12.122. An aerosol is a gaseous suspension of fine liquid or solid par­ticles. Spherical diameters may vary from 1 mm to as small as 1 nm, but the range of interest is normally from 0.01 to 100 |xm. Degraded core materials making their way into the containment as a result of a severe accident tend to form aerosols by several processes. For example, the volatile fission products, such as cesium, iodine, and tellurium, are likely to condense or chemically react to form lower-volatility compounds and then condense. Nonvolatile fission products and other materials also form solid aerosols or combine with the steam present to form liquid aerosols. Although solid aerosols can have various shapes, a spherical shape is first assumed in analysis and correlation factors applied.

12.123. The amount of fission products available for release from the containment depends greatly on aerosol formation and deposition pro­cesses that occur following a degraded core accident. However, the picture is complicated. The suspending gas is likely to be in turbulent motion while the particles change size and may deposit by several processes.

12.124. Deposition by sedimentation as a result of the action of gravity occurs in accordance with the Stokes law principle described in §12.121. However, both the particle density and diameter tend to change during
the course of an accident affecting the settling velocity. Particles having a diameter of about 1 |xm have a settling velocity of about 0.1 mm/s. A measure of fission product removal is the decontamination factor, defined as the ratio of activity before to that after a given process. In a PWR containment vessel having a volume of 70,000 m3, it would take 17 days to achieve a decontamination factor of 100 for such particles [5].

12.125. Modeling of the deposition of aerosols has been carried out by the TRAP-MELT code [9]. In addition to sedimentation, other deposition processes include thermophoresis, turbulent deposition, and diffusion. Thermophoresis is the result of a force in a nonisothermal gas arising from unequal molecular impacts on the particle. Turbulent deposition is a wall effect resulting from the particles not being able to follow the fluid eddy flow adjacent to the wall. Diffusion become significant for particles having a diameter of less than 0.1 |xm. It is the result of bombardment by the gas molecules, which results in a diffusive flux when the particle concentration varies in space. Aerosol behavior is the subject of continuing modeling development supported by experimental research.