Studying the Swirl Flow Hydrodynamics in the Guiding Channel Area Downstream of Mixing Spacer Grids of a PWR Reactor Fuel Assembly

The results from studying the coolant flow in fuel assemblies downstream of mixing spacer grids of a PWR reactor are presented. The aim of the study is to estimate the effect from using different designs of mixing grids. For this purpose, experimental investigations were carried out on an aerodynamic test facility with scaled models of fuel assembly fragments containing mixing spacer grids of different designs. Two adjacent cells of the guiding channel zone were selected as the representative study area. The specific structural feature of these cells is that the turbulizers installed on the intensifying grids have different spatial orientation. The general flow pattern is represented by the vector fields of tangential velocities and by the distribution of relative velocities in the gaps between the fuel rods and the guiding channel. For estimating the effect from using the grid designs with respect to coolant flow mixing, the intracell vortex formation coefficients and intercell mixing coefficients were analyzed. It has been shown from the performed analysis that, owing to the use of the alternative design with a pair of cell deflectors oriented in a changed direction, the intercell transfer coefficient and the intracell vortex formation coefficient were increased by a factor of 1.13 and 2.2, respectively, in comparison with those in the basic design. Hence, the use of the grid of the alternative design is more preferable for obtaining better mixing of the coolant flow. The accumulated database on coolant flow in the Square FA serves for engineering substantiation of the PWR reactor cores. The results from experimental studies are used for verification of CFD codes developed both in Russia and abroad and of programs for detailed cell-wise analysis of the cores for reducing the conservatism in substantiating their thermal reliability.