Livrables
The objective is to provide basic process understanding on the formation of secondary solid phases and the incorporation of radionuclides, and on the modelling of the solid solutions formed. The formation of solid solutions of radionuclides is studied for calcite and phosphate minerals. Substantial progress has been achieved in the following fields:
- The fundamental mechanistic study has been focussed on the incorporation of trivalent actinides (Cm(III) and their homologues Eu(III)) in calcite as a model mineral system. By using a combination of wet-chemistry methods, various spectroscopic and microscopic techniques and thermodynamic modelling, a breakthrough has been achieved in understanding the incorporation mechanism on a molecular level. The derived thermodynamic based quantification of the strong interaction of actinides with calcite is important to model their retention in clay and cement waters.
- Relevant secondary phases such as phosphates, barite and Ca-silicate hydrates, have been included in the study on secondary phases. By studying incorporation of radionuclides in synthetic and natural minerals important results have been obtained.
The objective was to eliminate the most serious deficiencies in the thermodynamic database for the actinides in aqueous solution. Substantial progress has been achieved in the following fields:
- Systematic study on solubility, colloid formation, hydrolysis and ternary hydoxo carbonate complexation of tetravalent actinides (Th, U, Np and Pu) in aqueous solution. Results included in NEA-TDB update.
- Complexation of actinides (U(VI), Th(IV) and Cm(III)) with small organic ligands, such as á-hydroxy carboxylic acids, with emphasis on formation of ternary complexes at high pH. Speciation by NMR, EXAFS and TRLFS. Comparison with quantum chemical calculations. Determination of thermodynamic data.
- Complexation of actinides by minor ligands, such phosphate, which is poorly known, but may be important for the speciation in groundwaters and for the assessment of phosphate minerals as engineered barrier.
- Redox behaviour of actinides, which is not well known on a molecular level, but necessary to predict the geochemical behaviour of actinides, which is dominated by their oxidation state.
- Development and application of computational tools for the prediction of structural, spectroscopic and thermodynamic data of actinides. Strong and successful interaction with a group of theoretical chemists, working outside the ACTAF project.
The objective is to provide basic understanding of sorption mechanism, sound thermodynamic data for the sorption of actinides on relevant mineral phases, and validation and development of appropriate models. Substantial progress has been achieved in the following fields:
- Comprehensive experimental data and state-of-the-art analysis by surface complexation model (SCM) for the sorption of U(VI) and Pu(IV) on magnetite and goethite, and for the sorption of U(VI) and Eu/Cm(III) on illite.
- Identification of sorption mechanism (ion exchange, adsorption, incorporation etc) and of spectroscopic characterization of surface complexes by TRLFS on Cm(III) at trace concentration range. Elucidation of interface reaction at molecular level by studying single crystals with various spectroscopic and microscopic methods.
- Sorption study of U(VI) on rock (phyllite and granite) and their mineralogical constituents and alteration products. Successful parameterisation of composite SCM for the prediction of sorption behaviour on rock based on their mineralogical composition.
The general progress achieved in this field has to be considered as a breakthrough for a reliable thermodynamically based modelling of actinide retention by sorption on mineral phases.
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