Development of good practice guidance for quantification of thermal-hydraulic code model input uncertainty

•Taking into account uncertainties is a key issue in nuclear power plant safety analysis using best estimate plus uncertainty methodologies. The OECD/NEA PREMUM project devoted to input uncertainty quantification has shown that inverse methods can exhibit strong user-effect. One of the main reasons was the lack of a clear guidance to perform a reliable analysis. This work is precisely devoted to the development of a first good practice guidance document for quantification of thermal-hydraulic code model input uncertainty.•The good practice guidance is structured following 5 elements that are summarized in the paper. Recommendations and open issues for future developments are also given.•This work emphasized that IUQ is a general process that involves a clear specification of the problem and efficient strategies to construct adequate experimental database. It also requires assessing the simulation model before quantifying input uncertainties. Finally, the validation of the quantified model input uncertainties has to be taken into account in order to check the acceptability of the results for the intended use. This work revealed the need to use mathematical tools integrating the physical knowledge in order to be rigorous, transparent and reproducible.•Even if part of the material to perform model IUQ is available, several remaining open issues should be tackled. This paper includes some development axes for future works. They concern the adequacy analysis of the experimental database, the construction of inverse propagation methods and the evaluation of the acceptability of the results in the validation and application domains. Taking into account uncertainties is a key issue in nuclear power plant safety analysis using best estimate plus uncertainty methodologies. It involves two main types of treatment depending on the variables of interest: input parameters or system response quantity. The OECD/NEA PREMIUM project devoted to the first type of variables has shown that inverse methods for input uncertainty quantification can exhibit strong user-effect. One of the main reasons was the lack of a clear guidance to perform a reliable analysis. This work is precisely devoted to the development of a first good practice guidance document for quantification of thermal-hydraulic code model input uncertainty. The developments have been done in the framework of the OECD/NEA SAPIUM project (January 2017–September 2019). This paper provides a summary of the main project