This work provides new experimental studies and improved thermodynamic descriptions of U(IV), Np(IV) and Pu(IV) solubility and speciation in alkaline to hyperalkaline carbonate containing solutions. The new experimental data derived for U(IV), Np(IV) and Pu(IV) obtained in the present work are compared to the comprehensive solubility study and thermodynamic model derived by Altmaier et al. (2004, 2005) on the Th(IV)–OH–CO3 system. Undersaturation solubility batch experiments were performed with UO2(am,hyd), NpO2(am,hyd) and PuO2(am,hyd). Independent batch samples were prepared in 0.1 mol L−1 ≦ [NaCl] ≦ 5.0 mol L−1 solutions in the presence of 0.1 mol L−1 and 0.04 mol L−1 Na2CO3/NaHCO3 at 8 ≦ pHm ≦ 12.5 (pHm = −log [H+]). Redox conditions in the investigated systems were fixed by the addition of reducing chemicals. Measurements of pHm, redox potential and An concentration were performed on a regular basis until attaining equilibrium conditions. Slope analyses and systematic evaluation of the solubility data at different carbonate concentrations and ionic strengths result in consistent chemical models for U(IV), Np(IV) and Pu(IV), where the enhanced solubility can be properly explained by the formation of the complex An(OH)2(CO3)34− as main An(IV) aqueous species in the investigated pH-range. The chemical model derived for U(IV), Np(IV) and Pu(IV) in the present work is significantly different from the one previously reported for Th(IV), where the predominant role of Th(OH)(CO3)45− as well as a greater impact of ionic strength and carbonate concentration on ThO2(am,hyd) solubility were observed. Based upon our new experimental studies and thermodynamic evaluation, we propose a new comprehensive thermodynamic model for the investigated systems based upon the SIT approach favored by NEA-TDB.

Studies on An(IV)-hydroxo-carbonate complex formation along the An(IV) = Th, U, Np, Pu series

256th National Meeting (2018)

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