Transplutonium actinide elements

The research programme of the European Institute for Transuranium Elements was, from its very beginning, devoted to both basic research on advanced plutonium containing fuel and to fundamental research on actinide elements. Non-fuel actinide research in Europe started more than 20 years ago with the reprocessing of irradiated actinide samples. Since the first isolation and purification of transplutonium elements, actinide research developed steadily in close contact and cooperation with specialised laboratories in Western Europe and in the United States. 

The oxidation-reduction behavior of plutonium is described by the redox potentials shown in Table I. (For the purposes of this paper, the unstable and environmentally unimportant heptavalent oxidation state will be ignored.) These values are of a high degree of accuracy, but are valid only for the media in which they are measured. In more strongly complexing media, the potentials will change. In weakly complexing media such as 1 M HClOq, all of the couples have potentials very nearly the same as a result, ionic plutonium in such solutions tends to disproportionate. Plutonium is unique in its ability to exist in all four oxidation states simultaneously in the same solution. Its behavior is in contrast to that of uranium, which is commonly present in aqueous media as the uranyl(VI) ion, and the transplutonium actinide elements, which normally occur in solution as trlvalent... 

In contrast to the early actinide compounds already discussed, the iodates of the transplutonium actinides that have been prepared contain trivalent oxidation states for the actinide elements. For Am, several higher oxidation states are possible, but attempts to prepare Am(V) or Am(VI) analogs of these iodates... 

The acid dependency observed in practice hal been only approximately inverse third power. Impurities in Cleanex feed solutions often cause a departure from ideality (e.g., by common-ion effect or by consumption of some of the HDEHP), and we have not been able to control the extraction of the actinide elements solely by monitoring the aqueous-phase acidity. Fortunately, when processing transplutonium elements, the high specific activity of 21+I+Cm facilitates the detection of that isotope in both phases, thus permitting a rapid determination of the degree of extraction. The extraction coefficients of the trivalent actinides and lanthanides are all quite similar, so the 21+1+Cm serves as an excellent marker for all the extracted ions. 

O Table 18.9 gives oxidation states of actinide elements (Katz et aL 1986). Actinium and transplutonium elements (from Am to Lr) take 3+ as the most stable oxidation state and they behave similar to the lanthanide elements, except element 102, No, which seems to prefer the 2+ state. Because of the itinerancy of the 5f electrons, the lighter actinide elements take broad range of oxidation states. The light transuranium elements, Np, Pu, and Am can behave as 3+ to 7+ cations and the most stable oxidation states of these three elements are 5+, 4+, and 3+, respectively. 

It has been more than fifty years since the discovery of the transuranium elements. The initial activities in this field established the fundamental solution and solid-state chemistry of the first two of these elements and their compounds under the auspices of the Manhattan Project. New separation methods including solvent extraction techniques and uranium isotope separation played a leading role in these programs. Tracer techniques were widely used to determine solubilities (or solubility liinits) of transuranium compounds as well as to obtain information about the coorination chemistry in aqueous solution. A little later, special solvent extraction and ion-exchange techniques were developed to isolate pure transplutonium elements on the milligram and smaller scale. The second edition of The Chemistry of the Actinide Elements, published in 1986 (i), covers most of these topics. A detailed overview of the history of transuranium chemistry is given in Transuranium Elements A Half Century (2). 

The separation of the lanthanides from thorium, uranium, plutonium, and neptunium can fairly readily be achieved by exploiting the greater extractability of the higher oxidation states of the light-actinide elements. However, the transplutonium actinides do not have stable higher oxidation states. In this case, separation of the lanthanide fission products from the transplutonium actinides must exploit the small differences in the solution chemistry of the lanthanides and actinides in the trivalent oxidation state. It is the separation of the lanthanides from the trivalent actinide cations that is the focus of this chapter. 

The strong similarity in the solution chemistry of the 4f and the 5f elements is most evident for the trivalent oxidation state of both families. The discovery experiments of the transplutonium actinides depended directly on this similarity as it allowed very accurate predictions of the chemical properties of the to-be-discovered elements. However, the actinide series is not an exact analog of the lanthanide elements. For example, while the stability of the trivalent oxidation state is a primary characteristic of all the lanthanide elements, trivalency is not the most stable state for the actinide elements of Z = 90-94 and 102. The greater stability of Nof q, relative to No, j,+ is not observed in the 4f analog although Yb " can be present in reducing systems. Such differences are related to the dififerences in the relative energies of the (n)f, (n -I- l)d and (n -I- 2)s orbitals when n = 4 (Ln) and 5 (An). 

The fate of actinide elements introduced into the environment is of course not merely a scientific issue. The disposal of the by-products of the nuclear power industry has become a matter of public concern. For each 1000 kg of uranium fuel irradiated in a typical nuclear reactor for a three-year period, about 50 kg of uranium are consumed. In addition to a large amount of energy evolved as heat, 35 kg of radioactive fission products and 15 kg of plutonium and transplutonium elements are produced. Many of the fission-product nuclides are stable, but others are highly radioactive. All of the fission products are isotopes of elements whose chemical properties are well-understood. The transuranium elements produced in the reactor by neutron capture, however, have unique chemical properties, which are reasonably well-understood but are not always easily inferred by extrapolation from the chemistry of the classical elements. Plutonium is fissile and can be recycled as a nuclear fuel in conventional or breeder reactors, but the transplutonium elements are not fissile to the extent of supporting a nuclear chain reaction, and in any event they are produced in amounts too small to be of interest for large-scale uses. The transplutonium elements must therefore be secured and stored. 

