Experimental study on the flow characteristics of 7-pin hexagonal bundle assembly with grid spacers

Lead-cooled Fast Reactors (LFRs) are of great significance for future nuclear power systems. With the advantages of great neutronic properties and high passive safety, LFR has been selected as the reactor of China initiative Accelerator Driven System (CiADS), which has the potential to transmute long-lived nuclear waste into shortlived ones and enhance the utilization of nuclear fuel. The hexagonal bundle with grid spacers has been identified as a promising design of the CiADS fuel assembly. The new design of grid spacers has the advantages of reliable fixation, small flow resistance, and not easy to cause blockage. Flow characteristics in this fuel assembly model were revealed by the particle image velocimetry (PIV) method. The results showed the presence of crossflow and high axial velocity areas around the grid spacers. The strongest cross-flow mixing was observed downstream of the grid spacers at a distance of approximately z/Dh = 7.5 (axial height/hydraulic diameter). Although the influence of the grid spacers was weak, it was still detectable downstream at a distance of z/Dh = 32. The axial velocity loss rates were obtained and they increased with Reynolds number. The highest axial velocity loss rate was in the edge subchannel. Measurements of axial pressure drop uncovered the pressure loss characteristics of grid spacers and a new correlation was proposed. This work contributes to the understanding of the flow and pressure drop characteristics of the CiADS fuel assembly and can facilitate the design of more efficient nuclear reactors.