Development of a Simplified Soft-Donor Technique for Trivalent Actinide-Lanthanide Separations

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Abstract

The necessity of reprocessing spent nuclear fuel has arisen from increasing awareness and concern for the environment, in addition to the potential of minimising proliferation. A number of different reprocessing techniques are currently being developed around the world to allow useful spent nuclear fuel (SNF) to be recycled and reused and the remaining waste to be treated. One such technique, currently being developed in the USA is the TALSPEAK process, an advanced reprocessing method for the separation of trivalent lanthanide (Ln3+) and minor actinide (MA3+) components. This process, developed in the 1960s at Oak Ridge National Laboratory, uses DTPA to act as a holdback reagent for MA3+, in a lactate buffered aqueous phase at pH 3.6, allowing Ln3+ to be selectively extracted by organophosphate HDEHP into an organic phase of DIPB or dodecane. TALSPEAK is one of the most promising techniques being researched due to its numerous advantages, particularly its relative resistance to radiolysis and its ability to be carried out without the need for high reagent concentrations. Additionally it gives high separation factors, in the region of ~50-100, comparable to other advanced reprocessing methods under development. The chemistry of the process is very complex and not particularly well understood so it would be advantageous to simplify the process by removing the need for a separate holdback reagent and buffer.In collaboration with colleagues at the Idaho National Lab, the use of amino acids as a potential combined buffer and soft donor was investigated. Although it was found that amino acids do not act as holdback reagents in their own right, optimisation of an L-alanine buffered TALSPEAK system with DTPA was found to allow the process to be carried out effectively at a lower pH of 2, which is more preferable for industrial application.As an extension of this, separation studies were carried out using the tripeptide L-glutathione (GSH) to determine its potential for use as a combined buffer and soft-donor. As with the studies with amino acids, it was found that GSH also does not act as a holdback reagent in its own right, however it does interact with Ln-DTPA complexes at pH 4. When optimised at this pH, separation factors of up to 1200 were achieved for Eu3+/Am3+, whilst still maintaining low MA3+ partitioning. However, further studies by ICP-MS and luminescence spectroscopy showed that a GSH buffered system was not effective for extraction of heavier lanthanides, although the results show the potential for further investigation into other short and longer chain peptide buffered systems and possibly lanthanide-lanthanide separations.Further studies were carried on amino acid appended DTPA ligands which were synthesised in a one step reaction in order to create a combined buffer and soft donor. The ligands were found to self-buffer at around pH 2 and allow successful separation of Eu3+/Am3+ (SF ~ 100). The results from initial investigations by luminescence spectroscopy and solvent extraction are promising and are presented here. Further work is needed on these systems in order to optimise their extraction capability and minimise Am3+ partitioning. In the future this work could promote studies for better understanding of TALSPEAK chemistry that could be used in industrial partitioning processes.

Keyword(s)

Actinide; Amino Acid; DTPA; Lanthanide; Separations; Solvent Extraction; TALSPEAK

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