Tricyclopentadienide complexes of many of the actinides are known (Ac = Th, U, Pu, Am, Cm, Bk, Cf). Indeed, these are the only cyclopentadienide complexes known for the transplutonium elements, where -(-3 is the most stable oxidation state. The transplutonium elements were all prepared by a microchemical procedure which utilized a melt of biscyclopentadienyl beryllium (6) according to ... 

Recently there has been interest in the sorptive behavior of natural clays toward metal ions potentially present in radioactive wastes. Initial studies of the transplutonium elements have been carried out to define their sorption behavior with such materials ( ). However, it is also important to understand the stability of the clay-actinide product with regard to radiation damage and to be able to predict what changes in behavior may occur after exposure to radiation, so that accurate transport models may be constructed. 

Today, it is accepted that lanthanides (4/elements) and transplutonium actinides (5/ elements) possess relatively similar physical and chemical properties (28-31, 63) including ... 

The ZEALEX Process Researchers from KRI have shown that the zirconium salt of dibutyl phosphoric acid (ZS-HDBP) was soluble in Isopar-L in the presence of 30% TBP. This super PUREX solvent, known as ZEALEX, extracts actinides (Np-Am) together with lanthanides and other fission products, such as Ba, Cs, Fe, Mo, and Sr from nitric acid solutions. The extraction yields depend on both the molar ratio between Zr and HDBP in the 30% TBP/Isopar-L mixture and the concentration of HN03 (232). Trivalent transplutonium and lanthanide elements can be stripped together from the loaded ZEALEX solvent by a complexing solution, mixing ammonium carbonate, (NH4)2C03, and ethylenediamine-N.N.N. N -tetraacetic acid (EDTA). An optimized version of the process should allow the separation of... 

Scientific Committee of International Conferences First International Transplutonium Element Symposium, Argonne, IL, USA, 1963 Third International Transplutonium Element Symposium, Argonne, IL, USA, 1971 Fifteenth International Conference on Coordination Chemistry, Moscow, USSR, 1973 First International Symposium on the Electronic Structure of the Actinides, Argonne, IL, USA, 1974... 

Treatment of irradiated targets. The chemical operations relative to the production of transplutonium elements (americium 243, curium 244) are all performed using a nitric acid medium. The highly corrosive nature of the solutions concentrated with Cl" ions, which were used in the USA for the development of the Tramex process (JO, and the instability of SCN" ions to radiation (12), led us to select nitric acid solution to perform the chemical separations. Once the medium was selected, it was necessary to find an adequate additive which, in combination with a suitable extractant, would allow solution of the main problem namely separation of the trivalent actinides from triva-lent lanthanides. 

Collins, E. D. Benker, D. E. Chattin, F. R. Ore, P. B. Ross, R. G., "Multigram Group Separation of Actinide and Lanthonide Elements by Li Cl-Based Ion Exchange", paper presented at Symposium on Industrial-Scale Production-Separation-Recovery of Transplutonium Elements, 2nd Chem. Congr. North American Continent, Las Vegas, NV, 1980. 

The transplutonium elements and the rare earths, or lanthanides, are so similar chemically that what is true for one group is generally true for the other. In practice, process development work is usually carried out with lanthanides, and frequently, all the solutions end up as analytical samples. Transplutonium elements, in contrast, are so valuable that the goal is the maximum yield of pure products. Accordingly, the methods and equipment developed with rare earth separations are applied directly to heavy actinide production separations. These may be quite small in scale, but this is "production" for some of these elements. 

One possible application in which large amounts of rare earths and actinides would be processed occurs in some schemes for nuclear waste management. If it should prove to be advantageous to remove transplutonium elements from nuclear waste, for example, the recovery of Am and Cm from the much larger amounts of rare earths would be required. This problem has been investigated by the author in tracer tests with rare earth mixtures typical of fission products, using a heavy rare earth such as holmium as a stand-in for Am and Cm (Fig. 5). It is clear that the bulk of the holmium can be recovered in reasonable purity, and that the bulk of the lighter rare earths is effectively separated from the very small amount of heavy rare earths, Am, and Cm. 

September 13—17, 1975 W. Mbllei and H. Blank, eds., Heavy Element Properties, 4th International Transplutonium Element Symposium, 5th International Conference on Plutonium and Other Actinides 1975, Proceedings of the Joint Session of the Baden Baden Meetings September 13, 1975, Nortli-Holland Publishing Co., Amsterdam,. American Elsevier Publishing Co., Inc., New York. 

Choppin, G. R., and Unrein, P. J. Thermodynamic study ot actinide fluoride complexation, p. 97-107, in Muller, W. and Lindner, R., eds., "Transplutonium Elements," North-Holland Publ. Co., Amsterdam, 1976. 

Extraction processes (TRUEX, PUREX, Talspeak, DIAMEX, PARC, etc.) generally involve complexation of transplutonium elements by alkyl phosphines, phosphine oxides, phosphoric acids, carbamoyl phosphonates, diamides, and thiophosphinates in aqueous/organic extractions, within derivatized solid supports, or on coated particles. There are excellent reviews of the processes and significant complexes by Mathur et al. and selected chapters in The Chemistry of the Actinide and Transactinide Elements to be published in 2003. " Work on the separation for nuclear waste management in the United States, France, and Russia have been reviewed. " ... 

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