Abstract <p>Results are presented of the experimental investigation into peculiarities of the coolant flow in a fuel-rod bundle of a fuel assembly in the RITM reactor of the retrofitted floating power unit. The purpose of this study was to experimentally determine hydrodynamic characteristics of the flow in a fuel-rod bundle of a fuel assembly. The experiments were performed in an experimental facility with air as a working fluid using a model of a fragment of a fuel-rod bundle in a fuel assembly on the basis of the hydrodynamic simulation theory. The experimental model included a bundle of fuel-rod simulators, burnable poison rod simulators, spacer grids simulators, a central displacer, and stiffening angles. Invasive methods, such as the pneumometric method and the method of tracer injection, were employed in the study. The flow features were visualized using axial and tangential velocity maps and the tracer distributions. The experiments revealed the axial flow features and the structure of transverse coolant flows. Three zones are identified in the flow structure: in the region of regular cells, at the central displacer, and at the periphery (near the fuel-assembly casing). The flow velocity in them differs by 25–30%. The arrangement of plates in the grids considerably affects the flow structure at a distance greater than <i>L</i>/<i>d</i>_h ≈ 10.0. The found flow features should be taken into account in substantiating the thermal reliability of new cassette-type cores using cellular thermohydraulic codes. The procedure of thermohydraulic calculation in cellular thermohydraulic codes should be revised to increase the number of types of calculation cells and to consider the flow maldistribution among the regions in a fuel-rod bundle and the cell types.</p>