Separating actinide elements from

The Au foil Is quickly dissolved In aqua regia and the Au extracted with ethyl acetate for 1 min. The aqueous phase is then eluted through a Dowex-1 anion resin column (2 zmn diameter, 1 cm long) with 6 M HC1 to complete the removal of Au end other impurities. The dropB. containing the actinide fraction are evaporated and the activity is then eluted through a Dowex-50 resin cation column with 0.4 M soln. of ammonium Q -hydroxy-isobutyrate, which is adjusted to pH 4.0 with HH4OH to separate the various actinide elements from each other, (Mv-Fm-E-Cf)... 

Perchlorates and iodates. Thorium perchlorate forms upon dissolution of thorium hydroxide in perchloric acid and crystallizes as Th(C104)4 4H20. The precipitation of tetravalent actinides as iodates has long been used to separate these elements from lanthanides at low pH. One of the earliest forms that Pu was isolated in was that of Pu(I03)4. The structure and most properties of Pu(103)4 are currently unknown, but a remarkable feature is that it is insoluble in 6M HNO3. 

Origins. Most of the radioactive waste at SRP originates in the two separations plants, although some waste is produced in the reactor areas, laboratories, and peripheral installations. The principal processes used in the separations plants have been the Purex and the HM processes, but others have been used to process a variety of fuel and target elements. The Purex process recovers and purifies uranium and plutonium from neutron-irradiated natural uranium. The HM process recovers enriched uranium from uranium—aluminum alloys used as fuel in SRP reactors. Other processes that have been used include recovery of and thorium (from neutron-irradiated thorium), recovery of Np and Pu, separation of higher actinide elements from irradiated plutonium, and recovery of enriched uranium from stainless-steel-clad fuel elements from power reactors. Each of these processes produces a characteristic waste. 

An extraction process for separating actinide elements (principally uranium, U, and plutonium, Pu) from fission products in an aqueous solution of spent fuel rods is illustrated in Figure 5.31. The extraction solvent is 30% tributyl phosphate (TBP) in kerosene. The most extractable of the fission products are zirconium, niobium and ruthenium. Zirconium, Zr, is used herein to represent the fission products. Determine the number of stages required in the wash section and in the extraction section. Determine the percentage of the Pu in the feed which is recovered in the extract product. V denotes the relative volumetric flowrate. 

The main problems in the separations are (a) separation of the actinides as a group from the lanthanide ions (which are formed as fission fragments in the bombardments which produce the actinides) and (b) separation of the actinide elements from one another. 

The problems of practical separation of these elements are (1) the separation of actinide elements fi om the lanthanide group (trivalent actinide ions behave like lanthanide ions), and (2) the separation of the actinide elements from each other. Ion-exchange and solvent extraction methods have so far been extensively studied. 

Nitrogen-based extractants, especially tertiary amines and quaternary ammonium compounds, are particularly effective in separating and recovering americium and other actinide elements from aqueous media. 

The Soviet scientists have performed experiments aimed at chemical identification, and have attempted to show that the 0.3-s activity is more volatile than that of the relatively nonvolatile actinide trichlorides. This experiment does not fulfill the test of chemically separating the new element from all others, but it provides important evidence for evaluation. 

Ion-exchange separations can also be made by the use of a polymer with exchangeable anions in this case, the lanthanide or actinide elements must be initially present as complex ions (11,12). The anion-exchange resins Dowex-1 (a copolymer of styrene and divinylben2ene with quaternary ammonium groups) and Amherlite IRA-400 (a quaternary ammonium polystyrene) have been used successfully. The order of elution is often the reverse of that from cationic-exchange resins. 

Americium may be separated from other elements, particularly from the lanthanides or other actinide elements, by techniques involving oxidation, ion exchange and solvent extraction. One oxidation method involves precipitation of the metal in its trivalent state as oxalate (controlled precipitation). Alternatively, it may be separated by precipitating out lanthanide elements as fluorosilicates leaving americium in the solution. Americium may also he oxidized from trivalent to pentavalent state by hypochlorite in potassium carbonate solution. The product potassium americium (V) carbonate precipitates out. Curium and rare earth metals remain in the solution. An alternative approach is to oxidize Am3+ to Am022+ in dilute acid using peroxydisulfate. Am02 is soluble in fluoride solution, while trivalent curium and lanthanides are insoluble. 

