Technetium aquatic chemistry studies in the context of nuclear disposal. Redox behavior, solubility and complex chemistry of Tc(IV) in electrolyte solutions

The presented work investigates the aquatic chemistry of technetium in the context of nuclear waste disposal. Due to their sometimes very long half-lives, the radionuclides stemming from research and technology require safe long-term storage. The international consensus lies with deep geological storage. Due to the properties of the selected host rock and the technical barriers used, (highly) saline solutions may form as a result of water intrusion. Investigations into the source term of the respective disposed radionuclides are therefore of great relevance for the long-term safety assessment of potential repositories. With roughly 6 %, ⁹⁹Tc is one of the most common fission products of ²³⁵U and ²³⁹Pu in nuclear reactors and is thus, due to its high abundance in spent nuclear fuel and its long half-life (∼ 2.1.10⁵ a), a relevant radionuclide in the context of nuclear waste disposal. Three independent studies on the behaviour of ⁹⁹Tc(IV) in aquatic systems were performed in the presented work. (1) The solubility of ⁹⁹Tc(IV) in KCl solutions was investigated under strongly reducing conditions (pe + pH_m ∼ 2, with pH_m = -log[H⁺]) and different ionic strengths (I = 0.1-4.0 M) at 1 ≤ pH_m ≤ 14.5. (2) The solubility and carbonate complexation of ⁹⁹Tc(IV) was investigated in dilute to concentrated carbonate solutions (0.01 M ≤ C_tot ≤ 1.0 M, with C_tot = [HCO₃^-] + [CO₃^2-]) at varying ionic strengths (I = 0.3-5.0 M NaHCO₃-Na₂CO₃-NaCl-NaOH) and different pH_m. (3) The redox behaviour and solubility of ⁹⁹Tc(IV) was investigated in nitrate-containing solutions with 0.1 M ≤ [NO₃^-] ≤1.0 M and varying ionic strength I = 0.1-5.0 M NaNO₃-NaCl-NaOH at 8.5 ≤ pH_m ≤ 14.5.