Radionuclides behaviour after deposition

Initial Behaviour of Radionuclides after Deposition on Foliage. 195... 

Following a nuclear accident, deposited radionuclides may be present in different physico-chemical forms, ranging from mobile low molecular mass (LMM) ionic species to inert high molecular mass (HMM) colloidal forms or particles. Even in areas far from the actual site, the relative fraction of radionuclides associated with HMM formed in rain-water may be substantial (Salbu, 1988). The size distribution patterns of radionuclides deposited, the composition of the fallout, level of activities and the activity ratios, will depend on the accident scenario, course of event, distance from the source, wind dispersion and climatic or microclimatic conditions. Spatial and temporal variations in the behaviour of deposited radionuclides with respect to mobility and bioavailability are to be expected and may in part be attributed to differences in the physico-chemical forms of radionuclides in the fallout, at least during the first years after deposition (Salbu et al., 1994). 

Most models assessing the long-term behaviour of Cs and Sr from fallout include processes relevant to LMM ionic species only. However, radionuclides may be associated with particles due to (a) release of fuel matrix or clusters, (b) condensation of volatiles on available particle surfaces after the release of (c) interactions with aerosol particulates during atmospheric transport. For volatile radionuclides (e.g., Cs, °Sr), all three mechanisms may be equally relevant, while the deposition of nuclides of refractory elements (e.g., " Ce, Nb, Zr) indicates the release of fuel particles. For fuel particles, depletion of volatiles relative to refractory elements would be expected to depend on the temperature reached during the releases, whereas the activity ratios for refractory elements should reflect the reactor fuel bum-up. 

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