Interaction of Hexafluoroacetylacetone with Metals and Their Alloys in the Medium of Supercritical Carbon Dioxide in the Processes of Equipment Decontamination

In the course of treatment of stainless steel with supercritical CO₂ containing -diketones in combination with pyridine and water, these agents react mainly with the oxide films. Therefore, in industrial use of such media, e.g., for decontamination of the equipment, the corrosion is expected to be low. Under these conditions, uranium does not dissolve from the surface of metallic uranium, and iron does not dissolve from the surface of pure metallic iron. In treatment of a copper or nickel surface with supercritical CO₂ containing -diketons and modifiers, both the oxide film and metals are involved in the reaction. It is presumed that water facilitates ionization of the complexing agent, and pyridine (or another amine with pK _a 5-6) creates a medium favorable for complexation. Furthermore, pyridine molecules may be incorporated in the complex as additional ligands.     

Deactivation in Sub- and Supercritical Carbon Dioxide

Solutions of hexafluoroacetylacetone and a modifier such as, e.g., pyridine in supercritical CO₂ allow 97-99% removal of actinides from the stainless steel surface. The deactivation efficiencies were compared for liquid and supercritical CO₂. Single treatment run with solutions of HDEHP and DCH18C6 in liquid CO₂ removes 70-80% of transuranium elements and over 50% of strontium and cesium from the stainless steel surface. Deactivation of real contaminated radioactive samples was studied. Methods such as supercritical fluid extraction and extraction with liquid CO₂ are suitable for deactivation of surfaces and porous materials.     

Deactivation of metals,fabrics, and soils in a liquid CO<Subscript>2</Subscript> medium

Liquid (as well as supercritical) carbon dioxide can be used in deactivation. Solutions of TBP and phosphorus-and fluorine-containing acids in CO₂ in the presence of alcohols make it possible to achieve the deactivation coefficients equal to 50 and above for surfaces polluted with weakly fixed α-, β-, and γ-radiating nuclides by two successive treatments. When performing the double treatment of metals contaminated with strongly fixed nuclides using solutions of chelating agents in CO₂, it is possible to achieve deactivation coefficients equal to 2–3 for aluminum, 10–20 for brass (90–95 of radionuclides are removed), and 10–30 for stainless steel (90–97% of radionuclides are removed). Using an enlarged pilot apparatus, the possibility of deactivating protective clothes and metal products in liquid carbon dioxide down to the natural radioactive background is shown.     

Effect of the Nature of Fluid and Structure of β-Diketone on Supercritical Extraction of Metal β-Diketonate Complexes

Extraction of uranium with trifluoromethyl alkyl diketones differing in the length of the alkyl chain was studied. The size of the alkyl group only slightly affects the extraction. Extraction of complexes formed by uranyl nitrate with -diketones in pentafluoroethane (Freon-125) was studied. The complexes are efficiently extracted with both super- and subcritical Freon-125. In contrast to extraction of these complexes in supercritical CO₂, in extraction with Freon-125 the complexes do not decompose.