Density-Wave Instability Characterization in Boiling Water Reactors under MELLLA+ Domain during ATWS
Density wave oscillations (DWO) are a class of two-phase flow instabilities which can pose significant safety concerns to boiling water reactors (BWR). During an anticipated transient without scram (ATWS) while operating in the proposed extended operating domain MELLLA+, natural circulation conditions can potentially lead to DWO-type instabilities which have the capability to develop into cycles of fuel surface dryout and rewet, damaging core integrity. In order to provide data on these phenomena, a series of tests were performed at the KATHY facility during which DWO was developed with and without simulated neutronic feedback. In this dissertation, the data provided by these tests is analyzed to determine the onset conditions for DWO. Following this, several models are assessed for their capability in predicting this stability boundary compared to the experimental results. The models were chosen in order to provide a suitably large range of prediction methodologies. Two analytical drift-flux models developed with and without thermal equilibrium are shown, with respective differences compared. A computational model of the full KATHY natural circulation loop is built using the 1D thermal-hydraulics code TRACE. This is adapted with a point-kinetics model for neutronic feedback for experimental comparison. With both the analytical models and the TRACE model, a series of parametric studies are performed showing the effects of inlet/outlet flow restrictions, pressure, channel geometry, and axial power profile on the stability boundary. Finally, two machine learning neural network-based models are developed and trained on various subsets of the experimental data. The results from each model showed certain benefits and drawbacks based on model complexity and physicality.