Scaling Analysis and Distortion Quantification in Nuclear Reactor Thermal-Hydraulics: French Advances and Perspectives

With the advent of many new nuclear reactor designs and the increasing cost of building scaled facilities, the choice of a scaling methodology must benefit from recent advances in this domain. The State of the Art on Scaling elaborated in 2017 within OECD-NEA-CSNI-WGAMA concluded that guidelines for scaling are necessary. It has been shown that using mature system codes extensively validated on a very large database may provide a deeper transient analysis and a more detailed and accurate scaling analysis for various reactor transients. It was also highlighted that methodologies were required to better quantify the distortions. This paper gathers some reflections and some progress in the scaling issue conducted at CEA, France, and draws some perspectives for further improvements. Attention is first paid to identification of the thermal-hydraulic phenomena (THPs) occurring during the different phases of a reactor accidental transient. Approaches for identifying a list of THPs to be used in a best estimate plus uncertainty methodology are briefly presented. The bifurcating events (BEs) that drastically change the phenomenology need to be identified as part of the scaling analysis. A set of scaling equations governing the evolution of the most important parameters of interest (PoIs) are given with all source and sink terms that can be evaluated by code simulations as function of time. Some approaches such as the “generalized distortion correction strategy” or the investigation of the complementarity of the scaling analysis and the general sensitivity analysis are proposed to analyze a specific transient and quantify the impact of scaling distortions on the PoI, on the occurrence and timing of BEs, and on the figure of merit. Ongoing developments to implement some instruments in the CATHARE code for scaling analysis are briefly described. Once validated against the analyzed systems and transients, this tool enables faster and more precise scaling analyses. This paper presents the remaining open issues and the future perspectives stemming from the work begun at the CEA.