Chemical and Ion Exchange Uranium Enrichment
Chemical-exchange isotope separation requires segregation of two forms of an element into separate but contacting streams. Since many contacts are required to achieve the desired separation, the contacting process must be fast and achieve as much separation as possible. For heavy elements such as uranium, achieving a suitable separation factor involves contact between two valence (oxidation state) forms such as hexavalent [U 6+ as in uranyl chloride (UO 2 Cl 2 )] and the quadrivalent [U 4+ as in uranium tetrachloride (UCl 4 )]. The 235 U isotope exhibits a slight preference for the higher valence, for example, the hexavalent over the quadrivalent in the Asahi process or the quadrivalent over the trivalent (U 3+ ) in the French solvent-extraction process.
The chemical-exchange process, developed by the French, is commonly referred to as CHEMEX. It uses the exchange reaction that takes place between two valence states (U 3+ and U 4+ ) of uranium ions in aqueous solution. Isotopic enrichment results from the tendency of 238 U to concentrate in the U 3+ compound while 235 U concentrates in the U 4+ compound. It is therefore possible to obtain enriched uranium by removing the U 4+ ions with an organic solvent that is immiscible with the aqueous phase (concentrated hydrochloric acid). Several possible extractants are available; however, tributyl phosphate (TBP), the choice of the French, is typically used. TBP is diluted with an aromatic solvent, and this organic phase moves countercurrent to the aqueous phase through a series of pulsed columns.
In the pulse column, the heavier aqueous phase is fed into the top of the column, and the lighter organic phase is fed into the bottom of the column. A rapid reciprocating motion is applied to the contents of the column, providing efficient and intimate contact of the two phases. In an HEU plant, centrifugal contactors might be employed particularly for the higher assay sections, since the stage times and corresponding specific uranium inventory could be reduced significantly. After passing through the column, the enriched and depleted uranium streams must be chemically treated so that they can be recirculated through the column again (refluxed) or sent to another column for additional enrichment. This requires complicated refluxing equipment at both ends of the column.
The ion-exchange process was developed by the Asahi Chemical Company in Japan and uses the chemical isotope effect between two valences (U 4+ and U 6+ ) of uranium. In this process, the organic phase is replaced by a proprietary ion-exchange resin. The aqueous phase flows through the stationary resin held in a column, and the net effect of all the chemical reactions is a "band" of uranium that moves through the ion-exchange column. The exchange between the unadsorbed uranium flowing through the band and that adsorbed on the resin enhances the isotopic separation. In this continuous separation system, 235 U and 238 U tend to accumulate respectively at the entrance and exit ends of the adsorption band. In this process, it is economical to regenerate many of the chemicals by reaction with oxygen and hydrogen in separate equipment.
The development and manufacture of the appropriate adsorbent beads are based on technology and know-how gained by Asahi in over 25 years of ion-exchange membrane development and manufacture. The adsorbent is a spherical bead of porous anion-exchange resin with a very high separation efficiency and an exchange rate over 1,000 times faster than the rates obtained in most commercially available resins. The two exchange processes discussed here are representative of exchange processes now under study in several countries. At present, no country has built or operated a full-scale uranium enrichment plant based on an exchange process. The primary proliferation concern is that they are based on standard chemical engineering technology (except for the proprietary ion-exchange resins).
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