Americium and other actinide elements may be separated from lanthanides by solvent extraction. Lithium chloride solution and an eight to nine carbon tertiary amine are used in the process. Americium is then separated from curium by the above methods. 

The XPS valence band spectra for the dioxides of the transuranium elements (from Np to Bk) have been presented in an extensive and pioneering work that also includes core level spectra and has been for a long time the only photoemission study on highly radioactive compounds. High resolution XPS spectra (AE = 0.55 eV) were recorded on oxidized thin metal films (30 A) deposited on platinum substrates with an isotope separator. (The oxide films for Pu and the heavier actinides may contain some oxides with lower stoichiometry, since starting with Pu, the sesquioxides of the heavier actinides begin to form in high vacuum conditions.)... 

The lanthanide elements are very difficult to separate because of their highly similar chemistry, but the earlier actinide elements have sufficiently different redox chemistry to allow easy chemical separations. This is important in the nuclear power industry, where separations have to be made of the elements produced in fuel rods of nuclear power stations as fission products, and of the products Np and Pu, which arise from the neutron bombardment of the uranium fuel. 

Uranium and thorium are actinide elements. Their chemical behavior is similar under most conditions. Both are refractory elements, both occur in nature in the +4 oxidation state, and their ionic radii are very similar (U+4 = 1.05 A, Th+4 = l.lOA). However, uranium can also exist in the +6 state as the uranyl ion (U02 2), which forms compounds that are soluble in water. Thus, under oxidizing conditions, uranium can be separated from thorium through the action of water. 

Neutral extracting agents possessing oxygen-donor atoms (hard bases) in their structure easily coordinate trivalent lanthanide and actinide cations, but do not discriminate between the two families of elements, because the ion-dipole (or ion-induced dipole type) interactions mostly rely on the charge densities of the electron donor and acceptor atoms. As a result, the similar cation radii of some An(III) and Ln(III) and the constriction of the cation radius along the two series of /elements make An(III)/Ln(III) separation essentially impossible from nitric acid media. They can be separated, however, if soft-donor anions, such as thiocyanates, SCN-, are introduced in the feed (34, 35, 39, 77). 

The SETFICS process (Solvent Extraction for Trivalent /-elements Intragroup Separation in CMPO-Complexant System) was initially proposed by research teams of the former Japan Nuclear Cycle Development Institute (JNC, today JAEA) to separate An(III) from PUREX raffinates. It uses a TRUEX solvent (composed of CMPO and TBP, respectively dissolved at 0.2 and 1.2 M in -dodecane) to coextract trivalent actinides and lanthanides, and a sodium nitrate concentrated solution (4 M NaN03) containing DTPA (0.05 M) to selectively strip the TPEs at pH 2 and keep the Ln(III) extracted by the TRUEX solvent (239). However, the DFs for heavy Ln(III) are rather poor. An optimized version of the SETFICS process has recently been proposed as an alternative process to extraction chromatography for the recovery of Am(III) and Cm(III) in the New Extraction System for TRU Recovery (NEXT) process. NEXT basically consists of a front-end crystallization of uranium, a simplified PUREX process using TBP for the recovery of U, Np, and Pu, and a back-end Am(III) + Cm(III) recovery step (240, 241). 

Opportunities remain to develop new or improved separation chemistries, and the engineered solid-phase materials that utilize them, in order to support radioanalyti-cal needs. Selectivity remains a key issue in order to support isolation of particular elements or groups of elements from complex sample matrixes. Selectivity to support determination of TRU actinides in the presence of large excesses of uranium is often required. The engineered form is also significant, as it must enable the separation chemistry to work while providing a robust material that can be used over and over again. 

The lanthanide and actinide elements are located at the bottom of the periodic table in two rows separate from the rest of the elements. By atomic number, they should be located in Periods 6 and 7, but they have special properties that distinguish them from elements in those periods. Lanthanides are very similar to each other and have some industrial uses. Many of the actinides were discovered as part of the first atomic bomb experiments. They are highly radioactive and have few uses. The transuranium elements were mostly created in the laboratory and are very short-lived. 

The purification procedures outlined above provide separation of berkelium from all trivalent lanthanides and actinides with the notable exception of cerium. Since berkelium and cerium exhibit nearly identical redox behavior, most redox separation procedures include a Bk-Ce separation step (21, 27, 37-41). Separation of Bk(III) from Ce(III) and other trivalent lanthanide and actinide elements can also be accomplished without the use of redox procedures (37, 39-47). 

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. 

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