Nuclear Proliferation TechnologyTrends Analysis - International ...

PNNL -14480
Chemex, the chemical exchange process developed by the French, 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+ compounds with an organic solvent that
is immiscible with the aqueous phase (concentrated hydrochloric acid). Several possible
extractants are available; however tributyl phosphate (TBP) 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
(pulsing) is applied to the contents of the column, providing efficient and intimate contact
of the two phases.
After passing through the column, the enriched and depleted uranium streams must be
chemically treated to be re-circulated through the column again (refluxed) or sent to
another column for additional enrichment. This requires complicated refluxing
equipment at both ends of the column.
3.4.1.2.2 Ion-exchange
The ion-exchange process developed by the Japanese (the Ashai Chemical Exchange
Process, ACEP) 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 ionexchange
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 adsorbent in the columns is a spherical bead of porous anion-exchange resin with
both a very high separation efficiency and exchange rate. The development and
manufacture of the appropriate adsorbent beads has required years of experimental
research and development.
3.4.1.3 Important components/materials
Components and materials necessary to develop a chemical or ion-exchange enrichment
plant include:
• Liquid-liquid exchange columns
• Liquid-liquid centrifugal contactors
• Electrochemical reduction systems and reduction cells
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