors. Finally, PVST scaling analyses capabilities are assessed on two post-test experiments that were carried out at the Large Scale Test Facility (LSTF) within the framework of the OECD/NEA ROSA and ROSA-2 projects. Results demonstrated the high reliability of the software for identifying those particular phenomena (environment heat losses, broken legs flow regime transitions, subcooled liquid vapour mixing, …) that modify the experiment simulations because of the PtoV scaling criterion.