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Geochemical Modelling as a Tool for Actinide Speciation during Anoxic Leaching Processes of Nuclear Fuel

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Abstract

The behaviour of many radiotoxic elements, which are contained within spent nuclear fuel planned for direct disposal, is strongly dependent on the redox chemistry of the aqueous media present in the deep geological systems, where such disposal options are investigated. Especially actinide elements are known to be sensitive. The knowledge of the speciation of actinides in the case of a possible mobilisation by intruding groundwater is therefore crucial and part of the source term calculation of many countries' performance assessment procedures. In this study, the influence of redox conditions and complexant concentration in two selected groundwaters upon the species distribution of U and Pu is assessed by using the thermodynamic modeling code PHREEQC version 2.3. The modeled systems were derived from laboratory leaching experiments testing nuclear fuel dissolution. For the calculations, critically reviewed data for U and Pu complexation were used. The results show that under the conditions calculated, U and Pu are present mainly as U(VI) and Pu(IV). The presence of solid UO2 substrate is supposed to change this distribution. The calculational use of different groundwaters as aqueous phase, implying a slightly different solution composition, affects U and Pu speciation: U species distribution is affected mainly by complexant concentration, Pu species are influenced by pH changes. A sensitivity study on Pu speciation is performed; constants changed within their confidence intervals were applied. The results are compared with experimental observations, and differences and possible expected effects during the dissolution of real, complex materials are discussed.

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  1. European Commission, Joint Research Centre, Institute for Transuranium Elements, Postfach 2340, D-76125, Karlsruhe, Germany

    M. Amme

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Amme, M. Geochemical Modelling as a Tool for Actinide Speciation during Anoxic Leaching Processes of Nuclear Fuel. Aquatic Geochemistry 8, 177–198 (2002). https://doi.org/10.1023/A:1024302211597

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