Simulation of Single-Phase Natural Circulation within the BEPU Framework: Sketching Scaling Uncertainty Principle by Multi-Scale CFD Approaches

TL;DR

This paper investigates the simulation of natural circulation in nuclear reactors using multi-scale CFD approaches, highlighting the importance of high-fidelity models for understanding scaling uncertainties and physical distortions.

Contribution

It introduces a methodology combining high-fidelity and low-fidelity CFD models within the BEPU framework to quantify scaling uncertainties and physical distortion effects in reactor safety simulations.

Findings

HF simulations are resilient to physical distortions and have correlated numerical uncertainties.

LF models, suitable for reactor scale, may have reduced predictability due to scaling uncertainties.

The concept of scaling uncertainty links LF simulation growth to physical distortion effects.

Abstract

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.