For the long-term safety assessment of nuclear waste repositories, a thorough understanding and thermodynamic description of the principal geochemical processes of actinides in aqueous solutions is highly relevant. Solubility phenomena strongly control the radionuclide source term, i.e. the maximum amount of dissolved radionuclide species potentially mobilized from a repository in the case of water intrusion. Due to its long half-life and abundance in spent fuel, 237Np is of high relevance in this context. In aqueous solutions, Np(V) is thermodynamically stable over a wide range of Eh and pH conditions. According to the NEA–TDB thermodynamic selection, the solubility of Np(V) in dilute to concentrated NaCl–NaOH solutions is controlled by the binary phase NpO2OH(am). Very comprehensive solubility experiments with Np(V) in alkaline, dilute to concentrated NaCl, MgCl2 and CaCl2 solutions have been performed at KIT–INE, which reveal that the current chemical model for Np(V) needs to be extended. The formation of so far not considered ternary M–Np(V)–OH(s) phases with M = Na, Ca readily takes place under these conditions. Our experimental data show that the latter have significantly lower solubilities compared to NpO2OH(am). A mild hydrothermal treatment of NpO2OH(am) in NaCl and CaCl2 solutions further promotes the formation of ternary M–Np(V)–OH(s) solid phases above pHm = 11. No such ternary phases are observed under weakly alkaline pH conditions, although Np phases exposed to elevated temperature within this pH-range show also a decreased solubility compared to NpO2OH(am). The presence of borate in NaCl and MgCl2 solutions induces the transformation or NpO2OH(am) into ternary Na/Mg–Np(V)–borate phases. These solid phases readily form at room temperature, and lead to a decrease of up 3 log-units in the solubility of Np(V) under weakly alkaline pH conditions. Although An(V) are normally considered as highly soluble and mobile, the results presented in this contribution show that sparingly soluble Np(V) solid phases can also form under boundary conditions relevant in the context of nuclear waste disposal. The combination of solubility experiments with extensive solid phase characterization allows the determination of thermodynamic models for the investigated systems. These models can be implemented in thermodynamic databases and geochemical calculations, and thus represent a very useful tool in the safety assessment of repositories for nuclear waste disposal.

Role of Np(V) solid phases in the solution chemistry of neptunium under alkaline pH conditions

254th National Meeting (2017)

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