Evaporation of liquid lead–bismuth eutectic (LBE) produces submicrometre aerosols in the cover-gas space of lead-cooled reactor systems. Cover-gas pressure simultaneously affects vapor diffusion, natural circulation, and particle wall loss, but its net effect on the measurable aerosol inventory remains unresolved. An LBE evaporation–aerosol facility with a 0.24 m³ pressure vessel was used to study argon cover gas at 700 and 800℃ and gauge pressures of 0, 20, 40, 60, 80, and 100 kPa. Complementary tests at 0 and 60 kPa from 650 to 900℃ and air cover-gas tests were conducted to separate pressure-dependent transport from atmosphere-dependent source suppression. Particle number concentration, optically inferred mass concentration, logarithmic size spectra, and SEM morphology were measured. Compared with 0 kPa(g), positive gauge pressure markedly increased suspended number and inferred mass concentrations at 700 and 800℃ while the response from 20 to 100 kPa(g) was non-monotonic and time-dependent. At 700℃ and 4 h, the modal diameter shifted from 0.25 μm at 0 kPa(g) to 0.26-0.33 μm at positive gauge pressure, and the fraction below 0.30 μm decreased from 90.2% to 43.5-63.3%. At 60–100 kPa(g), the spectral area was 34-45% larger than the 0 kPa(g) baseline. In the extended 700℃–60 kPa test, spectral intensity first increased and then declined, with the mode shifting to 0.33 μm, indicating a transition from formation-dominated to condensation/coagulation-deposition-dominated behavior. Air cover gas suppressed peak size-bin counts by 3 orders of magnitude, while pressure still broadened the spectrum. A source-growth-loss framework is proposed: higher absolute pressure reduces vapor and particle diffusivities, modifies buoyancy-driven circulation and thermophoretic transport, prolongs particle residence, and alters delivery to cooler walls. Overall, cover-gas pressure governs the measurable LBE aerosol inventory through coupled source-growth-loss mechanisms, yielding a non-monotonic and time-dependent response.

Lead–bismuth cooled fast reactor; Lead–bismuth evaporation; Cover-gas pressure; Aerosol concentration; Particle-size distribution; Coagulation and deposition