Surface Decontamination of Radioactive Metal Wastes Using Acid-in-Supercritical CO2 Emulsions

Supercritical fluid surface decontamination (SFSD) of radioactive metal wastes was successfully demonstrated using acid-in-CO 2 emulsions. NP-series(ethoxylated nonyl phenol series) formed stable acid-in-CO 2 emulsions, contrary to F-AOT(sodium bis(2,2,3,3,4,4,5,5-octafluoro-1-pentyl)-2-sulfosuccinate). An ultrasonic horn was utilized to enhance mechanical and chemical reactions in Sc-CO 2 emulsions. A Cu-coated specimen for a mock-up test was prepared and examined with acid-in-CO 2 microemulsions. Over 90% of the Cu coating was easily dissolved away from the specimen. For actual radioactive contaminants, 6 M HNO 3 in Sc-CO 2 microemulsions enhanced by ultrasonic force was more than 95% decontaminated. Sc-CO 2 macroemulsions were also applied with 0.1 M HNO 3 or 5% oxalic acid. The decontamination efficiencies were about 90% with HNO 3 and over 95% (up to background level) with oxalic acid. After decontamination, the used surfactants were recovered up to about 73% through the recovery process, and they can be reused as well as CO 2 . So the volume of radioactive wastes can be positively reduced. This SFSD technique may be an effective candidate for surface decontamination of radioactive metals in the near future.