Effect of complicated coolant flow behavior in the ABWR lower plenum on flow distribution to the core

Abstract

One of the cost reduction strategies in nuclear power generation is the augmentation of power outputs by increasing the coolant flow rate. To achieve augmentation of the power outputs in an Advanced Boiling Water Reactor, it is extremely important to evaluate the coolant flow from the lower plenum to the core inlet, which dominates the coolant flow distribution over the rod bundle of the core. In the lower plenum, there are a number of structures, such as control rod driving mechanisms and guide tubes. In addition, core support beams and side entry orifices are placed at the core inlet. Therefore, it is estimated that the coolant flow becomes very complicated in these areas. To predict complicated coolant flow in the lower plenum and core inlet, simulation using a Computational Fluid Dynamics (CFD) code is helpful. However, previous experimental data are not enough to verify the simulation results for the intended flow channel in this study. Hence, this study focuses on establishing a benchmark for the CFD code using the visualization method. Based on the validation process, the functions of the complicated flow structure at the core inlet and through the lower plenum are demonstrated in detail in the coolant flow distribution to the core. A 1/10th model of a lower plenum was constructed, and the velocity profiles were measured using Particle Image Velocimetry (PIV) and Laser-Doppler Velocimetry (LDV). Each measurement was performed for a Reynolds number of 2620. From the flow velocity measurement, the vertical velocity at the center of the lower plenum is determined to be faster than the velocity near the shroud, resulting in a parabolic-like distribution. Cross flow through the control rod guide tubes is also observed. The vertical velocity profiles showed a tendency to flatten around the core support beam. Side entry orifices at the inlet of the core are installed in the region of the core support beam, resulting in locally complicated flows. Vortices were observed around the side entry orifices, and a noticeable pressure drop was observed. The CFD analysis results closely agreed with the experimental profiles. Using the CFD results, the coolant flow distribution at each orifice is evaluated. The analysis shows a significant difference in the amount of coolant flowing to the core depending on the locations of the orifices relative to the core support beams. The macro-scale flow distribution into the core through the side entry orifices was uniform. And local flow structures around the side entry orifices were experimentally observed and were reproduced by the CFD analysis.