Experimental Investigations of Fuel Assemblies for Fast Reactor Applications

Abstract

Hexagonal rod bundles arranged in a tightly packed triangular lattice are the subject of extensive investigation for their applications related to energy conversion. Liquid metal fast reactors (LMFRs) and gas-cooled fast reactors (GFRs) are two types of Generation-IV nuclear reactor designs that utilize such rod bundles, due to their enhanced heat transfer and flow characteristics. Experimental measurements are critical to study the thermal-hydraulic behavior of these fuel assemblies. Wire-wrapped hexagonal fuel bundles in LMFRs, which use thin wires as spacers, have minimal experimental data available on the local subchannel pressure drop. In this research, experimental measurements of subchannel pressure drop were conducted in a 61-pin wire-wrapped rod bundle replica, for Reynolds numbers between 190 and 22,000. Specialized instrumented rods were utilized to measure the local pressure drop and estimate the subchannels’ friction factor. Five distinct subchannels were selected to study the effects of location on flow regimes and transition boundaries. The results of this experimental study provide a unique experimental data set to improve the predictive capabilities of specialized correlations and validate computational tools. In contrast, staggered spacer grids are used to maintain the structural integrity of GFR fuel assemblies, while inducing localized turbulence in the flow. Experimental flow visualizations are critical to identifying the differences in local flow properties that structural damage to the spacer grid may cause. The presented research investigates the flow-field characteristics at a near-wall and center plane in a prototypical 84-pin GFR fuel assembly. Velocity fields were acquired using the matched-index-of-refraction (MIR) method to obtain time-resolved particle image velocimetry (TR-PIV) measurements for a Reynolds number of 12,000. Reynolds and Galilean decomposition, and proper orthogonal decomposition (POD) analysis demarcated the influence of spacer grid damage and elucidated mechanisms of turbulence and flow instabilities. Reduced order flow reconstructions with vortex identification determined the spatio-statistical characteristics of generated vortices. Dynamic mode decomposition (DMD) analysis revealed the time-dependent vorticity spatial modes along with their oscillatory characteristics. The stability of the modes along with their growth and decay rates were also observed. The results offer a deeper understanding of fluid dynamics behavior to support LMFR and GFR rod bundle design efforts, and computational fluid dynamics model validation.