99Tc 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(am)) 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) solubility and complexation of Tc(IV) in dilute to concentrated NaCl–NaHCO3–Na2CO3–NaOH systems; (ii) solubility and complexation of Tc(IV) in dilute to concentrated NaCl–Na2SO4–NaOH systems; (iii) solubility and redox behaviour of Tc(IV) in dilute to concentrated NaCl–NaNO3–NaOH systems. This systematic work contributes 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.

Solubility, complexation and redox behaviour of Tc(IV): Effect of carbonate, sulfate and nitrate

256th National Meeting (2018)

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