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International audience; When looking for a validation experiment for CFD application to reactor simulation for either design or safety studies, it appears that available data often suffer from a lack of local measurements, an insufficient number of measured flow variables, a lack of well-defined initial and boundary conditions, and a lack of information on results uncertainty. Therefore the working group on CFD application to nuclear safety of the OECD-NEA-CSNI-WGAMA decided to establish some requirements for CFD-grade experiments able to validate properly the single phase CFD tools. The SILENCE network also supported this initiative and contributed to this work. This paper is a first attempt to establish such requirements. Several steps of an ideal experimental program are considered from the design to the data analysis in order to give recommendations and guidelines for future experiments. Clear objectives should be first defined with reference to a reactor application, an analysis of the process to be investigated and a selection of the type of turbulence model which may be used for the simulation. Then a discussion between experimentalists and the CFD code practitioners is necessary to define the test section geometry, the initial and boundary conditions, the quantities of interest with their locations, to define the requirements on the measurement uncertainty. The choice of measurement technique stem from these specifications. The acceptance criteria should be defined in accordance with the required accuracy and the sensitivity of the measurements to the uncertainty in the experimental conditions. Preliminary CFD simulations are necessary to confirm the most appropriate measurement locations, experimental conditions and overall the interest of the experiment for the physics to validate. Iterations may be necessary to get an optimized design of the experiment. CFD grade experiments should be able to validate CFD and one important concern is to minimize the validation uncertainty on some selected figures of merit.

International audience; Uncertainty assessment is a key step in nuclear applications to ensure that a system cannot move towards unsafe conditions. This topic has already been addressed by several OECD/NEA projects such as UMS or BEMUSE. However, if uncertainty propagation methods have now become mature for industrial applications, the input uncertainties quantification on the physical models still requires further investigations. It is precisely in this context that the SAPIUM project has been proposed in order to reduce as much as possible (or at least better understand) the user-effect observed in the previous PREMIUM activity that was a first attempt to analyze available methods to handle this issue. The underlying idea of the proposed work is not to focus on method benchmarking but on the construction of a clear and shared systematic approach for input uncertainty quantification as it is already addressed in industries and RandD for related topics. The main outcome of the project is a first good practices document that can be exploited for safety study in order to increase the agreement among experts on recommended practices as well as on remaining open issues for further developments. End users are therefore the developers and the users of BEPU methodologies, as well as the organizations in charge of evaluating them. Since it is an on-going activity, this paper describes the general content of the SAPIUM activity. All the details of the contributions will be available in the final document that will be issued in 2019.