In the framework of deterministic safety analysis, Best Estimate Plus Uncertainty methodologies can provide a more informative detailed analysis of transients than conservative approaches. However, one important step in the application of such methodologies is the derivation of the uncertainty of the physical models, which may have a large impact on the final uncertainty of the figures of merit. The probability density functions of the physical model parameters are obtained by applying Inverse Uncertainty Quantification (IUQ) methods to experimental data. Following the recommendations of the OECD/NEA SAPIUM project, Exercise 1 of the recent OECD/NEA project ATRIUM (Application Tests for Realization of Inverse Uncertainty quantification and validation Methodologies in thermal-hydraulics) aims at applying the SAPIUM guidelines for the choked flow phenomenon. This publication presents the first steps in the application of the IUQ method, which are the evaluation of the experimental database, the assessment of the simulation model and the assessment of the base case results. The US-NRC system code RELAP5 has been applied for the simulation of three experimental databases on choked/critical flow: Sozzi-Sutherland, Super MobyDick and Marviken CTF. A first assessment of the results along with sensitivities improves the understanding of the phenomenon, and the capabilities and limitations of the code. Afterwards, the adequacy of the experimental data to the specific target domain has been carried out. As a first step to apply IUQ methods, metamodels have been build using a Gaussian Process. Next, the assessment of the simulation models has been performed by evaluating 5 statistical indicators. For this comparison, the input model parameters have been calibrated by combining the use of Gaussian Process metamodels and optimization schemes. The overall error of the calibrated model was slightly above 10% which indicated the need to reassess the simulation model.