INFLUENCE OF RADIOACTIVITY ON PARTICLE-SURFACE INTERACTIONS
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In case of a nuclear plant accident or a deliberate explosion of a radiological dispersion device, human health and environmental risk assessments caused by radiation exposure are based on theoretical predictions of atmospheric dispersion of radionuclides. Because of large discrepancies between theory and observations after nuclear accidents, however, improving predictive modeling of radioactivity transport is a top priority. In general, it is assumed that the microphysical processes of atmospheric particles containing radionuclides are independent of electrostatic surface interactions. Radioactive decay of radionuclides, on the other hand, can lead to significant surface charging; therefore, electrostatic particle-surface interactions should be included in predictive models of radioactivity transport to reduce uncertainty in radiation risk assessments. This study investigates the influence of radioactivity on particle-surface interactions in the atmosphere. The investigations include (i) verification of radioactivity-induced charging mechanisms, (ii) evaluation of radioactivity-induced charging effects on particle size growth, and (iii) development of theoretical frameworks to couple the charging effects with the microphysical behavior of radioactive particles. Radioactivity-induced charging is examined using scanning surface potential microscopy. It is found that radioactivity induces charge accumulation on the surface of particles via emission of electrons and diffusion of negative and positive ions. These charging mechanisms should be included as competing terms in a charge balance. Analysis of the charge balance, including the charging mechanisms, suggests that radioactive particles can be highly charged in open air and thus may experience strong electrostatic interactions during short- and long-range transport. As an example, it is shown that because of surface charging caused by radioactive decay, the size growth rates of radioactive particles by aggregation can be significantly different from those of typical atmospheric particles. Systematical approaches with a wide range of complexity are developed to incorporate electrostatic interactions produced by radioactivity-induced charging into microphysical aerosol-transport models of any scale. Examples are given to show that the approaches developed in this work can also be applied to several investigations using radiation sources. This study contributes toward a better understanding of the microphysical behavior of radioactive particles in environmental systems, and the models developed can be employed to reduce the uncertainty in estimations of local and global radioactivity levels and the corresponding environmental and health risks. The results are useful in atmospheric dispersion modeling of radioactive plumes released by severe nuclear events, such as the recent accident of the Fukushima nuclear power plant in Japan.