One way to increase the margin to critical heat transfer is investigated -closer lattice spacing on the axial section of a fuel assembly with the minimum margin to critical heat emission. Experiments were performed on the KS facility at the Russian Science Center Kurchatov Institute on two 19-rod VVER-1000 fuel-assembly models with lattice spacings 340, 255, and 170 mm. For coolant mass flow rate 4000 kg/(m 2 ·sec) and relative enthalpy 0-0.1, the critical flow rate increases by 15-20% because the lattice spacing decreases from 340 to 170 mm. As the mass flow velocity and relative enthalpy of the coolant decrease, the critical heat flows do not increase as much. A generalizing relation is obtained for the gain in the critical heat flux as a function of the distance to a spacing lattice and the main regime parameters.As a result of the need to increase VVER-1000 power and develop the new VVER-1200 reactor, a great deal of attention is being devoted to studying heat and mass transfer intensification in the core. The designs of the intensifying facilities, which increase the transverse mixing of the coolant as well as the margin to critical heat transfer with acceptable characteristics of the fuel assemblies, are under development. Cellular or plate structures are being used as the basis for the designs of the heat and mass transfer intensifiers -the mixing lattices.One way to increase the margin to critical heat transfer is to arrange the spacing lattices more closely on the axial section of the fuel assemblies with the minimum margin to critical heat transfer. The spacing lattices in the VVER-1000 fuel assemblies designed by Gidropress OKB and VVER-1200 fuel assemblies are spaced by 340 mm with cell height is 30 mm. In the Gidropress fuel assemblies, the step of the spacing lattices is 255 and 510 mm.The effect of an intensifying lattice or ordinary spacing lattice on the flow is expressed in three effects which are favorable for heat and mass transfer: 1) additional agitation of the flow in the cells; 2) additional intensification of the transverse turbulent mixing with neighboring cells; and 3) directed convective flows in accordance with the slope of the lattice elements. The studies described investigated the influence of the first effect on the crisis of heat transfer. It appears when intensifying lattices or ordinary spacing lattices are used.The experiments were performed on the KS facility with 19-rod bundles, which simulated VVER-1000 fuel assemblies. Heat release appeared in the tubular fuel-element simulators when a rectifying electric current was passed through them. Fuel-element simulators with a diameter of 9.1 mm were arranged in a triangular lattice with step 12.8 mm. Their relative arrangement was fixed by means of spacing lattices arranged with steps 340, 255, and 170 mm.The experiments were performed on two bundles which differed by the axial distribution of the heat release. The heat-release profile characteristic for the end of the operation of the fuel load with a maximum in the top of half o...

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