Thorium purification

West CM. 1962. An evaluation of the health physics problems from thorium and its daughters in a thorium purification and fabrication process. Health Phys 8 279-297. 

Thorium concentrate produced by the processes described in Sec. 8 is too impure to be used as nuclear fuel. Especially objectionable impurities, which frequently are associated with thorium in its ores, are neutron-absorbing rare earths and uranium, the latter because it would dilute isotopically U formed in thorium during subsequent neutron irradiation. The objective of thorium purification is removal of these and other impurities to concentrations below a few parts per million. 

Solvent extraction with TBP has become the standard procedure for purifying thorium, just as for uranium. Processes used in different countries differ, however, in details such as the solvent used to dilute TBP, its concentration, and the means used to strip thorium and coextracted uranium from TBP. Table 6.20 summarizes the main features of processes used for purification of thorium on an industrial scale in the principal thorium-producing countries. Wylie [W5] gives more detail on early pilot-plant thorium-purification runs. Most of the published U.S. work on thorium purification on an industrial scale deals with irradiated thorium rather than natural this will be described under the Thorex process, in Sec. 5 of Chap. 10. 

Figure 6.8 Thorium purification by solvent extraction with TBP. Circles, relative flow ------------...

Various thorium purification processes have involved the use of ion-exchange indirectly, e.g. to remove the uranium which is normally associ-... 

The fuel for the Peach Bottom reactor consisted of a uranium-thorium dicarbide kernel, overcoated with pyrolytic carbon and silicon carbide which were dispersed in carbon compacts (see Section 5), and encased in graphite sleeves [37]. There were 804 fuel elements oriented vertically in the reactor core. Helium coolant flowed upward through the tricusp-shaped coolant channels between the fuel elements. A small helium purge stream was diverted through the top of each element and flowed downward through the element to purge any fission products leaking from the fuel compacts to the helium purification system. The Peach... 

Figure 1. Schematic diagram showing a TRU-spec extraction chromatography method for separation of uranium, thorium, protactinium, and radium from a single rock aliquot. Further purification for each element is normally necessary for mass spectrometric analysis. Analysis of a single aliquot reduces sample size requirements and facilitates evaluation of uranium-series dating concordance for volcanic rocks and carbonates. For TIMS work where ionization is negatively influenced by the presence of residual extractant, inert beads are used to help remove dissolved extractant from the eluant.

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). 

Table 12.6 contains, in a simplified way, the composition, location, and treatment of main thorium ores. The purification of thorium by TBP extraction is illustrated in Fig. 12.4. This purification takes place after the dissolution of Th in nitric acid, generally from a hydroxide cake. When the Th is dissolved in sulfuric acid, purification is achieved by extraction with long-chain alkylamines. 

This article presents a general discussion of actinide metallurgy, including advanced methods such as levitation melting and chemical vapor-phase reactions. A section on purification of actinide metals by a variety of techniques is included. Finally, an element-by-element discussion is given of the most satisfactory metallurgical preparation for each individual element actinium (included for completeness even though not an actinide element), thorium, protactinium, uranium, neptunium, plutonium, americium, curium, berkelium, californium, and einsteinium. 

If an actinide metal is available in sufficient quantity to form a rod or an electrode, very efficient methods of purification are applicable electrorefining, zone melting, and electrotransport. Thorium, uranium, neptunium, and plutonium metals have been refined by electrolysis in molten salts (84). An electrode of impure metal is dissolved anodically in a molten salt bath (e.g., in LiCl/KCl eutectic) the metal is deposited electrochemically on the cathode as a solid or a liquid (19, 24). To date, the purest Np and Pu metals have been produced by this technique. 

Since the radioactivity of thorium salts is smaller than that of the minerals, B. B. Boltwood (93) thought that some of the radiothorium must have been lost during the purification process. On the assumption that radiothorium was formed directly from thorium, he computed that the half-life period of the former ought to be at least six years, whereas... 

Hahn obtained an experimental value of only two years. Hahn therefore assumed that there must exist between thorium and radiothorium an unknown rayless product, mesothorium, which can easily be separated from thorium in the purification process. 

Yaftian, M.R. Hassanzadeh, L. Eshraghi, M.E. Matt D. Solvent extraction of thorium (IV) and europium (III) ions by diphenyl-N,N-dimethylcarbamoylmethylphosphine oxide from aqueous nitrate media, Sep. Purif. Tech. 31 (2003) 261-268. 

Govindan, R Palamalai, A. Vijayan, K.S. Raja, M. Parthasarathy, S. Mohan, S.V. Subba Rao, R.V. Purification of 233U from thorium and iron in the reprocessing of irradiated thorium oxide rods, J. Radioanal. Nucl. Chem. 246 (2000) 441 144. 

Since the chemistry of actinium is confined to the Ac + ion, it can readily be separated from thorium (and the lanthanides, for that matter) by processes like solvent extraction with thenoyltrifluoroacetone (TTFA) and by cation-exchange chromatography. The latter is an excellent means of purification, as the Ac + ion is much more strongly bound by the resin than its decay products. 

A similar scheme has been used for example, for the recovery on anion exchanger of uranium as a nitrate complex from its mixture with thorium [17, p. 317], or for the purification of nickel from calcium on cation exchanger[18]. 

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