Evaluation of fundamental radionuclide extraction data for UREX

Abstract

The speciation of technetium and actinides in advanced solvent extraction systems is the basis for their manipulation in separations. The ability to understand and predict radionuclide speciation is paramount to successful modeling of proposed separation systems. This project will examine the speciation of radionuclides in different stages of the UREX separation scheme, providing data useful to modeling. The areas to be examined include the speciation of U and Pu with tributylphosphate and the kinetics and thermodynamics of lanthanides and actinides in the TALSPEAK system. The complexation constants ofU and Pu with tributylphosphate will be evaluated. In the TALSPEAK system, studies will elucidate the difference in complexation kinetics for the lanthanides and actinides. Computational studies based on density functional theory will be performed for both systems. Evaluation of fundamental radionuclide extraction data for UREX ................................................ 1 1. Background and Rational ......................................................................................................... 1 1.1. Tribuylphosphate based extractions ................................................................................. 1 1.2. Lanthanide-Actinide Separations ..................................................................................... 1 1.3. Evaluation of Chemical Thermodynamic and Kinetic Data ............................................ 3 2. Research Objectives and Goals ................................................................................................ 4 3. Technical Impact ...................................................................................................................... 5 4.1. Determination of complexation constants for the U and Pu TBP system ........................ 5 4.2. Collection of thermodynamic and kinetic data for T ALSPEAK ..................................... 5 4.3. Task 3. Computational Studies ......................................................................................... ? 5. Project Timeline ....................................................................................................................... 8 6. References ................................................................................................................................ 8 1. Background and Rational 1.1. Tributylphosphate based extractions The extraction of tetravalent Pu in nitric acid solution by tributylphosphate (TBP) is well known and has been the basis of Pu purification for a number of decades. The extraction is based on the formation of an organic phase neutral complexes such as Pu(N03)4•2TBP. Upon reduction, the trivalent Po species is back extracted into an aqueous phase. However, the data needed for detailed modeling of Pu extraction is not available for all conditions of concern to the AFCI. Based on the direction from Dr. George Van de grift of ANL, this proposal was developed to obtain the necessary data for Pu extraction modeling. Understanding the role of nitrate in Pu speciation is important for determining the necessary data for Pu extraction modeling. Data indicates the dinitrate complex of Pu is strong and may account for difficulties in modeling Pu extraction under some nitrate conditions. The formation of hydrolysis products may also form an extractable species that will need to be included in modeling. In the UREX process, acetohydroxamic acid (AHA) is expected to be used (Figure I). The interaction of AHA with tetravalent Pu decreases extraction into the organic phase by either complex formation or reduction of Pu to the trivalent state. For the separation of Pu in a solvent extraction system using acetohydroxamic acid, it is necessary to determine the complexation kinetics and thermodynamics [I]. In addition, since Puis a redox sensitive element, the change in speciation due to oxidation-reduction reactions upon complexation needs to be evaluated. This project will experimentally evaluate the fundamental speciation of Pu and U in the TBPdodecane-nitric acid system, with the main emphasis on nitrate speciation and subsequent third phase formation at high nitric and metal ion conditions. Experiments on the AHA systems will be conducted in collaboration with the and with input from DOE separations group. For this project the focus will be on the evaluation on U and Pu complexation constants with TPB. Figure 1. Acetohydroxamic acid 1.2. Lanthanide-Actinide Separations A schematic of a proposed UREX separation (Figure 2) demonstrates the desired separations of actinides and fission products for the advanced fuel cycle. Some of the separations are well developed (i.e., UREX, essentially a modified PUREX step) while others require fmther investigation. The separation of trivalent lanthanides from trivalent actinides, and even the actinides Am from Cm, has been identified as a topic for investigations by GNEP. These separations are based on slight differences in ionic radii, small variations in Lewis acidity between the lanthanides and actinides, and redox state speciation. Numerous ligands have been examined for actinide and lanthanide separations, some of which are the basis of separation schemes (Figure 3). In this work efforts will focus on the TALSPEAK process through the determination of thermodynamic and kinetic data necessary for

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Cite this paper

@inproceedings{Czerwinski2015EvaluationOF, title={Evaluation of fundamental radionuclide extraction data for UREX}, author={Kenneth R. Czerwinski and Ken R Czerwinski and Jamie Lee Warburton and Cynthia L Gong}, year={2015} }