Electrochemical Behavior of Metallic Technetium in Aqueous Solution

Full text of publication follows: The isotope 99Tc is a problematic fission product due to its high fission yield, long half-life, and complex solution chemistry. In the proposed UREX process, 99Tc is separated from other elements, converted to the metal, and included in a waste form. Waste forms containing Tc metal are convenient for disposal or eventual transmutation into stable 100Ru. However, little data on the behavior of Tc metal in aqueous media exist. In order to better understand its corrosion mechanisms, a Tc metal electrode was constructed. The electrochemical studies were performed in Yucca Mountain water (pH = 7.65) under aerobic conditions at room temperature. The electrode was immersed for various time intervals and potentiodynamic measurements (log(i) = f(E)) were performed. Results indicate that the corrosion potentials (Ecor) shift to more positive values as the immersion time increases, e.g., Ecor = -54 mV/SCE for t = 2 min to Ecor = +46 mV/SCE for t = 15 h. Further analysis of the potentiodynamic curve after 15 hours of immersion indicate a trans-passivation behavior beginning at E = +150 mV/SCE, which is characteristic of oxide formation at the surface of the metal. In view of these results, a mechanism of corrosion was proposed for Tc metal that involves the formation of TcO2. The behavior of Tc metal in acidic conditions (nitric, chloride and acetic) is under investigation. Speciation and kinetic data for the dissolution of Tc metal will be presented. (authors)