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-CO2 emulsions. NP-series(ethoxylated nonyl phenol series) formed stable acid-in-CO2 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-CO2 emulsions. A Cu-coated specimen for a mock-up test was prepared and examined with acid-in-CO2 microemulsions. Over 90% of the Cu coating was easily dissolved away from the specimen. For actual radioactive contaminants, 6 M HNO3 in Sc-CO2 microemulsions enhanced by ultrasonic force was more than 95% decontaminated. Sc-CO2 macroemulsions were also applied with 0.1 M HNO3 or 5% oxalic acid. The decontamination efficiencies were about 90% with HNO3 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 CO2. 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.