Technetium-99 is one of the main fission products of 235U and 239Pu in nuclear reactors. Due to its long half-life (2.1x105 a) and redox-sensitive character, 99Tc is a very relevant radionuclide in the safety assessment of repositories for radioactive waste disposal. Tc(VII) is the prevailing oxidation state under oxidizing and redox-neutral conditions and exists as soluble and mobile TcO4–. Tc(IV) forms sparingly soluble hydrous oxides (TcO2:xH2O(s)) under reducing conditions as those expected in deep underground repositories. An appropriate knowledge of the stability fields of Tc redox states, solubility, hydrolysis and complexation behaviour of Tc(IV) in dilute to concentrated saline systems is required for a correct assessment of Tc source term in repositories for radioactive waste disposal. Studies developed within this framework can also provide relevant insights for understanding Tc chemistry in legacy sites such as Hanford nuclear tank wastes. This contribution summarizes recent and on-going research activities at KIT–INE, Germany, on the solution chemistry of Tc: (i) redox chemistry of Tc(VII)/Tc(IV) under acidic to strongly alkaline pH conditions, in dilute to concentrated NaCl and MgCl2 systems, with focus on the definition of the redox boundary between both oxidation states; (ii) solubility and hydrolysis of Tc(IV) under acidic to strongly alkaline pH conditions, in dilute to concentrated NaCl, KCl, MgCl2 and CaCl2 solutions; (iii) complexation of Tc(IV) with carbonate in dilute to concentrated NaCl–NaOH solutions. The results obtained provide quantitative tools for (iv) the assessment of redox processes involving Tc(VII) and Tc(IV) in the absence and presence of nitrate. This work contributes also to an improved thermodynamic description of Tc(IV) in dilute aqueous solutions to concentrated brine systems, which can be implemented in geochemical models/calculations, thus providing further confidence in the safety analysis of repositories for the disposal of radioactive waste.

Aquatic chemistry and thermodynamics of technetium: Redox processes, solubility and complexation

254th National Meeting (2017)

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