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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. [ABSTRACT FROM AUTHOR]

The results of experimental studies of local hydrodynamics and mass exchange of coolant flow behind spacer and mixing grids of different structural versions that were developed for fuel assemblies of domestic and foreign nuclear reactors are presented in the article. In order to carry out the study the models of the following fuel assemblies have been fabricated: FA for VVER and VBER, FA-KVADRAT for PWR-reactor and FA for KLT-40C reactor. All the models have been fabricated with a full geometrical similarity with full-scale fuel assemblies. The study was carried out by simulating the flow of coolant in a core by air on an aerodynamic test rig. In order to measure local hydrodynamic characteristics of coolant flow five-channel Pitot probes were used that enable to measure the velocity vector in a point by its three components. The tracerpropane method was used for studying mass transfer processes. Flow hydrodynamics was studied by measuring cross-section velocities of coolant flow and coolant rates according to the model cells. The investigation of mass exchange processes consisted of a study of concentration distribution for tracer in experimental model, in determination of attenuation lengths of mass transfer processes behind mixing grids, in calculating of inter-cellar mass exchange coefficient. The database on coolant flow in fuel assemblies for different types of reactors had been accumulated that formed the basis of the engineering substantiation of reactor cores designs. The recommendations on choice of optimal versions of mixing grids have been taken into consideration by implementers of the JSC “OKBM Afrikantov” when creating commissioned fuel assemblies. The results of the study are used for verification of CFD-codes and CFD programs of detailed cell-by-cell calculation of reactor cores in order to decrease conservatism for substantiation of thermal-mechanical reliability.