The studies of KIT-INE presented in this contribution are developed with the aim of improving the scientific basis for assessing nuclear waste disposal scenarios at elevated temperature conditions. Adequate scientific tools must be available to assess the potentially relevant chemical effects and their impact upon repository safety. Furthermore, this work is contributing to an advanced scientific understanding of general fundamental chemical processes in aqueous media. Studies are performed within the German ThermAc project, funded by BMBF (coordinator contact: marcus.altmaier@kit.edu). ThermAc is extending the chemical understanding and available thermodynamic database for actinides, long-lived fission products and relevant matrix elements in aquatic systems at elevated temperatures. A clear focus of ThermAc is on long-lived actinides in oxidation states III, V and VI, with selected fission products and important redox controlling matrix elements like Fe also receiving attention. ThermAc mainly addresses the temperature range from ~5°C up to ~90°C, focusing on systems at low or intermediate ionic strength. ThermAc is following an integrated concept with the following strategic components: (a) systematic use of estimation methods for thermodynamic data and model parameters, (b) comprehensive experimental validation of the estimations, (c) fundamental studies for improved process understanding of actinide chemistry at elevated T, and (d) comprehensive critical evaluation of the work performed within (a-c). Examples of research studies performed by KIT-INE include (i) assessment of the impact of elevated temperatures on the solubility and speciation of U(VI) in dilute to concentrated NaCl solution - experimental data and modeling, (ii) studies at elevated T on the ternary Ca/Mg-U(VI)-CO3 systems (iii) solubility phenomena of trivalent actinides at elevated temperatures, and (iv) stability/transformation of solubility limiting solid phases at RT, exposed to elevated T conditions.

Effect of elevated temperatures on actinide solubility and speciation–studies performed by KIT-INE within the German project ThermAc

253rd National Meeting (2017)

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