An accurate knowledge of the aquatic chemistry and thermodynamics of actinides is fundamental in the context of nuclear waste disposal. The availability of complete and correct chemical and thermodynamic models represents a key milestone in the development of reliable geochemical model calculations and source-term estimations and, thus, in the safety assessment of repositories for nuclear waste disposal. Uranium contributes with the largest inventory to the radioactive waste. U(VI) is the most stable oxidation state of uranium under anoxic and oxidising conditions, whereas U(IV) prevails in strongly reducing environments, forming the sparingly soluble UO2(am,hyd) ([U] &lt; 10–8 M) in solutions at pH ≥ 5 and in the absence of complexing ligands. The predominance of the latter oxidation state is expected in deep geological repositories, where very reducing conditions are foreseen. The geochemical conditions in the nuclear waste repositories which may provide dilute to concentrated dissolved salts (I =0.1 M–15 M) as well as the strong complexing ligands like CO32- and EDTA, can significantly alter the chemical behaviour and, therefore the migration of actinides. In this framework, an appropriate understanding of the redox behaviour of U(VI)/U(IV) as well as solubility and complexation behaviour of U(IV) in dilute to concentrated saline systems is essential for the long term performance assessment calculations. In the present study, the redox behaviour of U(VI)/U(IV) was investigated from oversaturation and undersaturation conditions in dilute to concentrated NaCl (0.1–5.0 M), MgCl2 (0.25–4.5 M) and CaCl2 (0.25–4.5 M) solutions using different reducing chemicals in the presence and absence of complexing ligands. The experiments in the presence of EDTA were performed with variable ligand concentration ([EDTA]tot = 0.001–0.05 M) and ionic strength (I = 0.5–5.0 M) in neutral to alkaline pH conditions. Complexation behaviour of U(IV) was investigated from undersaturation conditions as a function of carbonate concentration (Ctot =0.01 M–1.5 M), ionic strength (Ctot = 0.1 M–0.3 M, I = 0.5 M–4.0 M (NaCl–NaHCO3–Na2CO3)) and OH concentration (Ctot = 0.1, 0.01 M ≤ [NaOH] ≤ 0.6 M). The redox boundaries between U(VI)/U(IV) in relevant geological repository conditions were assessed with the intent of extending the chemical, thermodynamic, and activity models for An(IV).

Redox behavior of U(VI)/U(IV) and solubility of U(IV) in repository relevant dilute to concentrated solutions in the absence and presence of complexing ligands

253rd National Meeting (2017)

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