Aerosols transportation mechanisms

The analyses of accidental sequences here described reveal that revolatilization of volatile FP species from deposits on the RCS surfaces could occur during the last period of the in-vessel phase. It has also been found that thermal revaporization of other core materials is negligible. At this time, favored by the thermal and hydrodynamic conditions of the carrier gas through the RCS, the revolatilized FP vapors could nucleate and/or condensate onto particles. Thus, aerosol transport mechanisms could determine the release of radioactivity to the correspondent atmosphere during this period. In connection with this phenomenon, results obtained for the V, SGTR and AB scenarios are summarized. 

Several groups investigated the use of liposomes for the intra-pulmonary delivery. Farr et al. (1985) showed that the deposition of aerosolized liposomes in the human lung depends on the aerosol particle size. Short-term retention profiles for MLVs and SUVs deposited in the lung were indicative of clearance via the mucociliary transport mechanism. 

The rate of removal of mucus from the airways is determined by such factors as mucus viscosity, the amount of mucus produced, and the degree of ciliary activity. These processes may be influenced by a variety of diseases, including asthma, cystic fibrosis, and chronic bronchitis [82,83], In patients suffering from cystic fibrosis or chronic bronchitis, mucus hypersecretion is evident and mucociliary function is impaired. The failure to clear mucus from the airways leads to airway obstruction and to chronic colonization of the airways with bacterial organisms (which leads to lung infections and airway inflammation and damage). In asthmatic subjects, airway mucus is more viscous and ciliary transport mechanisms are inhibited [82,83]. In these diseases, the therapeutic objective is to improve mucus clearance from the airways. For example, aerosols of water or saline (especially hypertonic saline) promote clearance of mucus by... 

The dominant transport mechanism for both aerosol and gaseous agents in the atmosphere is advection associated with the bulk motion of the atmosphere. Since airflows in the planetary boundary layer exhibit signihcant turbulence under most conditions (though turbulence may be suppressed under conditions of temperature inversion), this will cause aerosol releases to disperse into a plume or puff that expands... 

In the sections that follow, mathematical methods for characterizing aerosol size and chemical properties are discussed. These are primarily of a definitional nature and are needed to provide a common basis for discussing the broad range of aerosol properties and behavior. However, aerosol characterization does not provide, directly, information on the mechanisms of aerosol formation, or temporal and spatial changes in the aerosol—that is, aerosol transport processes and aerosol tlynatnics. These and related topics are covered in later chapters. Advances in aerosol instrumentation have made it possible to measure many of the most important parameters necessary to characterize aerosols (Chapter 6). However, much rejnains to be done in developing aerosol instrumentation for research as well as industrial and atmospheric applications. 

Note that the magnitude of diffusion coefficients of gases is on the order of 0.1 cm2 s 1. Therefore a 0.1 pm particle diffuses in a quiescent gas roughly 10,000 times more slowly than a gas molecule, and Brownian diffusion is not expected to be an efficient transport mechanism for aerosols in the atmosphere. 

Diffusion is the primary transport mechanism of a nanoaerosol. The thermal velocities of nanoaerosol particles follow certain distribution due to the random motion of the surrounding gas molecules. The actual velocity distribution is not well determined, but most researchers assume it to follow the Maxwell-Boltzmaim distribution [1]. The diffusion coefficient of aerosol particles in the air with low Reynolds number is determined by the Stokes-Einstein equation... 

Persistent (slowly bio-degrading) organic chemicals, such as pesticides, PCBs, etc. occur throughout the environment, and in a matter of years, by many different transport mechanisms, they appear in all environmental compartments air, water, soil, sediment, aerosols, water-suspended particulates and vegetative and animal biota. Mackay [43] developed the so-called... 

The shared-cost action programme for 1989 included contracts for the development of ESTER, as described above. It also included the CEC s participation in the development of VICTORIA, a code from the USNRC which was originally part of MELPROG. VICTORIA calculates fission product release from the fuel and control rods, and its transport in dry conditions within the circuit as aerosols or vapours. The code models the aerosol transport and deposition mechanisms to be found e.g. in RAFT, but places much more emphasis on the chemistry. Two hundred chemical species or more can be considered in the equilibrium calculations, and the aerosols are treated as multicomponent i.e. aerosols of different chemical composition may have different size distributions. VICTORIA development for the CEC is undertaken by the UKAEA, and is part of a wider progranmie of development by laboratories in the US and Canada. 

As it can be seen, vapor depletion phenomena control FP retention before the failure of the core support plate, since temperatures of the RCS atmosphere are too high, what inhibits vapor nucleation and condensation onto Inert aerosols. On the contrary, core slump at the same time that FP vapors revolatilize promotes their conversion into the condensed phase therefore, aerosol generation and transport mechanisms determine the final escape of radioactivity to the auxiliary building atmosphere before vessel breach. 

The first purified and characterized drug substances were administered as aerosols as a topical treatment for asthma approximately 50 years ago. More recently, drugs have been evaluated for systemic delivery. For each category of drug the mechanism of clearance from the airways must be considered. These mechanisms may be listed as mucociliary transport, absorption, and cell-mediated translocation. The composition and residence time of the particle will influence the mechanism of clearance. 

Radon-222, a decay product of the naturally occuring radioactive element uranium-238, emanates from soil and masonry materials and is released from coal-fired power plants. Even though Rn-222 is an inert gas, its decay products are chemically active. Rn-222 has a a half-life of 3.825 days and undergoes four succesive alpha and/or beta decays to Po-218 (RaA), Pb-214 (RaB), Bi-214 (RaC), and Po-214 (RaC ). These four decay products have short half-lifes and thus decay to 22.3 year Pb-210 (RaD). The radioactive decays products of Rn-222 have a tendency to attach to ambient aerosol particles. The size of the resulting radioactive particle depends on the available aerosol. The attachment of these radionuclides to small, respirable particles is an important mechanism for the retention of activity in air and the transport to people. 

Cataldo DA, Ligotke MW, Bolton H, et al. 1989. Evaluate and characterize mechanisms controlling transport, fate and effects of army smokes in the aerosol wind tunnel. Pacific Northwest Laboratory ADA-215 415. 

Substances that can be inhaled include gases, vapors, liquid aerosols (both liquid and solid substances in solution), and finely divided powders/dusts (dust aerosols). Such substances may be absorbed from the respiratory tract or, through the action of clearance mechanisms, may be transported out of the respiratory tract and swallowed. This means that absorption from the gastrointestinal tract will contribute to the total body burden of substances that are inhaled. 

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