FEATURES OF THERMAL HYDRAULICS OF ACTIVE ZONES OF FAST LOW-POWER AND HIGH-POWER SODIUM PRODUCTION REACTORS FOR A CLOSED FUEL CYCLE SYSTEM

The paper considered the features of heat transfer of two fast sodium reactors of large and low power (1000 and 190 MW(e.)), which are developed for use in the production of secondary nuclear fuel. The design of a case fuel assembly for a higher power reactor is a combination of thin fuel elements (6.1 mm) with U-Pu fuel and a wide grating (relative pitch 1.39) with spacing of the gratings. For a low-power reactor, in this paper, three fuel assemblies (diameter of fuel rod 8.1 mm, relative pitch 1.16) were considered: without a cover, with a cover 1 mm apart from the peripheral row of fuel rods and with a cover spaced 0,5 mm from the peripheral row of fuel rods. Calculations were carried out using the COBRA-IV-I code. Data were obtained on the azimuthal temperature distributions on the claddings of the corner, peripheral and central fuel rods, temperatures on the surfaces of the covers, and calculations were performed to optimize the designs of fuel assemblies in order to reduce the temperature difference on the claddings of the peripheral fuel rods. For a large-power reactor, optimization of the configuration of the corner cell was proposed in order to reduce the azimuthal temperature difference at the corner and peripheral fuel rods. For a low-power reactor, optimization of the fuel assembly design is recommended by replacing the spacer grids with wire spacing and equalizing the temperature field by mixing the coolant, as well as increasing the relative pitch of the fuel element grid from 1.16 to 1.19.