The separation of radioactive isotopes

In forming artificial radioactive isotopes, problems of isolation are often encountered. For example, a product may decay quickly with the result that the initial product is contaminated with the daughter nuclide. 

The methods used to separate a desired isotope depend on whether or not the starting material and the product are isotopes of the same element (e.g. equation 3.14). If they are not, the problem is essentially one of chemical separation of a small amount of one element from large amounts of one or more others. Methods of separation include volatilization, electrodeposition, solvent extraction, ion-exchange or precipitation on a carrier . For example, in the process foZn(n,p) Cu, the target (after bombardment with fast neutrons) is dissolved in dilute HNO3 and the Cu is deposited electrolytically. This method is successful because of the significant difference between the reduction potentials °(Cu /Cu) = -I-0.34V and (Zn +/Zn) = -0.76 V (see Chapter 8). 

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Sintered membranes are made on a fairly large scale from ceramic materials, glass, graphite and metal powders such as stainless steel and tungsten.9 The particle size of the powder is the main parameter determining the pore sizes of the final membrane, which can be made in the form of discs, candles, or fine-bore tubes. Sintered membranes are used for the filtration of colloidal solutions and suspensions. This type of membrane is also marginally suitable for gas separation. It is widely used today for the separation of radioactive isotopes, especially uranium. 

Until the advent of modem physical methods for surface studies and computer control of experiments, our knowledge of electrode processes was derived mostly from electrochemical measurements (Chapter 12). By clever use of these measurements, together with electrocapillary studies, it was possible to derive considerable information on processes in the inner Helmholtz plane. Other important tools were the use of radioactive isotopes to study adsorption processes and the derivation of mechanisms for hydrogen evolution from isotope separation factors. Early on, extensive use was made of optical microscopy and X-ray diffraction (XRD) in the study of electrocrystallization of metals. In the past 30 years enormous progress has been made in the development and application of new physical methods for study of electrode processes at the molecular and atomic level. 

For use in a radioactive environment the gas centrifuge must be completely maintenance free. It has been used for the separation of xenon isotopes and consideration has been given to its application for separation of fluorohydrocarbons. Worldwide, in the region of a quarter of a million gas centrifuges have been manufactured. As an order of magnitude figure, an investment of 1000 is necessary to obtain 0.3 g/s (10 g/h) of product. 

Closely related to tracer analysis is the method of isotopic dilution analysis. Here, instead of checking the effectiveness of a method from known amounts of an element in the sample, and of its radioactive isotope, one knows only the amount of radioactive isotope added, and by precipitating or otherwise separating the total amount of that element present, and then measuring its radioactivity, one determines its amount, and hence the amount present in the original sample. 

The meta-elements of Crookes anticipated the idea of isotopes. The most readily available sources of separated stable isotopes are lead 206 from uranium minerals and lead 208 from thorium minerals, and it is interesting enough that before 1910 several successful chemical separations of radioactive isotopes of the same element were reported, involving both thorium 230 (ionium) and lead 210 (radium D). In otir opinion, this can only be due to kinetically metastable chemical nonequivalency in the mixture, for instance, due to colloidal or oligomeric complexes. The valuable conclusion of this story is that the chemical similarity of trivalent rare earths is so striking that doubts have been expressed whether they deserved more than one place in the Periodic Table, a situation isotopes later had to accept. Such a doubt has never been expressed for any other elements, not even for a pair of elements like vanadium and chromium, which were confused at the time of their discovery (7). Nevertheless, studies based on the possibility of metaelements continued rather late for instance, Debierne attempted to separate neo-radium from conventional radium 226 and to perform nuclear reactions on charcoal cooled with liquid helium (77). 

Successful experiments on the photoionization detection and separation of radioactive isotopes were conducted by Zherikhin et al. (1984). Later on it became clear that atoms could be ionized most effectively not in a beam by the scheme presented in Fig. 9.7(a), but in a hot cavity in accordance with the scheme shown in Fig. 9.7(c),... 

Production and Separation of Radioactive Isotopes of the Pt Metal Group Elements... 

A compilation of six procedures for the separation of radioactive Ir isotopes from different target materials and a review of the chemical properties of Ir which might be useful for the isolation of the element, are given by Leddicotte [142]. 

Although many fluorocarbon polymers are commercially available (Appendix 16.H), poly(tetrafluoroethylene) (PTFE) is estimated to command about 90% of the market. This polymer, under DuPont s trade name. Teflon, has been made since the early 1940s. Both PTFE and poly(chlorotrifluoroethylene) were developed to meet wartime demands, especially for the corrosive processes involving separation of radioactive isotopes. Many other fluorinated polymers have been commercialized over the years. 

The licensing process consists of two steps construction and operating license that must be completed before fuel loading. Licensing covers radiological safety, environmental protection, and antitru,st considerations. Activities not defined as production or utilization of special nuclear material (SNM), use simple one-step. Materials Licenses, for the possession of radioactive materials. Examples are uranium mills, solution recovery plants, UO fabrication plants, interim spent fuel storage, and isotopic separation plants. 

The method of assaying the radioactivity of the bound and/or unbound fraction following separation, solely depends on the nature of the isotope and on the method utilized for the separation of the bound and unbound fractions,... 

Crystallisation was one of the earliest methods used for separation of radioactive microcomponents from a mass of inert material. Uranium X, a thorium isotope, is readily concentrated in good yield in the mother liquors of crystallisation of uranyl nitrate (11), (33), (108). A similar method has been used to separate sulphur-35 [produced by the (n, p) reaction on chlorine-35] from pile irradiated sodium ot potassium chloride (54), (133). Advantage is taken of the low solubility of the target materials in concentrated ice-cold hydrochloric acid, when the sulphur-35 as sulphate remains in the mother-liquors. Subsequent purification of the sulphur-35 from small amounts of phosphorus-32 produced by the (n, a) reaction on the chlorine is, of course, required. Other examples are the precipitation of barium chloride containing barium-1 from concentrated hydrochloric acid solution, leaving the daughter product, carrier-free caesium-131, in solution (21) and a similar separation of calcium-45 from added barium carrier has been used (60). 

The other major springboard for the fluorocarbon chemical industry was the "Manhattan Project to develop the atomic bomb. This required the large-scale production of highly corrosive elemental fluorine and uranium(VI) fluoride for the separation of the radioactive 235U isotope. Oils capable of resisting these materials were needed to lubricate pumps and compressors, and polymers were needed to provide seals. Peril uorinated alkanes and polymers such as PTFE and poly(chlorotrifluoroethylene) (PCTFE) proved to have the appropriate properties so practical processes had to be developed for production in the quantities required. In 1947 much of this work was declassified and was published in an extensive series of papers3 which described the fundamental chemistry on which the commercial development of various fluoro-organic products, especially fine chemicals, was subsequently based. 

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