TRACE Assessment for Simulating Density Wave Oscillations in a Full-Scale BWR Rod Bundle Under Natural Circulation

Density wave oscillation (DWO) is one of the most extensively studied dynamic two-phase flow instabilities. The accurate prediction of these phenomena is important to ensuring safety in two-phase flow systems, such as boiling water reactors (BWRs). Recent reactor power uprates have led to the need for more accurate simulations at the system scale. For reactor licensing, the thermal-hydraulic computational code TRACE, developed by the U.S. Nuclear Regulatory Commission, is used for best-estimate predictions of light water reactors. One BWR power uprate condition of recent interest is the Maximum Extended Load Line Limit Analysis Plus, or MELLLA+, which allows BWRs to operate at lower core flow rates while maintaining the same power levels. Experiments performed at the Karlstein thermal-hydraulic test facility (KATHY) have shown that an anticipated transient without scram while operating under these conditions can lead to the development of DWOs.

This technical note assesses the capability of TRACE V5P7 to simulate DWO onset and development by comparison to the KATHY experimental data under natural circulation, focusing only on the thermal-hydraulic mechanisms. This study shows the analysis of DWO development from this data set, which utilized electrically heated fuel rods with a nonuniform axial power profile in a full-scale BWR rod bundle. The developed TRACE model is shown to be capable of producing DWO-type instability under the experimental conditions, while also allowing for an expanded parametric study on factors impacting stability.