Large-scale dispersion

Engesgaard P., Jensen K. H., Molson J., Erind E. O., and Olsen H. (1996) Large-scale dispersion in a sandy aquifer simulation of subsurface transport of environmental tritium. Water Resour. Res. 32, 3253-3266. 

Jensen K. H., Bitsch K., and Bjerg P. L. (1993) Large-scale dispersion experiments in a sandy aquifer in Denmark observed tracer movements and numerical analysis. Water Resour. Res. 29, 673 -696. 

Releases may take place inside buildings or outdoors. This influences the dispersion behaviour. Outdoor releases often remain without grave consequences because of quick dilution of the released materials. A release of the same quantity indoors, however, may have grave consequences because of toxic impacts and the possibility of mixtures with air within the hmits of flammability. The elevation of the point of release also influences the dispersion behaviour. A release of a liquid below ground level may remain completely contained. On the other hand, a release of a gas or vapour above ground may lead to a large-scale dispersion. 

Resuspension of deposited radionuclides is generally not taken into account, as it is usually of less importance during the early phases of an emergency (with the possible exception of large scale dispersion of plutonium). The effect of sheltering may be taken into account provided that data are available and that sheltering countermeasures have been effective. The effects of prophylaxis with stable iodine may also be taken into account provided that the exact time of its apphcation is available. 

A further complication is that the specific toxicity of a trace element may vary widely for different species of organisms. Thus it is virtually impossible to compare, in any meaningful way, the global effect of large-scale dispersal of an element like zinc, which is a relatively non-toxic and common environmental contaminant, with the effect of dispersal of beryllium, a highly toxic element in limited use. 

Triaryl phosphates are also used on a large scale as flame-retardant hydrauhc fluids (qv), lubricants, and lubricant additives (see Lubrications and lubricants). Smaller amounts are used as nonflammable dispersing media for peroxide catalysts. 

Thermal or Flame Spray Process. The earliest experiments in metal spray used molten metal fed to a spray apparatus, where it was dispersed by a high speed air jet into tiny droplets and simultaneously blown onto the surface of the part to be covered. The metal solidified on contact. Modem processes use a more convenient source than premelted metal. Spray heads using a flame or an electrical arc to melt metal wires or powders directly are much more convenient. These are the only types used on a large scale in the United States. 

Suspended Particle Techniques. In these methods of size enlargement, granular soHds are produced direcdy from a Hquid or semiliquid phase by dispersion in a gas to allow solidification through heat and/or mass transfer. The feed Hquid, which may be a solution, gel, paste, emulsion, slurry, or melt, must be pumpable and dispersible. Equipment used includes spray dryers, prilling towers, spouted and fluidized beds, and pneumatic conveying dryers, all of which are amenable to continuous, automated, large-scale operation. Because attrition and fines carryover are common problems with this technique, provision must be made for recovery and recycling. 

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