Discussion on the stability of natural circulation loops with supercritical pressure fluids

The paper presents a methodology for the analysis of the flow stability in natural circulation loops containing fluids at supercritical pressure. The work was made possible by the availability of experimental data collected on an experimental apparatus at the China Institute for Atomic Energy, showing the onset of unstable behaviour. The RELAP5 code is firstly used for predicting the observed phenomena, obtaining a qualitative prediction of instabilities with quantitative discrepancies in relation to the conditions observed for the onset of unstable behaviour. In the aim to allow for a systematic discussion of stability phenomena, dimensionless parameters previously proposed to analyse the stability of single heated channels with fluids at supercritical pressures are now adapted and applied to natural circulation loops. The proposed dimensionless numbers address purely thermal-hydraulic phenomena, disregarding the presence of heating structures. The combined analyses with the system code and an in-house code written in dimensionless form and based on the mentioned parameters allowed highlighting the importance of heat transfer to heating structures as a phenomenon contributing to stabilise natural circulation loops.