Turbulent Flow Characteristics in a Blocked Exterior Subchannel of a Helically Wrapped Rod Bundle

Abstract

Abstract Liquid metal fast reactor using sodium as a coolant typically utilize a tightly packed triangular lattice of fuel pins enclosed in a hexagonal duct. During the reactor operation, partial or total flow blockage of coolant channels, may occur at different spatial locations within the fuel assembly, due to potential isolated or combined causes, including collection and accumulation of debris, and cladding deformation. Previous studies have shown that the flow characteristics within the wire-wrapped fuel assembly, i.e., including the interior and exterior subchannels, are very complicated and strongly influenced the flow and heat transfer phenomena between the coolant fluid and fuel rods. It is important to understand and characterize the effects of channel blockage to the flow mixing characteristics in the exterior sub-channels (or bypass channels) of the wire-wrapped fuel bundle. Texas A&M University has conducted isothermal flow experiments in a wire-wrapped 61-pin hexagonal fuel bundle to support the research on advanced nuclear fuel development sponsored by the US Department of Energy (DOE). The experimental facility employs matched-index-of-refraction (MIR) techniques and laser diagnostic velocity measurement techniques. In this article, we present the time-resolved particle image velocime-try (TR-PIV) measurements to characterize turbulent flow characteristics in the exterior subchannels of the wire-wrapped fuel bundle, under the presence of a localized total blockage of one of the exterior subchannels. From the obtained TR-PIV velocity fields, turbulent flow characteristics including mean velocity, root-mean-square fluctuating velocity, and Reynold stress, are computed and presented. In addition, spectral analysis to the turbulent velocity fields is performed to investigate the characteristic flow frequencies associated to different flow conditions with the presence of the blockage. Finally, proper orthogonal decomposition (POD) analysis is performed to the velocity snapshots to reveal the dominant flow structures that play important roles in the flow dynamics and heat transfers of the fuel bundle.