Inter fuel-assembly thermal-hydraulics for the ELSY square open reactor core design

The lead-cooled reactor is one of the six proposed innovative reactor types by the Generation IV International Forum (GIF). In Europe, the lead-cooled reactor design is known as the European Lead-cooled System (ELSY), which is a 600 MWe medium size fast reactor. The reference design of the ELSY core foresees square open (wrapper-less) fuel-assemblies with a staggered arrangement. In this design, the fuel rods in a fuel-assembly are separated by 3.4 mm. The gap between fuel rods of neighboring fuel-assemblies is 5.5 mm. In other words, the reference gap size between fuel-assemblies is larger than the gap between fuel rods within a fuel-assembly.This article discusses the involved inter fuel-assembly thermal-hydraulics between neighboring fuel-assemblies in the ELSY core. For this purpose as a starting point a validated Reynolds Averaged Navier Stokes (RANS)-based Computational Fluid Dynamics (CFD) approach is adopted. Moreover, bare fuel rods are considered in the present analyses that serve as a step towards inclusion of a spacer grid when its design is fixed. As the next step, the fuel-assemblies are numerically arranged with different gap sizes of 2.1 mm and 3.4 mm in order to analyze the influence of gap size on the inter fuel-assembly thermal-hydraulics. As a final step, analyses on the influence of different power levels of neighboring fuel-assemblies in the ELSY core are presented based on the reference ELSY core design. These inter-fuel assembly thermal hydraulic analyses lead to a conservative Nusselt number correlation for calculating maximum surface temperature of bare fuel rods that are located in the gap region between neighboring fuel-assemblies having different power levels. Such correlations, when implemented, will improve the applicability of system codes.