Analysis of Flow Instability Onset in a BWR Rod Bundle Geometry

Prediction of unstable flow operating conditions is important to maintaining reactor safety, particularly during anticipated transient without scram (ATWS) scenarios. The maximum extended load line limit analysis plus (MELLLA+) domain is an extended operating domain for boiling water reactors (BWRs) that allows operation at lower mass flow rates and higher reactor powers. While assessing safety considerations before allowing operation in the MELLLA+ domain, density wave flow instabilities are shown to arise during an ATWS that leads to an eventual failure-to-rewet (FTR) if the unstable flow conditions are not reversed. One hypothesized method of determining onset of instability is by denoting a 180° out-of-phase oscillation between the inlet mass flow and total pressure drop across a BWR rod bundle. As part of an ongoing analysis of these instabilities, the onset of density wave oscillation is predicted by analyzing the phase shift between inlet mass flow and total pressure drop in a BWR rod bundle. Experimental data is provided by tests performed at the Karlstein Thermal Hydraulic Test Facility (KATHY) under the proposed MELLLA+ operating conditions. The instability onset boundary determined from phase shift is compared to instability models derived using perturbation methodology under thermal equilibrium and non-equilibrium conditions. Current results for definition of instability boundary show good agreement with both instability models.