APPLICATION OF SUPERCRITICAL CARBON DIOXIDE TO DECONTAMINATION OF RADIOACTIVE CONTAMINANTS FROM METAL SURFACES

摘要

Supercritical carbon dioxide (SF-CO{sub}2) with its moderate critical constants, nonflammable nature, high diffusivity and low cost provides us an alternative for replacing various solvents traditionally used in chemical processes. It is also an attractive media for decontamination of radioactive contaminants deposited or formed on the surfaces of carbon or stainless steels, because amount of secondary radioactive waste in chemical treatments can be minimized through complete separation of the dissolved contaminants from CO{sub}2 by depressurization of the high-pressure fluid. For practical usage of SF-CO{sub}2 to waste decontamination, a problem arising from low solubility of solutes in SF-CO{sub}2 should be overcome; we found a couple of technical solutions were feasible for removal of typical contaminants by ferrous oxides. We studied two chemical systems for this purpose in this paper. Acetic acid was known and pentadecafluorooctanoic acid was found to be soluble in SF-CO{sub}2 with water. Polyoxyethylene nonylphenyl ether (NP-2) was found to form reversed micelles of citric acid solution in SF-CO{sub}2. Solubilities and stabilizing amounts of water in the high-pressure fluids were measured using a phase equilibrium analyzer. These chemical systems were tested as media for removing ferrous oxide fixed on the carbon steel, and SF-CO{sub}2 containing acetic acid with water and NP-2 with citric acid were found to be useful.