Particle growth and gravitational sedimentation

Coagulation of aerosol particles occurs when particles cross streamlines of flow and come into contact. Such contacts between particles can be caused by  

Gravitational collisions between particles can be quite an important mechanism of coagulation in reactor containments because of the nature of the aerosol particle size distribution that develops in the containment atmosphere. Such size distributions are often predicted to be bimodal when there is an operating source of aerosol to the containment. The small size mode of the bimodal distribution is produced by particles freshly injected into the atmosphere from the operating source. The larger size mode is made up of particles that have aged in the atmosphere. Once sources of aerosol to the containment become small, the size distribution quickly becomes unimodal. 

Other phenomena can accentuate the agglomeration of aerosol particles and, thus, accentuate the rates of particle removal from a containment atmosphere. Agglomeration of particles by sonically driven processes has been considered as an accident management measure. Agglomeration can occur in a standing sound wave in a reactor containment because there is a phase lag between the responses of particles of different size to the gas vibrations. 

Calculation of gravitation sedimentation rates is usually based on the assumption that the containment atmosphere (or, more recently, the individual compartment atmosphere) is well mixed so that the particle concentration is spatially uniform. This need not be the case. Instances where stratification of the atmosphere is possible at least in particular compartments of containments have been identified. More sophisticated models would be needed to predict accurately the gravitational sedimentation of particles from these poorly mixed atmospheres. Accident management measures may, in fact, be used to prevent the development of stratified regions from which radioactive aerosol are removed slowly. 

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