Peculiarities of Hydrodynamics of Lead and Lead-Bismuth Coolant Flows of Reactor Loops

Adequate design engineering and maintenance of circuits with fast neutron reactors cooled with lead and lead-bismuth coolants require considering the peculiarities of hydrodynamics of these coolant flows. It is traditionally reputed that the hydrodynamic characteristics of heavy liquid-metal melts are analogous to the characteristics of water and primary sodium, which is practically valid for the conditions of part of the equipment and channels of a reactor circuit. The main peculiarities of heavy liquid-metal coolants compared to water and primary sodium, which affect the flow characteristics, are: - unwettability of channels with oxide protective coatings of reactor circuits by lead and lead-bismuth eutectic melts; - high boiling temperature exceeding the fusion temperature of steel; - high density exceeding by an order the densities of water and natrium; - low solubility of impurities in lead and lead-bismuth eutectic melts; - higher surface tension coefficient. The design value of saturated vapors of lead and its alloys at the temperatures 400–550 °C is 10⁻¹⁸–10⁻¹⁰ at (1 at = 0.1 MPa), which is essentially less than the values of natrium and water. Processes of traditional cavitation in the flow of heavy liquid-metal coolants cannot occur because of their specific character. The main circulation pumps are a basic element of reactor circuits. In fact, the flow sections of these pumps and those of other vane-type pumps operating in lead and its alloys cannot be calculated by traditional methods as far as cavitation characteristics are concerned; adequate calculation formulas are not available now. In a channel with walls unwettable by a flow of heavy liquid metal, this flow contacts with walls by means of the boundary layer having specific properties (surface energy, etc.) analogous to those of free surfaces of melts contacting with gas. Internal pressure in the flow forces liquid metal against walls, thus the liquid metal speed in the region of their contact is zero. As the pressure in the flow decreases due to growth of speed or other effects, the outer layer of the liquid metal flow can move away from the wall; in this case water appears on its surface. To study cavitation processes in a heavy liquid-metal coolant flow, the authors have carried out the following experiments: - determining the conditions of disconnection of liquid lead and lead-bismuth eutectic column; - determining the cavitation characteristics of the centrifugal pump pumping lead at the temperature 500 °C; - comparative investigation of the characteristics of Venturi nozzle in water and liquid metal. The experimental study of the characteristics of disconnection of heavy liquid-metal coolant column has shown that disconnection occurs at the boundary of liquid and cold metals; the reason of disconnection is leakage of gas from melt volume and, perhaps, from the near-wall region; disconnection occurs at negative voltages in the cross section of the column. The experimental study of the cavitation characteristics of the centrifugal pump at the temperature of pumped lead 500 °C and the circumferential speed of about 15 m/s has show that failure (cease) of pumping takes place at the pressure at the impeller inlet of about 19.6–24.5 kPa. Continuous operation of pump in the regime of pumping failure does not lead to destruction of the flow part surfaces of the pump. The character of the process corresponds to the so-called gas cavitation and is completely inconsistent with traditional cavitation. The experimental comparative study of the hydrodynamic characteristics of the same Venturi nozzle for water current at the temperature T = 20 °C and lead-bismuth eutectic at T = 350 °C without gas supply and with gas supply at the speeds 10–20 m/s has shown the following. The hydraulic resistance of the eutectic nozzle is more than an order higher than the analogous value for water under the same test conditions. This is, probably, due to flow disconnection and jet contraction in the narrow part of the nozzle with formation of water on its surface and backflows in the nozzle diffuser. Supply of relatively small amounts of gas into the narrow part slightly varies the characteristics of the processes. The consideration of specific character of heavy coolant flow hydrodynamics is required for adequate design engineering and maintenance of some elements of reactor circuit.