Coolant hydrodynamic features at the outlet of a RITM SNPP fuel cassette

The paper presents the results of studies connected with the features of coolant hydrodynamics at the outlet of various design heads of a RITM fuel cassette for a nuclear icebreaker and an SNPP. Experiments were carried out using a research bench with an air working medium on a model of the fuel cassette outlet section by pneumometric and contrast injection methods based on the theory of hydrodynamic similarity. The hydrodynamic pattern of the coolant flow is visualized using cartograms of the axial velocity at the outlet of the fuel rod bundle, behind the FA heads of various designs, in front of the letdown pipe, as well as in the upper support plate. The determination of fuel rod bundle areas, where the coolant can flow into the letdown pipe, was carried out by the contrast injection method. The results of the studies establish the influence of the FA head design on the heterogeneity of the axial flow at the outlet of the fuel rod bundle and in front of the upper support plate. The more intensive mixing of the coolant behind the head of the fuel cassette of a nuclear icebreaker as compared to the head of the SNPP fuel cassette is due to the inhomogeneity of the flow leaving the passage holes of the heads.