Distribution of americium

Various cases of internal exposure to americium have been reported in which the exposures resulted from skin punctures with materials also containing plutonium. Information on the distribution of americium in these cases has been derived from the analysis of autopsy tissues. In most cases, the largest fraction of the 241 Am activity measured in the body was associated with tissues near the puncture wound. In one case,... 

The Mewhinney and Griffith (1983) model was developed to predict lung retention and tissue distributions of americium in people who may be exposed to americium. Descriptions of applications of the model in risk assessment have not been reported. 

Distribution. Bone constitutes the largest fraction of the deposited body burden of americium in all mammalian species that have been studied. The mechanisms by which americium is taken up and retained in bone are only partially understood. The distribution of americium in bone initially is confined to bone surfaces, including endosteal and periosteal surfaces, and adjacent to vascular canals in cortical bone (Polig 1976 Priest et al. 1983, 1995 Schlenker et al. 1989). Deposition appears to be favored at sites of active... 

Biotic Transport Biotic transport can be defined as the actions of plants and animals that result in the transport of a radioactive material or other substance from a waste site to locations where it can enter pathways that may result in exposure to humans. Small mammals are ubiquitous and inhabit areas containing radioactive contamination or radioactive waste sites. Mammals inhabiting these areas may become contaminated with americium by consuming contaminated soil or plants and disturb americium-contaminated soil through their burrowing and excavating activities. These animals may therefore affect the distribution of americium within the waste site or transport americium to previously uncontaminated areas. In addition, small mammals may be consumed by animals higher in the food chain such as hawks and coyotes, which would add to the dispersal of americium from disposal areas. However, results of... 

Bioavailability from Environmental Media. The absorption and distribution of americium as a result of inhalation and ingestion exposures have been discussed in Sections 3.3.1 and 3.3.2. EPA lists identical uptake factors for inhaled and ingested americium (and all the other transuranics other than plutonium) regardless of compound solubility, indicating that the knowledge base for americium is not sufficiently developed to quantify the differences that are recognized for most other elements. 

One of the limitations of the portable field survey instruments in the measurement of americium is that their quantitative accuracy depends on how well the lateral and vertical distribution of americium in the soil compares with the calibration parameters used. These methods can provide a rapid assessment of americium levels on or below surfaces in a particular environment however, laboratory-based analyses of samples procured from these environmental surfaces must be performed in order to ensure accurate quantification of americium (and other radionuclides). This is due, in part, to the strong self absorption of the 59.5 keV gamma-ray by environmental media, such as soil. Consequently, the uncertainty in the depth distribution of americium and the density of the environmental media may contribute to a >30% error in the field survey measurements. Currently, refinements in calibration strategies are being developed to improve both the precision and accuracy (10%) of gamma-ray spectroscopy measurements of americium within contaminated soils (Fong and Alvarez 1997). 

Boocock G, Popplewell DS. 1966. In vitro distribution of americium in human blood serum proteins. Nature 210 1283-1284. 

Durbin PW, Schmidt CT, Mclnroy JF, et al. 1985. Estimation of initial distribution of americium in adult male human skeleton. Health Phys 49(1) 162. 

Hammarstrom L, Nilsson A. 1970a. Radiopathology of americium 241 I. Distribution of americium in adult mice. Acta Radiol 9 433-442. 

Schmidt CT, Durbin P. 1985. A five-compartment model for the kinetic distribution of americium in man. Health Phys 49(1) 161. 

Taylor GN, Jee WSS, Dockum N, et al. 1969. Microscopic distribution of americium-241 in the beagle thyroid gland. Health Phys 17 723-725. 

After the twenty-four hour period of contact, the solution in the fissures was rapidly withdrawn. The surfaces of the fissures were dried under vacuum at 25 °C and then dismantled. The distributions of americium on the fissure surfaces were determined first qualitatively using autoradiographs of the fissure surfaces with POLAROID LAND black and white 3000 ASA type 47 film. The autoradiographs are shown in Figures 5, 7, and 9. The lighter areas on the autoradiographs represent areas of americium adsorption. 

The distributions of americium on the fissure surfaces were then quantitatively determined by scanning the face of each fissure with a Nal scintillation crystal through a 0.3 cm slit in lead shielding. The 59 keV gamma ray emitted by Am was monitored. Histograms of the americium distributions on the fissure surfaces were produced and are presented in Figures 5, 7, and 9. 

The fissures were dried under vacuum at 25°C and were dismantled. The distributions of americium on the fissure surfaces in the second set of experiments were determined as they were in the first set, that is first qualitatively by autoradiography and then quantitatively by gamma scanning the fissure faces through a 0.3 cm slit. 

Autoradiographs of the fissure surfaces from the second set of experiments are presented in Figures 6, 8, and 10. Histograms representing the quantitatively determined distribution of americium on the fissure surfaces are also presented in Figures 6, 8, and 10. 

Figure 5. Distribution of americium on fissure surface, experimental and predicted, after 0.67 fissure volume elution. Flow rate of 1.13 cm/hr. (A), Americium distribution on surface of fissure (B), model prediction of americium on surface of fissure (C), autoradiograph of fissure surface showing americium distribution.

Model Predictions. The rate for desorption of americium from the fissure surfaces into solution was assumed to equal the rate for the adsorption of americium from solution by the fissure surfaces. The sorption rate and the equilibrium fractionation of americium that were determined in the static experiments were used to determine input parameters to the ARDISC model. The ARDISC model predictions for the distributions of americium on the fissure surfaces in both sets of experiments are presented in Figures 5 through 10 along with the autoradiographs and the experimental histograms representing the various distributions of americium on the fissure surfaces. 

Figures 6, 8, and 10 show that the distributions of americium on the fissure surfaces after the addition of 0.67 fissure volumes of stock solution followed by the elution of 20 fissure volumes of "schist -equilibrated water through the fissures at flow rates of 1.13, 2.29, and 4.77 cm/hr. A comparison was made between the americium distributions found on the fissure surfaces after the addition of americium stock solution and that found after elution of the americium by 20 fissure volumes of "schist"-equilibrated water. It was found that after the initial loading of americium into the fissures in the first 0.67 fissure volumes of solution, the peak concentrations of americium that were sorbed at the top of the fissures decreased in their relative concentration. The leading edges of the detectable nuclide concentration extending into the fissures had increased in length and relative concentration with subsequent elution by rock equilibrated water through the fissures. The ARDISC model predicted the same relationships.

Ferris and his co-workers [30] determined the equilibrium distribution of americium (and other transuranium elements) between liquid bismuth and molten LiCl, LiBr, and several LiF-BeF2-ThF4 solutions at temperatures of 60O-75O°C. Some of the americium appeared to be in the divalent state in the Am/PuClj system [29]. The distribution coefficient, D — (gAm/g metal phase)/(g Am/g salt phase), of americium between molten aluminum metal and molten AlClj-KCl is 1.96 [31],... 

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