The research team showed that solid uranium dioxide, a nuclear fuel, dissolves in supercritical CO₂ containing a complex of tri-n-butylphosphate (TBP) and nitric acid. The resulting uranyl nitrate-TBP complex is recovered by reducing the fluid pressure to atmospheric pressure and then allowing the gas to expand into a collection vial.

The work was carried out by scientists at the University of Idaho, Moscow; the Vernadsky Institute of Geochemistry & Analytical Chemistry, Moscow, Russia; and British Nuclear Fuels Ltd. (BNFL), Sellafield, England [Chem. Commun., 2001, 1868].

"The extractant oxidizes uranium(IV) to uranium(VI) to form a complex that is very soluble in supercritical CO₂," explains Idaho chemistry professor Chien M. Wai. "This process does not require any aqueous solution or organic solvent to dissolve the UO₂.

"Dissolution of UO₂ in supercritical CO₂ minimizes waste generation," Wai continues. "The new extraction method can also be used for decontamination of actinide-contaminated solid wastes that conventionally require acid or aqueous solution leaching to remove the contaminants from solid matrices."

The report is significant, says William R. Pitner, a research chemist at Queen's University of Belfast, Northern Ireland, who carries out work on nuclear fuel reprocessing. "It demonstrates the applicability of an alternative technology to the nuclear industry," he says. "As an industry which will continue to evolve in the decades to come, it is important that it continue to investigate nontraditional methods that may complement or replace existing processes," Pitner adds.

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