Newly Developed Decontamination Technology Based on Gaseous Reactions Converting to Carbonyl and Fluoric Compounds

A new gas-phase decontamination technology is developed based on gaseous reactions utilizing the volatile properties of the carbonyl and fluoric compounds of radioactive transition elements and actinides (corrosion products, fission products, and transuranium) on a material's surface. The feasibility of this new technology is determined by removing nonradioactive (Co, Cr, Ni, Re, Mo, Mn, Ru, and Zn) and radioactive (⁶⁰Co, ⁶³Ni, and ¹⁰³Ru) nuclide transition elements as gaseous forms under high CO pressure (50 to 200 atm) and high temperature (~350°C). Experiments involving U and using fluoric gases are also performed. For radioactive nuclides existing in an oxide layer of stainless steel, pretreatment with supercritical CO₂ + I₂ + H₂O is used to remove the oxide layer completely, and by the subsequent gaseous reaction, 95 to 99% of ⁶⁰Co is removed from the layer by CO gas treatment at a pressure of 200 atm. The plasma treatment using fluorine gas results in U being removed with high efficiency (~60%) after only 5 min, even at a reduced pressure of 1 Torr and at room temperature. When the carbonyl and fluoric species generated from a nontoxic gas mixture (1 Torr) of CF₄ and O₂ is used, U and ⁶⁰Co are removed simultaneously with high removal efficiencies of 80 and 100% for ⁶⁰Co and U, respectively. The data provide evidence that chemically reactive plasma treatment is available as a gas-phase decontamination method that can be conducted using nontoxic gases under safe and mild conditions such as reduced pressure, shorter time periods, and ambient temperature. Finally, a fluoric chemical reaction can be used to remove solid U deposits by converting them to gaseous U compounds at room temperature and without using plasma treatment. The pressure of ClF₃ gradually affects the higher removal efficiency of U, and the removal efficiency is >90% under the conditions of 30 min and >100 Torr. The results verify that chemical reactions involving carbonylation and fluorination reactions can be utilized for gas-phase decontamination, and the potential for this new idea for decontamination is affirmed.

If gas-phase decontamination technology is further developed, it will be not only convenient but also economically advantageous because decontaminating and treating the large volume of nuclear wastes - especially nonincinerable radioactive wastes - are currently very difficult.