Resumen de Scaling-up methodology: a systematical procedure for qualifying NPP nodalizations. Application to the OECD/NEA ROSA-2 and PKL-2 Counterpart test
System codes along with necessary nodalizations are valuable tools for thermal hydraulic safety analysis. Qualifying both codes and nodalizations is an essential step prior to their use in any significant study involving code calculations. Since most existing experimental data come from tests performed on the small scale, any qualification process must therefore address scale considerations. Along these lines, the present thesis introduces a new scaling-up methodology that contributes to the qualification of Nuclear Power Plant (NPP) nodalizations by means of scale disquisitions. The "UPC Scaling-up methodology" is a systematic procedure based on the extrapolation of Integral Test Facility (ITF) post-test simulations. There are three main pillars that support this procedure: judicious selection of experimental transients, full confidence in the quality of the ITF simulations, and simplicity in justifying discrepancies that appear between ITF and NPP counterpart transients. The techniques that are presented include the so-called Kv scaled calculations as well as the use of two new approaches, ¿Hybrid nodalizations¿ and "Scaled-up nodalizations". These latter two methods have revealed themselves to be very helpful in producing the required qualification and in promoting further improvements in nodalization. "Scaled-up nodalizations" allow effects of the ITF scaling-down criterion to be checked. On the other hand, "Hybrid nodalizations" help the user to establish how design differences modify the results. In order to carry out these calculations, a Powerto-Volume-Scaling Tool (PVST) was developed. This software generates scaled-up input decks for RELAP5mod3 following the Power to Volume Scaling (PtoV) methodology. Within the presentation of this software, it is included a detailed description of the PtoV criterion together with the scaling distortions that are expected from its application to the RELAP5mod3 equations. PVST capabilities are also assessed on two post-test simulations that were carried out at the LSTF experimental facility within the framework of the OECD/NEA ROSA and ROSA-2 projects. Finally, an assessment of the present methodology was carried out by making use of the OECD/NEA ROSA-2 and PKL-2 Counterpart Tests (an exhaustive description of both facilities and experiments is also included). One of the limitations of scaling methodologies is the impossibility to qualify their predictions because of a lack of counterpart experimental data at NPP level. Thus, the ROSA-2 PKL-2 Counterpart Test was of great value because it allows an identical transient to be compared between two facilities with relevant differences in both design and scale. The study of both LSTF and PKL counterpart tests has enabled us to define which phenomena could be well reproduced by the nodalizations and which phenomena could not, and also to establish the basis for future extrapolation to a NPP scaled calculation. On the other hand, the application of the UPC scaling-up methodology has demonstrated the fact that selected phenomena can be scaled-up and explained between counterpart simulations by carefully considering differences in both scale and design. As future lines of research, in the short term it is planned to fully apply the present methodology to qualify NPP nodalizations for the correlation of core exit temperatures (CET) versus peak cladding temperatures (PCT). In the long term, it is also intended to fully integrate the "UPC scaling-up methodology" within scaling issue, and to focus the efforts on providing a definite answer to the scaling controversy on the extrapolation of code accuracy to NPP level.
