Actinide accumulation

The experimental measurements of SNF bumup fraction is carried out by means of recording fission product gamma-radiation [11, 12] and neutron radiation of actinides accumulated in the fuel [11, 13]. If SRP s are stored in the frozen heavy metal, these measmements are impossible. For this reason the experimental methods are mainly used for measuring subcriticality of SRP s being stored. 

Figure 11.17 reflects the effect that actinide partitioning and transmutation has on the actinide hazard index of only the HLW itself. If the total quantity of actinides accumulated in the HLW and in the fuel cycle is considered, the same equilibrium reduction factor will eventually be attained provided that a constant nuclear power level is assumed, but it will take a very long time. In the fuel-cycle study performed for the American Physical Society [P2], an example with recycling the actinides to a LMFBR has been calculated that is shown in Rg. 11.18. 

The project of BRUS-150 integral type reactor of 500 MW thermal power and 150 MW electric power has been developed. The core, steam generators, pumps and all lead-bismuth loops are located in the reactor vessel, so that leak-proof vessel contains total amount of lead-bismuth. BRUS-150 reactor can be also used for transmutation of minor actinides accumulated in WWER type reactors and for utilization of weapon grade plutonium (for the main characteristics -see Table 2.2)... 

The overall distribution of lanthanides in bone may be influenced by the reactions between trivalent cations and bone surfaces. Bone surfaces accumulate many poorly utilized or excreted cations present in the circulation. The mechanisms of accumulation in bone may include reactions with bone mineral such as adsorption, ion exchange, and ionic bond formation (Neuman and Neuman, 1958) as well as the formation of complexes with proteins or other organic bone constituents (Taylor, 1972). The uptake of lanthanides and actinides by bone mineral appears to be independent of the ionic radius. Taylor et al. (1971) have shown that the in vitro uptakes on powdered bone ash of 241Am(III) (ionic radius 0.98 A) and of 239Pu(IV) (ionic radius 0.90 A) were 0.97 0.016 and 0.98 0.007, respectively. In vitro experiments by Foreman (1962) suggested that Pu(IV) accumulated on powdered bone or bone ash by adsorption, a relatively nonspecific reaction. On the other hand, reactions with organic bone constituents appear to depend on ionic radius. The complexes of the smaller Pu(IV) ion and any of the organic bone constituents tested thus far were more stable (as determined by gel filtration) than the complexes with Am(III) or Cm(III) (Taylor, 1972). 

It is significant that oat plants, which are known to contain the Fe3+ complexor, 2,4-dihydroxy-7-methoxy-l,4-benzoxazin-3-one (128), do not show any significant accumulation of plutonium, or the other actinides. It is possible that this complexing agent is located within plant cells which do not come into contact with the cation transporting mechanisms. Although there is evidence of the existence of microbial hy-droxamates in soil and that hydroxamates do become concentrated in plants (129), there has been no evidence presented yet that hydroxamates are the agents responsible for plutonium uptake into plants. On the other hand there is evidence that EDTA and DTPA can stimulate actinide concentration in plants (See Table 6). 

The apparent failure of trivalent and tetravalent cations to enter plants could result from the interaction of the cations with the phospholipids of the cell membranes. Evidence for such interactions is provided by the use of lanthanum nitrate as a stain for cell membranes (143) while thorium (IV) has been shown to form stable complexes with phospholipid micelles (144). However, it is possible that some plant species may possess ionophores specific to trivalent cations. Thomas (145) has shown that trees such as mockernut hickory can accumulate lanthanides. The proof of the existence of such ionophores in these trees may facilitate the development of safeguards to ensure that the actinides are not readily transported from soil to plants. 

From an examination of Tables 7a and b it is apparent that in very young animals there is an increased uptake of actinides across the gastro-intestinal tract. It is known than in suckling animals there is a more rapid accumulation of iron than there is in adult animals. Presumably actinide uptake is a result of entrainment in the iron transporting processes. 

This competition, manifested in the structure of the actinide atomic spectra, also appears as a competition between configurations there is an accumulation of many configurations, both even and odd, at about the same height, leading to a high density of energy levels (in fact, we had arrived at the same conclusion by inspecting Fig. 1 in Sect. A.I.2). 

