Actinide assimilation

Modeling Conclusions. This exercise indicates that Th and U span the interface between the case where inhalation appears to dominate in the contribution of actinides in bone, and the case where ingestion is the more important pathway. The reasons for these differences lie in two important transfers The soil/sedi-ment to organism transfer (as it affects dietary concentrations) and the assimilation from the vertebrate GI tract. While terrestrial-derived foodstuffs dominate our diet, other components of the diet (i.e., aquatic-derived foods) may also make a contribution to intakes. Exceptions to the inhalation case may therefore occur in special populations where certain aquatic foods are consumed in greater amounts, i.e., shellfish (16). 

Assimilation from the Gastrointestinal Tract. There are several sources of variation in the assimilation of actinides from the mammalian intestine. Among the most important are chemical form and age of the animal. The comparative metabolism of different actinides administered to rats as nitrates has been studied by Sullivan and Crosby (25,26). The consistency of experimental technique in their administration of actinides to adult and newborn rats permits comparisons between the assimilation of different actinides. The fractional assimilation, expressed as the amount in liver and carcass together, 7 days post-dose, was approximately IO for isotopes of U, Pu, Am and Cm in adult rats. In young rats, the assimilation was about two orders of magnitude greater. 

Figure 3. Enrichment of trivalent actinides over Pu(IV) across biological membranes. The assimilation of Am-241 and Cm-244 from the rat GI tract is greater than for plutonium. When plutonium nitrate is inhaled by dogs, daughter Am-241 is preferentially transported to the liver, resulting in depletion in lung and lymph nodes.

Pu(IV), which forms highly charged polymers, strongly sorbs to soils and sediments. Other actinide III and IV oxidation states also bind by ion exchange to clays. The uptake of these species by solids is in the same sequence as the order of hydrolysis Pu > Am(III) > U(VI) > Np(V). The uptake of these actinides by plants appears to be in the reverse order of hydrolysis Np(V) > U(VI) > Am(III) > Pu(IV), with plants showing little ability to assimilate the immobile hydrolyzed species. The further concentration of these species in the food chain with subsequent deposit in humans appears to be minor. Of the 4 tons of plutonium released to the environment in atmospheric testing of nuclear weapons, the total amount fixed in the world population is less than 1 g [of this amount, most (99.9%) was inhaled rather than ingested]. 

Much of man s life-time exposure will be dependent on the assimilation of actinides by plants and on the eventual ingestion of vegetable foods by humans. 

In nearly all pot culture experiments, the presence of a chelator increased the uptake of Pu and Am by plants (33, 34, 3 , 36). The chelator likely favors mono-dispersed Pu forms and affects sorption of the elements to the solid soil phase. Because the chelator tends to promote mobility, it enables a greater fraction of the actinide to be assimilated by roots. The chelator is also able to complex soil Am directly and increase its uptake by plants (36). 

It is difficult to distinguish between the fraction of contaminant retained on the surface of leaves and the fraction assimilated metabolically into internal tissues of foliage. Such distinction may not be important if only concentration of the element in the food consumed by man or livestock is the important factor. In any case, chronic anthopogenic release of Pu to the atmosphere followed by direct contamination of foliage would lead to the highest levels of Pu in plants. Thus, incorporation of actinides into foods by this pathway followed by ingestion and deposition of Pu in bone and liver of human populations may result in levels of Pu in these organs that approach those derived from inhalation. 

The limited information on the plant uptake of other actinide elements (U, Np, Am, Cm) indicates that higher CR values can be expected relative to those observed for Pu. Values for Am based on uptake by roots and from deposition on foliage approach or exceed the 10-1 value used in the LMFBR assessment thus, the value used in dose assessments is probably realistic but not conservative. Price (32) reported CR values of 10-1 to 10-2 for 237Np assimilated by the root pathway. Based on these data and on the low K. for Np (Table II), it appears that this element exhibits a higher mobility than the other actinides. A potential CR > 10-1 due to uptake from soil and from direct contamination of foliage is hypothesized for Np. Curium-244 uptake by the root pathway yielded CR values of 10 3 to 10-1, according to pot culture experiments (32, 52). 

The array of information available on the uptake of actinide elements by plants permits qualified statements on the probable order of soil-to-plant transport. Plutonium exhibits the lowest level of uptake. Neptunium appears to be assimilated to the greatest extent, and U, Am, and Cm have intermediate values of... 

It appears that both the order of sorption of actinides to colloids and their order of uptake by plants is affected by oxidation state. The postulated order of uptake by plants (Np>Am=CmHI>Pu) is approximately the inverse of their order of hydrolysis and their order of adsorption of oxidation states to colloids [Pu(IV)>U(VI)2Cm(III)=Am(III)>Np(V)]. Based on reported adsorption results for Pu(VI) (48, 49), and on preliminary results from our sorption studies dealing with oxygenated-"yl" species, the position of Pu(VI) would be between U(VI) and Np(V). Results from one study (48) showed that Pu(VI) is assimilated by barley plants more readily than Pu(IV) or Pu(III). 

Recycling of refractory metals from wastes is an important issue today since they are in relatively low amount in the earth s crust and it should really be a substantial economy of expensive raw materials and also of energy. Molten salt electrolysis is proved to be appropriate to this function which can be assimilated in the case of metallic wastes to electrorefining. Today, waste treatment of other transition metals like actinides or lanthanides is a reality in the nuclear field, while other strategic elements such as silicon are expected to be recovered with a molten salt technology. 

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