Diluent and Solvent Effects in Liquid-Liquid Extraction Systems based on bis-triazine-bipyrididine (BTBP)-class Ligands

Licentiate thesis, 2012

Used nuclear fuel taken directly from a reactor is radiotoxic for mankind and its environment for a long time. One of the major contributions to the long time radiotoxicity is the presence of a group of elements called the actinides. If the actinides could be transmuted into less radiotoxic nuclides the strain of the final storage would decrease, both with respect to storage time and volume efficiency. However, this transmutation demands a partitioning of the actinides from the rest of the used fuel. This separation can be achieved by solvent extraction. This work focus on solvent extraction systems based on a special class of extracting molecules, so called BTBPs. The BTBPs extracts trivalent actinides, but it has earlier been showed that the extraction is affected by the diluent used. Therefore the aim of the work presented here has been to try to explain why the properties of diluent affect the extraction. Experiments have been made in order to achieve the basic understanding regarding the extraction, to investigate in what way a variation of the diluent affects the extraction and whether an exchange of the diluent cyclohexanone for hexanoic acid can improve a proposed Group ActiNide EXtraction (GANEX) solvent. Unfortunately it was found that this was not the case. During the basic studies of the extraction it was shown that the interfacial tension can be correlated with the rate of transfer of americium from the aqueous into the organic phase in alcohol diluents. This model will be further developed in the future. It has also been showed that the time needed to reach equilibrium extraction for actinides is shorter than that of lanthanides in two cyclohexanone based GANEX solvents. This makes separation of actinides from lanthanides higher at short contact times. This is valid both in trace concentration experiments and at higher metal concentrations. It has been shown that a mixture of tributyl phosphate (TBP) + cyclohexanone and a mixture of di(2-ethylhexyl) butyramide (DEHBA) + cyclohexanone behave differently with respect to the interfacial tension. In the DEHBA system the interface is depleted with respect to DEHBA, while the interfacial tension in the TBP system is linearly increased by the addition of TBP.