Validation of the System Code CATHARE3 on Critical Flow Experiments in the Framework of the OECD-NEA Atrium Project

Abstract

When applying the Best Estimate Plus Uncertainty (BEPU) methodology for the safety analyses of nuclear reactors, one of the major issues is to quantify the uncertainties associated to the physical models in thermal-hydraulic codes. A good practice guideline for Inverse Uncertainty Quantification (IUQ) was therefore developed during the OECD-NEA SAPIUM (Development of a Systematic APproach for Input Uncertainty quantification of the physical Models in thermal-hydraulic codes) project in 2020. A first application of this guideline is now carried out within the OECD-NEA ATRIUM (Application Tests for Realization of Inverse Uncertainty quantification and validation Methodologies in thermal-hydraulics) project, which was launched in 2022. The goal is to perform practical IUQ benchmark exercises to evaluate the applicability of the SAPIUM best-practices and suggest possible improvements. In this article, we describe part of the work performed at CEA on the first benchmark exercise on critical flow. In particular, we focus on the validation of the system code CATHARE3 against the available experimental data and the associated sensitivity analyses performed to better understand the simulation results and prepare the IUQ process. The 324 chocked flow experiments come from three different facilities: Sozzi-Sutherland, Super Moby-Dick and Marviken-CFT. The simulations are in very good agreement with the experimental data (maximum discrepancy of 23.3% on the critical flowrate). Based on the sensitivity analyses, two main influential parameters are identified: the wall-to-liquid friction and the flashing models in CATHARE3. The flashing is dominant for relatively short nozzles (L/D ≤ 18). For longer nozzles, the wall-to-liquid friction becomes more and more influential.