Inquiry on the Effect of Heating Structures in the Prediction of Flow Stability at Supercritical Pressures

Abstract Flow stability is an important concern in the design of apparatuses containing fluids with heating and cooling sections. Fluid density changes, even when they are minimal as in singlephase thermosyphon loops, are often responsible for the occurrence of flow oscillations, which are mostly unwanted while striving for stable operating regimes. Nuclear reactors may be prone to these phenomena, something that poses problems in view of the stability of operation requested, inter alia, by the General Design Criterion 12 in Appendix A to the USA 10 CFR 50 law. A design conducive to stable operation and an early detection and suppression of oscillations is therefore in order. In the frame of a third IAEA Coordinated Research Project on Supercritical Water Reactors (SCWRs), aiming to close some of the open issues in this technology, the problem of the effect of heating structures on the prediction of flow stability at supercritical pressure is now targeted for being attentively scrutinised. In front of the scarce experimental data available that sometimes provide counterfeiting information with respect to the one proposed by models, a varied modelling approach must be used, searching for possibly overlooked ingredients in the currently adopted numerical recipes. In the present paper, different modelling techniques have been adopted, ranging from the use of 1D time-domain and linear in-house programmes, to system codes and CFD. With reference to some experimental data and predictions appeared in literature, an attentive discussion is carried out on the possible reasons for discrepancies between observations and predictions, aiming to contribute to clarify this open issue.