Non-stoichiometry is a very important property of actinide dioxides. Small departures from stoichiometric compositions, are due to point-defects in anion sublattice (vacancies for AnOa-x and interstitials for An02+x )- A lattice defect is a point perturbation of the periodicity of the perfect solid and, in an ionic picture, it constitutes a point charge with respect to the lattice, since it is a point of accumulation of electrons or electron holes. This point charge must be compensated, in order to preserve electroneutrality of the total lattice. Actinide ions having usually two or more oxidation states within a narrow range of stability, the neutralization of the point charges is achieved through a Redox process, i.e. oxidation or reduction of the cation. This is in fact the main reason for the existence of non-stoichiometry. In this respect, actinide compounds are similar to transition metals oxides and to some lanthanide dioxides. 

Figure 4. Comparative accumulation of actinides by small mammals from contaminated soil or sediment relative to the accumulation of plutonium-239. Accumulation factor (AF) = concentration of nuclide in the internal small mammal body -- concentration of nuclide in dry soil. Twelve shrews and seven rats and mice from a floodplain forest were composited to yield four and three separate analyses, respectively. Twelve cotton rats inhabiting the banks of a liquid waste pond (3513) also were analyzed.

Table VI. The accumulation of actinides by small mammals and man from soil expressed as the concentration ratio of element in...

Summary. Through an examination of the comparative behavior of the actinide elements in terrestrial and aquatic food chains, the anticipated accumulation behavior of the transuranium elements by people was described. The available data suggests that Pu, Am and Cm will not accumulate to a greater degree than U in the skeletons of individuals exposed to environmentally dispersed activity. The nature of the contamination event, the chemical and physical associations in soils and sediments, the proximity to the... 

TBP is a sufficiently powerful extractant for actinides that it may be used in diluted form. Dilution improves the hydrodynamic properties of the solvent, allowing more complete and rapid phase disengagement. Typically concentrations of 20-30 v/o TBP in OK are used in process flowsheets. Although TBP is relatively stable as an extractant, radiolysis does lead to the formation of some acidic phosphate esters, HDBP and H2MBP, which can impair process performance.289 An aqueous alkali wash of the recycled solvent is usually carried out to remove those by products. Radiolytic degradation of the diluent in the presence of nitric acid can result in the formation of hydroxamic acids,290 which can lead to fission product retention by the organic phase. Again the solvent wash is used to prevent the accumulation of such species. A comprehensive account of the industrial utilization of TBP has recently been published.291... 

Because there are few data on the results of human exposure to actinides, the health effects of these radioelements are more uncertain than those discussed above for ionizing radiation, radon, and fission products. Americium accumulates in bones and will likely cause bone cancer due to its radioactive decay. Animal studies suggest that plutonium will cause effects in the blood, liver, bone, lung, and immune systems. Other potential mechanisms of chemical toxicity and carcinogenicity of the actinides are similar to those of heavy metals and include (i) disruption of transport pathways for nutrients and ions (ii) displacement of essential metals such as Cu, Zn, and Ni ... 

Suzuki and Banfield (1999) discuss the similarities between the uranium-microbe interactions and transuranic-microbe interactions. Macaskie (1991) notes that it is possible to extrapolate the data for microbial uranium accumulation to other actinides. Hodge et al. (1973) observe that the biological behavior of uranium, thorium, and plutonium resemble that of ferric iron. Microbes can also affect the speciation and transport of multivalent fission products. For example, Fe " -reducing bacteria and sulfate-reducing bacteria can reduce soluble pertechnetate to insoluble Tc(IV), as discussed by Lloyd et al. (1997). For additional information about these topics, the reader is referred to the references cites above. Applications of these principles are described in the section on bioremediation later in this chapter. 

Neptunium-237 Np-237 has two production routes. The first is rapid /3 decay of U-237 produced by nuclear reactions in the fuel. The second route is /3 decay of Pu-241 to Am-241, which decays losing an a particle to Np-237. This second route will lead to the in-growth of Np-237 in the waste during storage. Np-237 will only be present in waste contaminated with actinides. This is demonstrated by the waste streams that were found to exceed the GQ level. They were IX resin and sludge from BWA and SPF eontents from HPA. Adequate estimates of Np-237 can be made by ratio to Cs-137 determined by FISPIN in the first instance and then refined by taking account of the accumulation and chemistry of other transuranics already measured. At higher concentrations, it can also be measured by y spectrometry of the short-... 

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