Aerosols, removal

The theory of filtration of aerosols from a gas stream is much more involved than the sieving action which removes particles in a liquid medium. Figure 29-1 shows three of the mechanisms of aerosol removal by a filter. In practice, the particles and filter elements are seldom spheres or cylinders. 

Other lesser mechanisms that result in aerosol removal by filters are (1) gravitational settling due to the difference in mass of the aerosol and the carrying gas, (2) thermal precipitation due to the temperature gradient between a hot gas stream and the cooler filter medium which causes the particles to be bombarded more vigorously by the gas molecules on the side away from the filter element, and (3) Brownian deposition as the particles are bombarded with gas molecules that may cause enough movement to permit the aerosol to come in contact with the filter element. Browruan motion may also cause some of the particles to miss the filter element because they are moved away from it as they pass by. For practical purposes, only the three mechanisms shown in Fig. 29-1 are normally considered for removal of aerosols from a gas stream. 

The high efficiency, asbestos-cellulose filter (C.W.S. filter) is one of the best developed for aerosol removal. This filter, which shows a decontamination factor of nearly 5 X 108 f or 0.3 m particles, improves in performance within a short time after the beginning of service, because of particulate deposition. Usually a glass wool pre-filter is used before the CWS filter,... 

Additional practical methods of aerosol removal include filtration and centrifugation. Centrifugation, of course, is basically the same as sedimentation except that the force of gravity is replaced by artificial forces of greater strength. Equation (13.6) continues to apply in that case, although the value of g must be multiphed by the appropriate factor. 

Aerosol removal from bubbles is very dependent on bubble size. Removal is more efficient from smaller bubbles. Fortunately, bubbles rising in a suppression pool disintegrate to a eommon size of about 0.5 cm regardless of how they are injected into the pool [A-9]. 

Aerosol removal processes that oceur when a bubble rises through a suppression pool vary in efficiency with particle size. As with sprays, very fine and very large particles are efficiently removed. There is a partiele size that is minimally affeeted by the deeontamination processes. Aerosols that emerge from a suppression pool have sizes narrowly distributed around the minimally affected particle size (also called the maximum penetration size). These residual aerosols also resist removal by many other deeontamination proeesses so they can be quite persistent in the atmosphere. 

Aerosol removal by suppression pools has reeeived quite a lot of experimental and analytical attention. Computer models of the removal proeess inelude the SPARC model [A-lOa], the BUSCA model [A-10b], and the proprietary model SUPRA [A-lOe]. Exeellent experimental studies have been eondueted [A-11] and additional studies are underway in Europe and Japan [A-9a, b]. 

Similar aerosol removal can be expected in VVER-440 reactors equipped with bubbler condensers [A-12]. 

Some pressurised water reactor containments are equipped with fan coolers. These coolers would appear to offer the possibility of particle deposition by thermophoresis or even by difiusiophoresis. Analyses of the potential decontamination that could be achieved with such fan coolers have not been reported. They may not provide a long-term aerosol removal capability. Accumulation of insulating deposits of aerosol on their surfaces may limit effectiveness. Fan coolers do require power to operate and the necessary power may not be available under accident conditions. 

V.4 Annotated bibliography of aerosol removal from the containment atmosphere... 

A-7b. D.A. Powers and S.B. Burson, A Simplified Model of Aerosol Removal By Containment Sprays, NUREG/CR-5966, SAND92-2689, Sandia National Laboratories, Albuquerque, NM, June 1993. 

A-9b. J. Hakii, H. Kaneko, M. Fukasawa, Y. Masahiro, and M. Matsumoto, Experimental Study of Aerosol Removal Effect by Pool Scrubbing, Second Workshop on LWR Severe Accident Research at JAERI, Tokyo, Japan, 1991. 

Experimental programs that are intended to validate the models of aerosol removal by steam suppression pools and other water pools. 

Typical mechanisms for aerosol removal from gas streams by filters are diffusion to surfaces, interception and impaction. Very large particles can be removed by gravitational settling. These mechanisms are quite dependent on the particle size and it is usually found that conventional filters have a minimum in filter efficiency for particles in a narrow size range less than 1 im. When the gas is hot relative to the filter, thermophoresis can enhance particle removal. When the aerosol laden gas stream contains elevated concentrations of steam that condenses within the filter, difflisiophoresis will enhance particle removal. These phoretic enhancements of filtration are attractive because filtration efficiencies by these mechanisms are not especially dependent on the aerosol particle size. Washed Venturi scmbbers involve the injection of water droplets into the aerosol laden gas and these water droplets act much like spray water droplets to remove aerosol particles. Electrostatic precipitation is, in principle, a very attractive decontamination process, but it is difficult to assure that the necessary power will be available to operate the precipitators under accident conditions. 

For hydrosols in gremular filtration, the external force consists of gravitational force, particle-collector surface interaction forces, such as the unretarded London attraction force (defined in (3.1.16)) and electrokinetic force (3.1.17) in the double layer, and electrostatic forces, if any, such as coulombic attraction/repulsion forces (3.1.15) (usually important in aerosol-removal processes unless the collector particles are deliberately charged). In... 

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