Distribution of thermal and mass flow rate parameters of coolant in the cross-section of 7-rod bundle consisting of the tubes of 6.0-mm outside diameter and 1.0-m heated length for two types of discretization of its cross-section into characteristic cells is experimentally investigated by the method of isokinetic sampling. The range of operation parameters corresponds to the conditions of LOCA accompanied by pressure and flow rate decrease relative to the nominal ones and is as follows: bundle-averaged outlet vapor content varies from −0.6 to 0.2, outlet pressure is 6 and 10 MPa, bundle-averaged mass velocity varies from 300 to 1500 kg/(m2s), inlet temperature varies from deep subcooling to (ts − 20) °C, and heat flux rate reaches 1.0 kJ/kg.

It is confirmed by the experiment that the surface in the gap between the peripheral rods is most probable place of burnout in the bundle. Physical model of two-phase flow in the bundle that considers it as the equivalent annular channel with an eccentricity is proposed. The programs of thermal-hydraulic computation of fuel assembly and their adaptation to the experimental data for the two types of discretization into characteristic cells are performed and comparative assessment of them is made.

The data resulted from the study concerning one-phase coolant could be applicable for a reference estimation of heat transfer in future designs of the fuel bundles.

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