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The objective of the present work was to investigate experimentally the flow velocity of lead coolant in a ring-shaped gap on horizontal and vertical experimental sections during monitoring and regulation of the thermodynamic activity of oxygen. The experiments were motivated by an interest in validating the concept of the BREST-OD-300 fast reactor, in which a lead melt is to be used.The experiments were performed on an experimental stand. The stand contained experimental sections -horizontal and vertical with sensors for measuring the local velocity in the ring-shaped channel (Fig. 1). The low rate of the liquid metal was monitored with a magnetic flowmeter and by the volume method -calibration flow container; the thermodynamic activity of oxygen in the lead was monitored by sensors placed in flow and buffer containers. The equipment and pipelines were made of 08Kh18N10T steel and are electrically heated and possess thermal insulation.The sensors for measuring the flow velocity of the lead coolant were placed in two experimental sections: vertical and horizontal.The vertical section consisted of two coaxially arranged pipes (d 2 /d 1 = 4.2). The outer pipe, 45 mm in diameter, 2.5 mm thick, and 1570 mm long, was made of 12Kh18N10T austenitic steel. The inner pipe consists of a 2100 mm long tubular electric heater with a regular BREST-OD-300 fuel-element cladding made of 16Kh12MVSFBAR-Sh ferrite-martensite steel and a 1600 mm long active part. The flow direction in the ring-shaped gap is bottom to top.The horizontal section tilts at an angle of 3°along the direction of motion of the lead in the form of two coaxially positions pipes (d 2 /d 1 = 2.35). The outer tube is 1348 mm long, 45 mm in diameter, and 2.5 mm thick and consists of 12Kh18N10T austenitic steel. The inner pipe is 2095 mm long, 17 mm in diameter, and 3 mm thick; this pipe is a regular tube of a steam generator in the BREST-OD-300 reactor and is made of 10Kh9NSMFB ferrite-martensite steel.The sensor makes it possible to measure the potential and full head of the liquid-metal flow. The local velocity is calculated from the difference of these heads. The full head was measured with a capillary (tube with inner diameter 1 mm and wall thickness 0.25 mm), whose free end was directed opposite to the coolant flow. The coolant flows through the capillary and the system of pipes into the tank used to measure the full head. The potential head was measured through an opening in the wall of the experimental section. The free end of the capillary and the opening, for measuring the head, in the wall are located in the same transverse section of the tube. The lead flows through the measuring opening and through the system of pipes into the tank for determining the potential head.Using nuts, the entrance end of the capillary is moved in the radial direction over the cross section of the channel by means of a spacing plate. The plate is connected with a tie rod, whose other end is secured to a probe with the open end of the capillary.

Adequate design solution and maintenance of circuits with fast reactors cooled by lead and lead-bismuth coolants require taking into account the peculiarities of hydrodynamics of these coolant flows. The design pressure of saturated vapors of lead and its alloys at temperatures of 400–550 °C is 10 −18−10 −10 atm, which is significantly less than that of sodium or water. Processes of traditional cavitation cannot occur in a flow of heavy liquid-metal coolants because of their specific character. The main circulation pumps of reactor circuits are one of their basic elements. In fact, the flow-type parts of these pumps and other vane pumps operating in lead and its alloys cannot be calculated by traditional methods in terms of cavitation characteristics; appropriate calculation formulas are not currently available. To study cavitation processes in a heavy liquid-metal coolant flow, the authors have carried out the experiments aimed at: - determining the conditions of disconnection of liquid lead and lead-bismuth eutectic column; - determining the cavitation characteristics of a centrifugal pump transferring lead at a temperature of 500 °C; - studying the characteristics of ejector (Venturi nozzle) in a liquid metal; - studying the cavitation erosion effect of the lead coolant on impeller vanes of an axial-flow pump in a limited volume in the FT-4-A stand at a lead flow rate of up to 1200 t/h; - studying the cavitation characteristics of an axial-flow pump in the FT-4 stand at shaft speeds of 13.34–25 Hz. These studies are performed with the lead coolant at temperatures of 450°−550°C, oxygen in lead from 10−4−10−5 to 100, flow rates from 20 to 1800 t/h, which corresponds to velocities of the lead coolant flow from 1.0 to 26 m/s. The experiments have shown that as distinct from water, traditional cavitation processes in a heavy liquid-metal coolant (HLMC) flow are not recorded. The probable cavitation mechanism is gas cavitation. The allowance for the specific character of hydrodynamics of HLMC flows is necessary for adequate design engineering and maintenance of some elements of the reactor circuit.