Hydrodynamics of turbulent coolant flow at the outlet of the fuel assembly of the RITM reactor for a small nuclear power plant

The paper presents the results of an experimental study and comparative analysis of coolant hydrodynamics at the outlet of a fuel assembly (FA) with upper nozzles of three different designs. The FAs are intended for the core of the RITM reactor of a small nuclear power plant (SNPP). The aim of the work was to study the axial flow velocity at the outlet of the fuel rod bundle and behind the upper nozzles of different designs, as well as to identify regions of the fuel rod bundle from which flow could enter the sampling tube leading to a resistance thermometer (RTD). The experiments were conducted on an experimental test bench with an air working fluid using a model of the FA outlet section. The model consisted of a fragment of the fuel rod bundle, replaceable mock-ups of the upper nozzles for three design variants, as well as mock-ups of the mounting plate for the FA and the coolant sampling tube to the RTD. The pneumometric method was used to determine the axial flow velocity. The tracer injection method was employed to identify the regions of the fuel rod bundle from which flow could enter the sampling tube to the RTD. The hydrodynamic flow pattern is presented in the form of maps of axial velocity and tracer concentration. The experimental results allowed detailing the turbulent flow structure and led to the following conclusions. The shape of the flow passage of the upper nozzle confusor has little effect on the axial flow already before the outlet spacer grid. However, a significant effect of the flow path geometry of the upper nozzles is manifested when the flow exits the fuel rod bundle. The shape of the confusor flow passage and the flow path geometry of the upper nozzles have a considerable influence on the axial velocity magnitude and flow structure. The greatest contribution to the change in axial velocity comes from the reduction in the flow passage area of the upper nozzles. The non-uniformity of the flow structure from the upper nozzle windows is determined by the sampling tube, which blocks part of the flow passage. The flow exiting the upper nozzle with three windows is the most non-uniform. During flow through the upper nozzle confusors, a mixing process is observed, expressed as a displacement of the tracer toward the sampling tube. The greatest transverse tracer displacement occurs in the confusor of the three-window upper nozzle. To minimize the influence of flows with different temperatures on the RTD readings, it is necessary to use upper nozzles with three windows. The research results have been used by specialists of JSC “OKBM Afrikantov” to substantiate engineering solutions in core design.