The DIAMEX process is one of the most promising processes for partitioning the trivalent actinides from the highly active raffinates (HARs) coming from the PUREX process. In the present work, experiements for the definition of a DIAMEX process for treating high-active concentrates (HACs), the product after the concentration and denitration of HAR, have been performed. Within the DIAMEX process, oxalic acid has to be added to the DIAMEX feed in order to prevent third-phase formation due to co-extraction of some fission products, e.g. Zr and Mo. However, the experimental results indicate that after adding oxalic acid to the HAC a precipitate is formed which, in addition to Zr and Mo as the main constituents, also contains considerable amounts of lanthanides, and thus will lead perhaps to a loss of trivalent actinides. This unusual behaviour of Ln(III) coprecipitation was further investigated. Thus, other Zr, Mo complexing agents were tested and with D-mannitol promising results were achieved. With D-mannitol we did not observe any precipitates and the extraction experiments could be performed using the DIAMEX solvent composed of 1.0 mol/L DMDBTDMA in TPH. Extraction results as the basis for the development of a continuous counter-current test will also be presented.

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