Americium distribution

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. 

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.

Figures 5, 7, and 9 show the americium distributions on fissure surfaces after the administration of 0.67 fissure volumes of stock solution at flow rates of 1.13, 2.29, and 4.77 cm/hr. The peak concentrations of americium were adsorbed at the top of the fissures and leading edges of americium were extended into the fissures. In general, at faster flow rates the relative quantities of americium sorbed at the top of each fissure decreased and the length and relative amount of the leading edge extending into each fissure increased.

The numerical values of the extraction factor (a) are set by selecting the desired separation, the mode of extraction, and the number of extraction stages. For example, the value of a required for 90% americium removal by two stage countercurrent extraction is a = 2.54 (see Table I). As shown previously, the value of the americium distribution coefficient is a function of the salt composition i.e., the MgCl2 content of the salt and the composition of the diluent salt system. The value of the salt-to-metal ratio is set by the weight of salt and metal fed to the extraction. 

The usual effect of increasing the acid concentration is reported to be an increase in the (due to increased amounts of the extractable MA3 in the aqueous phase) followed by a decrease in the (due to formation of the extractant-HNC adduct), resulting in a maximum extraction at an acid concentration between 2 and 6 M. However, one study has noted an increase in americium extraction at nitric acid concentrations from 12 to 16 M. These data are not consistent with the usual view of americium distribution dependence on nitric acid and nitrate concentration, and the authors hypothesize that a TBP HN03 adduct, which is a stronger extractant for americium than TBP alone, is formed above 8 M HNO3 and an organic-phase complex of Am(N03)3 (TBP mHNC ) is formed rather than Am(N03)3 nTBP (26). While one may not absolutely discount this possibility, additional factors such as the extraction of HAm(N03) and deviations from ideal activities in such concentrated acid solutions should definitely be considered. 

Influence of oxidizing reagents on uranium (VI) and americium distribution coefficients. 

Am(iii) is sorbed much more strongly onto anion-exchange resins from concentrated lithium chloride solutions than are the lanthanides [61], Americium distribution ratios increase with increased lithium chloride concentration (Fig. 8.1), whereas increased temperature enhances the separation of americium from rare earths. A lithium-chloride-based anion-exchange process for separating multigram amounts of americium and curium from lanthanide fission products and to isolate an Am-Cm fraction free of heavier actinides is routinely operated at the Oak Ridge facility [14]. 

Each of the elements has a number of isotopes (2,4), all radioactive and some of which can be obtained in isotopicaHy pure form. More than 200 in number and mosdy synthetic in origin, they are produced by neutron or charged-particle induced transmutations (2,4). The known radioactive isotopes are distributed among the 15 elements approximately as follows actinium and thorium, 25 each protactinium, 20 uranium, neptunium, plutonium, americium, curium, californium, einsteinium, and fermium, 15 each herkelium, mendelevium, nobehum, and lawrencium, 10 each. There is frequently a need for values to be assigned for the atomic weights of the actinide elements. Any precise experimental work would require a value for the isotope or isotopic mixture being used, but where there is a purely formal demand for atomic weights, mass numbers that are chosen on the basis of half-life and availabiUty have customarily been used. A Hst of these is provided in Table 1. 

Figure 7. Distribution Coefficient for Americium Extraction vs MgC12 Concentration in the NaCl-KCl-MgCl2 and NaCl-CaCI2 MgC12 Salt Systems.

Durbin and Schmidt (1985) Model of Distribution and Excretion of Absorbed Americium in the Human... 

Several cases of accidental exposure to americium as a result of wound penetrations have been reported (Thompson 1983). These exposures have resulted in241 Am burdens in the liver and skeleton, indicating absorption and distribution from the wound site (Mclnroy et al. 1989). 

Information on the distribution of absorbed americium to mammary milk in humans is not available. Numerous studies in animals have shown that transfer to milk occurs and that neonates can be exposed to americium during lactation. These studies include experiments in which the animals were exposed by... 

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,... 

Skeletal deposition is assumed to distribute into two pools 50% goes to the trabecular bone surface and 50% to the cortical bone surface. A first order rate coefficient for elimination of americium from liver to plasma is assumed to be 0.0019 day 1 (half-time, 365 days). 

Excretion is assumed to be in feces, from either the intestine or from liver to the intestine, and from blood to urine (i.e., the kidney is represented as a distribution compartment). An absorption fraction of 0.0005 is assumed for americium in the small intestine... 

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. 

Durbin and Schmidt (1985) proposed a model for tissue distribution and excretion of absorbed americium in humans. A unique feature of this model is that transfers from plasma to tissues are assumed to be instantaneous therefore, a central plasma (and blood) compartment is not included in the model (see Figure 3-10). Tissue compartments included in the model are slow and fast turnover bone compartments, representing cortical and trabecular bone, respectively liver and slow and fast turnover for other soft tissue compartments. Excretion pathways include urine and feces. Urinary excretion is represented as a sum of the contributions from bone, liver, and other soft tissues. Fecal americium is assumed to be excreted from the liver. 

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... 

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