Separation transcurium

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. 

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. 

A second solvent extraction process (Pharex) was developed to partition the transcurium actinides from the americium and curium in the Tramex product ( 3) The Pharex process utilized 2-ethylhexyl phenylphosphonic acid as the extractant for the transcurium actinides. During early operations/ the selectivity of the Pharex extractant was found to be severely reduced by the presence of zirconium ions, which were introduced into the process solutions by corrosion of Zircaloy-2 equipment in TRU. At zirconium concentrations above 10 ppm, the achievable separation began to be diminished and, at 100 ppm, a practical separation could not be made (4). Thus, a replacement for the Pharex process was needed, and the LiCl AIX process was the most immediate alternative ... 

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. 

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. 

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. 

Only HDEHP has found large-scale use for the recovery and separation of americium however, other organophosphorus acids have been evaluated. Bis(2-ethylhexyOphenylphosphonic acid (HEH0P) was used to separate americium and curium from transcurium elements contained in 1 m HCI [52]. 

The separation procedure most suitable for californium isotopes generated in accelerators may not be the same as that used for californium produced in reactor targets. In some accelerator experiments the desired californium isotopes may be physically separated via recoil mechanisms, which simplifies the rapid separations required for short-lived isotopes. The need for nuclear or radioactive purity, as opposed to chemical purity, will also affect the particular separative processes to be used. A considerable amount of information on californium chemistry was determined using tracer levels of californium. The major purification schemes for californium at the tracer level involved ion-exchange techniques to separate californium from other transcurium elements. 

Most extraction procedures are useful for separating californium from americium/curium or from lighter actinides, but are limited for separating it from other transcurium elements. For example, HDEHP dissolved in an aromatic diluent has been used to separate Cf and Cm (separation factor 50). Efforts continue to find new and better extractants with the aim of improving separation factors and selectivity. It is unlikely that a specific extractant for californium will be developed but new materials may provide improved separation factors. Recent reviews that discuss californium extraction chemistry are available [32-34]. 

---