Plutonium-239 (t½ = 2.41x104 a) can potentially contribute to the long-term risk in underground repositories for the disposal of radioactive waste. In reducing aqueous environment as foreseen after the closure of a repository, the formation of Pu(III) and Pu(IV) is expected. Uncertainties in the thermodynamic data available for both oxidation states lead to a rather ill-defined redox transition border, especially under alkaline to hyperalkaline pH conditions as those defined by cementitious systems. Isosaccharinic acid (ISA) is the main degradation product of cellulose, which may be disposed of in large quantities along with low- and intermediate-level waste (L/ILW). The strong complexation of ISA with An(III) and An(IV) together with the large inventory of this organic ligand requires dedicated research efforts with focus on the Ca(II)–Pu(III/IV)–ISA system, for which virtually no experimental studies are available in the literature. This contribution summarizes recent and on-going research activities on the solution chemistry of Pu: (i) solubility and redox chemistry of Pu(III)/Pu(IV) in alkaline reducing systems, with focus on the definition of the redox boundary between both oxidation states; (ii) solubility, complexation and redox chemistry of Pu(III)/Pu(IV) in alkaline reducing systems in the presence of ISA and Ca(II); (iii) aqueous speciation of Cm(III) (as analogue of Pu(III)) in the presence of ISA by TRLFS. The results obtained in our study provide quantitative tools for the assessment of solubility and redox processes involving Pu(III) and Pu(IV) in the absence and presence of organic ligands. These studies benefit from the combination of systematic solubility experiments, advanced spectroscopic techniques available at KIT–INE and theoretical density functional theory calculations. This work contributes also to an improved thermodynamic description of Pu in alkaline reducing systems relevant in repositories for L/ILW, which can be implemented in geochemical models/calculations providing further confidence in the safety analysis of such repositories.

Solution chemistry of Pu in alkaline reducing systems: Redox, solubility and complexation with organic ligands

256th National Meeting (2018)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.