  In order to enhance safety, nuclear reactors in the design phase consider
natural circulation as a mean to remove residual power. The simulation of this
passive mechanism must be qualified between the validation range and the scope
of utilization (reactor case), introducing potential physical and numerical
distortion effects. In this study, we simulate the flow of liquid sodium using
the TrioCFD code, employing both higher-fidelity (HF) LES and lower-fidelity
(LF) URANS models. We tackle respectively numerical uncertainties through the
Grid Convergence Index method, and physical modelling uncertainties through the
Polynomial Chaos Expansion method available on the URANIE platform. HF
simulations are shown to exhibit a strong resilience to physical distortion
effects, with numerical uncertainties being intricately correlated. Conversely,
the LF approach, the only one applicable at the reactor scale, is likely to
present a reduced predictability. If so, the HF approach should be effective in
pinpointing the LF weaknesses: the concept of scaling uncertainty is inline
introduced as the growth of the LF simulation uncertainty associated with
distortion effects. Thus, the paper outlines that a specific methodology within
the BEPU framework - leveraging both HF and LF approaches - could pragmatically
enable correlating distortion effects with scaling uncertainty, thereby
providing a metric principle.
