Transcurium element

Ion exchange (qv see also Chromatography) is an important procedure for the separation and chemical identification of curium and higher elements. This technique is selective and rapid and has been the key to the discovery of the transcurium elements, in that the elution order and approximate peak position for the undiscovered elements were predicted with considerable confidence (9). Thus the first experimental observation of the chemical behavior of a new actinide element has often been its ion-exchange behavior—an observation coincident with its identification. Further exploration of the chemistry of the element often depended on the production of larger amounts by this method. Solvent extraction is another useful method for separating and purifying actinide elements. 

Thompson, S.G. Some aspects of the research on Transcurium elements at... 

The clarified Tramex product solution is divided into two or three batches (<35 g of curium or <19 g of 244 Qm pgr batch) and processed by LiCl-based anion exchange, which is discussed in detail in another paper at this symposium (10), to obtain further decontamination from rare earths and to separate curium from the heavier elements. In each run, the transplutonium and rare-earth elements are sorbed on Dowex 1-X10 ion exchange resin from a 12 hi LiCl solution. Rare earths are eluted with 10 hi LiCl, curium with 9 M LiCl, and the transcurium elements with 8 jl HC1. About 5% of the curium is purposely eluted along with the transcurium elements to prevent losses of 2498 which elutes immediately after the curium and is not distinguishable by the in-line instrumentation. The transcurium element fractions from each run are combined and processed in a second-cycle run, using new resin, to remove most of the excess curium. 

The location of berkelium, a beta emitter which cannot be monitored, can be estimated from the position of the californium in the column, as determined by the neutron peak (from 2 2cf), during the time that curium is in the effluent solution. Typical neutron peaks are shown in Fig. 2. By comparison of the relative distribution coeffients of the actinides, the berkelium location is known to be about midway between californium and curium. The last 5-10% of the americium-curium is purposely routed into the transcurium element product tank to minimize the berkelium loss. Subsequently, this americium-curium is recovered in a second-cycle LiCl AIX run. 

The process sequence now used is shown in Fig. 4. Since only about 5% of the fission products are disposed of in waste solutions from the Tramex batch extraction, that process serves primarily as a feed pretreatment for the LiCl AIX. The Tramex product contains about 98% of the transcurium elements and can be processed quickly to maximize the recovery of 253Es which has a 20-d half-life. As time permits, the "clean rework" can be processed to recover the remaining actinides. 

Because of its promise, the pressurized ion exchange approach was applied immediately to transcurium element production at TRU, and Fig. 2 indicates the sort of separation that was obtained. This shows the relative alpha count rate given by an in-line detector, and it demonstrates good separation of Fm, Es, Cf, and Cm. Berkelium is also well separated, appearing between Cf and Cm, but it is not shown because it is not an alpha-emitter. 

These columns are 1.2 m long and are made from stainless steel tubing up to 2.5 cm in diameter. The resin is graded into size ranges such as 25—50 or 70—100 ym diameter, the size selected depending on the application. Flow velocities are typically 15 cm min 1 for loading (they can be much higher if necessary) and 12 cm min 1 for elution. Full-scale separations of transcurium elements normally take less than 3 h and second-cycle purification of individual elements and separations of two elements require less than 1 h. These methods are entirely adequate for all present and planned production requirements. 

For separations involving large amounts of Am, Cm, or rare earths, displacement development provides a satisfactory first-cycle separation and yields Am and Cm products and a transcurium element fraction suitable for final separation by elution development. However, alternative methods for the first cycle (removal of the bulk of the lighter actinides and rare earths) are available besides displacement development chromatography, these include solvent extraction and the LiCl-anion exchange system. 

The latter system is used at TRU, while the SRL development program demonstrated the suitability of displacement chromatography. Both methods appear to be satisfactory. Until now, and for the foreseeable future, the quantity of transcurium elements has been too small to justify any process other than elution development for the final separation. 

R. J. Silva, Transcurium Elements, in International Review of Science, Inorganic Chemistry Series One, Vol. 8, Radiochemistry (Ed. A. G. Maddock), Butterworths, London, 1972... 

Plutonium Purification. The same purification approach is used for plutonium separated from sediments or seawater. In case reduction may have occurred, the plutonium is oxidized to the quadrivalent state with either hydrogen peroxide or sodium nitrite and adsorbed on an anion exchange resin from 8M nitric acid as the nitrate complex. Americium, curium, transcurium elements, and lanthanides pass through this column unadsorbed and are collected for subsequent radiochemical purification. Thorium is also adsorbed on this column and is eluted with 12M hydrochloric acid. Plutonium is then eluted from the column with 12M hydrochloric acid containing ammonium iodide to reduce plutonium to the non-adsorbed tervalent state. For seawater samples, adequate cleanup from natural-series isotopes is obtained with this single column step so the plutonium fraction is electroplated on a stainless steel plate and stored for a-spectrometry measurement. Further purification, especially from thorium, is usually needed for sediment samples. Two additional column cycles of this type using fresh resin are usually required to reduce the thorium content of the separated plutonium fraction to insignificant levels. 

Series on Rjadiochemisty, National Academy of Sciences. Reports available from Office of Technical Services, Department of Commerce, Washington, D.C. P. C. Stevenson and W. E. Nervik, Actinium (with scandium, yttrium, rare earths), NAS-NS-3020 E. Hyde, Thorium, NAS-NS 3004 H. W. Kirby, Protactinium, NAS-NS-3016 J. E. Gindler, Uranium, NAS -NS-3050 G. A. Burney and R. M. Harbour, Neptunium, NAS-NS-3060 G. H. Coleman and R. W. Hoff, Plutonium, NAS -NS-3058 R. A. Penneman and R. K. Keenan, Americium and Curium, NAS-NS-3006 G. H. Higgins, The Transcurium Elements, NAS-NS-3031. 

Techniques of Microchemistry and Their Applications to Some Transcurium Elements at Berkeley and Oak Ridge... 

Although space is not available for a complete discussion, the reader should be aware of the following factors which influence the choice of a particular technique for use in transcurium element research ... 

Figure 2. Schematic of preparation-vacuum system used for synthesizing transcurium element...

Am. The uotope Am is formed by the decay of Pu. It undergoes alpha decay, with a half-Ufe of 458 years, to form Np. Isotopically pure Am can be extracted from aged reactor-grade plutonium. Irradiation of separated Am is the basis of technology to produce gram quantities of Cm. This is also one route to the production of the transcurium elements. However, the first neutronfission cross sections and which result in considerable heat evolution as contrasted to the production of transcurium isotopes from the irradiation of plutonium rich in the isotope Pu. 

Am. The isotope Am is the 10.1-h beta emitter formed by neutron captme in Am. It is an intermediate in the nuclide chain leading to Cm and thence to the transcurium elements. 

Sec. 5.1 under Am. Repeated irradiation of the Pu and Am and extraction of Cm yields as much as 80 g Cm per kilogram of initial Pu [F2]. Further kradiation of the Cm yields the higher-mass curium isotopes and the transcurium elements. 

---