Session: VVUQ for Fluid Dynamics and Heat Transfer 2

Paper Number: 152557

152557 - Experimental Investigation of Pressure Drop and Flow Regimes in Helical Cruciform Fuel Rod Bundles for Simulation Validations 

Abstract: 

To significantly enhance the power density and overall performance of Light Water Reactors (LWRs), the helical cruciform fuel (HCF) rod assembly has been introduced as a novel alternative to conventional cylindrical fuel rods. The HCF assembly consists of 4-finned, axially twisted fuel rods closely packed in a square array, forming a self-supporting structure without the need for traditional spacer grids. This innovative geometry provides several key advantages, including an increased surface-to-volume ratio, a shorter radial heat conduction path, and enhanced coolant mixing. These characteristics are expected to not only improve heat transfer but also allow the reactor to operate at higher power densities, all while maintaining thermal-hydraulic safety margins.

However, despite the theoretical promise of HCF, the accuracy of subchannel analysis models used to predict their behavior has been limited by the absence of appropriate experimental correlations. Previous computational studies have struggled to fully capture the thermal-hydraulic characteristics of the HCF configuration. In response, experimental research has been conducted to measure coolant mixing and pressure drop in HCF bundles, focusing on rod bundles. These experimental results are critical for refining the models and advancing the understanding of HCF behavior under reactor conditions.

The experimental works of the study examines the flow characteristics in a square 9x9 HCF rod bundle by conducting axial and lateral differential pressure measurements at four distinct locations along the bundle walls. Axial pressure drop is measured across both the entire bundle length and a single helical pitch, while lateral pressure measurements are collected at taps positioned on adjacent walls, 90° apart, and opposite walls, 180° apart, spanning the bundle’s width, at the same axial elevation. Friction factors are determined for each axial pressure drop measurement, providing insights into frictional behavior along the rod bundle.

The hardness of the experimental investigation of the HCF bundle lies on the manufacturing rods with tolerance that is small enough to ensure low uncertainty. Due to the geometrical complexity of the helical cruciform fuel rod, significant uncertainties can arise during the manufacturing process, leading to wide distributions in cross-sectional geometry and helical pitch. This causes large variations in flow channel shape and flow rate which is fatal to calculate friction factor and Reynolds number (Re). To address these uncertainties and ensure reliable experimental results, we manufactured a total of 1,820 rods, carefully analyzing their geometrical distributions. Out of these, 81 rods with the smallest deviations in geometry (0.2% in diameter and 0.65% in wetted perimeter) were selected for the experimental campaign. This selection process allowed us to reduce error in Re from 30.0% to 9.6%, error in the friction factor from 19.9% to 9.4%, when it is compared to the randomly picked rods bundle. This reduction ensures the robustness of the results, particularly in the context of validating subchannel models for the HCF configuration.

The experimental results presented in this study provide essential insights into the thermal-hydraulic behavior of HCF rod bundles, offering a detailed analysis of frictional pressure losses compared to other rod bundle geometries. The derived correlations contribute valuable data for refining numerical simulations and future empirical models, significantly expanding upon the experimental dataset for HCF rod bundles.

Presenting Author: Hansol Kim Texas A&M University

Presenting Author Biography: Dr. Hansol Kim is a postdoctoral researcher at Texas A&M University. She earned her Ph.D. in Mechanical Engineering at Texas A&M University. Her research field includes nuclear reactor thermal-hydraulics, high-fidelity modeling and simulation, and design optimization of thermal-hydraulic systems.

Authors:

Dalton Pyle Texas A&M University
Matthey Kinsky Texas A&M University
Joseph Seo Texas A&M University
Hansol Kim Texas A&M University
Yassin Hassan Texas A&M University