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Thermohydraulic calculations of isolated and communicating cells of a rod bundle were performed by the channel method for CANDU-X fuel assemblies and by a three-dimensional method. It was established that in solving the problem for the tightest cell in the case q = const the azimuthal nonuniformity of the temperature was found to decrease by 77°C but it too was inadmissibly large. The temperature distribution along the surface of a fuel element in the case q = const was found to be different from the solution of the adjoint problem. A region with elevated coolant temperature, impeding heat exchange between two neighboring cells, was found between two adjoining cells. It was found that to evaluate computational reliability an experimental study must be performed on rod assemblies with supercritical coolant parameters.The heightened interest in reactors with supercritical parameters is due to the pursuit of higher efficiency. One variant of this direction is a technical work up of a CANDU-X type channel reactor [1]. The channel of this reactor is an assembly with 43 fuel elements -eight with outer diameter 13.5 mm and 35 with diameter 10.5 mm, which form a central group and two outer rows, respectively. The fuel assembly is encased in a channel tube with inner diameter 102 mm. The heated length of the channel is 5.77 m. Spacing lattices are used in the assembly.The conventional methods of hydraulic calculations are based on a representation of the reactor core as a system of parallel channels (channel method). In calculations, the thermohydraulic properties are averaged in the transverse section of the channels, which requires closure relations for the coefficients of hydraulic friction and heat emission and correction factors relating the parameters of the most highly heated cells and the fuel elements in a channel with the cross-sectional averages of the regime and geometric parameters. The development of channel methods for thermohydraulic calculations of rod assemblies makes it possible to obtain the spatial distribution of the properties. However, once again, the problem of the closure relations remains open but now at the level of the cells between fuel elements.Because no experiments have been done with rod assemblies, it is recommended that the relations obtained for tubes be used in the first approximation [1]. However, heat-transfer processes in assemblies more complicated than in tubes because of the strong change of the thermophysical properties of the coolant, the difference of the cell geometry, the nonuniformity of the energy release along and over the transverse cross section of the channel, and the presence of spacing lattices.In the present work, three-dimensional thermohydraulic calculations performed with the FLUENT code (USA) are used to study for the example of a CANDU-X channel the characteristics of heat transfer in a rod assembly cooled by water with supercritical parameters. Because we do not know the structural particulars of the spacing lattices in a CANDU-X channel the calculations were...