Plutonium resuspension

Anthropogenic (man-made) releases of plutonium are the primary sources of plutonium to the atmosphere. Atmospheric testing, fires involving plutonium-containing materials, and routine releases due to normal activities at processing and generating plants are all potential sources of airborne plutonium. Resuspension of plutonium sorbed to contaminated surface soils via fugitive dust emissions is an indirect pathway by which plutonium may be re-released into the atmosphere (Harley 1980). 

Garcia-Olivares, A. Iranzo, C. E. 1997. Resuspension and transport of plutonium in the Palomares area. Journal of Environmental Radioactivity, 37, 101-114. 

Plutonium is deposited on plant tissues by fallout and by resuspension from soil. Pinder et al. (1985) and Pinder McLeod (1988) measured 238Pu and 239+240pu in topsoil and in corn and sunflower foliage near the Savannah River Plant. Because the ratio of Pu isotopes in contemporary fallout was different from that in soil, Pinder et al. were able to estimate how much of the Pu on the foliage derived from the soil. They found that, at harvest, resuspended soil on foliage amounted to about 0.8 g per m2 of ground area. 

Most radioactive particles and vapours, once deposited, are held rather firmly on surfaces, but resuspension does occur. A radioactive particle may be blown off the surface, or, more probably, the fragment of soil or vegetation to which it is attached may become airborne. This occurs most readily where soils and vegetation are dry and friable. Most nuclear bomb tests and experimental dispersions of fissile material have taken place in arid regions, but there is also the possibility of resuspension from agricultural and urban land, as an aftermath of accidental dispersion. This is particularly relevant to plutonium and other actinide elements, which are very toxic, and are absorbed slowly from the lung, but are poorly absorbed from the digestive tract. Inhalation of resuspended activity may be the most important route of human uptake for actinide elements, whereas entry into food chains is critical for fission products such as strontium and caesium. 

Anspaugh et al. (1975, 1976) studied resuspension at the GMX location on the Nevada Test Site, where plutonium was disseminated by small non-nuclear explosions about 30 years ago. To obtain representative values of the resuspension factor, ideally a large and uniform area of deposited activity is required, in order that there should be a constant flux layer in the air near the ground. The fallout of Pu at the GMX site is non-uniform, so Anspaugh et al. analysed their measurements of x in relation to a model calculation of the concentration expected from the areal source. 

Resuspension of dust from the floor is an important source of indoor pollution. Jones Pond (1966) studied resuspension of plutonium from the floor of a laboratory which had been deliberately contaminated. Droplets were dispersed over the floor and allowed to dry out. Pu was present in the droplets either as a suspension of particles, mass median diameter 15 /mi, or as nitrate in solution. The activity per unit area was measured with a floor probe. After 16 h had elapsed, airborne Pu was measured while operators performed various tasks in the laboratory. 

Individuals living in Palomares, Spain, were exposed to plutonium after the dispersal of the plutonium in two bombs released during the midair collision of two airplanes (Iranzo et al. 1987). Exposure via inhalation due to the resuspension of contaminated soil was studied for 15 years following the release. Those individuals living near cultivated lands with the highest contamination received a cumulative total of 52.3 mrem (5.2x10 mSv) from 1966 to 1980 while those in the urban area of Palomares, farther away from the source, received 5.4 mrem (5.4x10 mSv) (Iranzo et al. 1987). 

Depending on the nature of the release, it may be advisable to set up ground based air samplers after a release has occurred to monitor for the presence of fallout and resuspended radionuclides. The resuspension of radionuclides does not usually give rise to an important pathway of exposure however, it can do so for plutonium or other actinides. 

In unpaved areas, long lived radionuclides gradually penetrate into the soil, which prevents their resuspension in the air. In the long term the sampUng and analysis of airborne radionuclides should therefore be regularly performed mainly in inhabited areas contaminated with plutonium and other actinides. 

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. 

Plutonium aerosols can be formed in various ways, including (a) oxidation or volatilisation of Pu metal, (b) oxidation or volatilisation of irradiated U or UO2, (c) droplet dispersion from aqueous solutions or suspensions of Pu, and (d) resuspension of soil or dust which has become contaminated with Pu. The particle size of Pu-aerosols is very variable, depending on the mode of formation. 

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