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Selective extraction of minor actinides from high activity liquid waste by organized matrices


Reduction of the volume of wastes to be disposed off in geological formations is one major strategic challenge for the nuclear community. A way to sharply decrease this volume is to selectively remove the long-lived elements (actinides, 137Cs, 90Sr). Hence, a large part of the initial wastes could be directed to a subsurface repository and a very small part containing most of the long-lived radionuclides to be disposed off, after conditioning, in geological formation. The removal of these long-lived elements from high sodium nitrate content acidic liquid wastes needs very selective extractants in order to minimize the volumes of wastes in geological formations. Former projects funded by EC have focused on the removal of 137Cs and 90Sr and have proposed 2 functionalised macrocycles presenting extraction properties for selective extractions. The purpose of this project was to find a selective extractant for removal of minor actinides from high activity liquid waste, and specially to separate minor actinides from lanthanides, both having similar chemical properties. During the CALIXPART project, 160 extractants were synthesised by the universities of Madrid, Mainz, Parma, Twente, Liege, Micromod, Katchem and the Institute of Inorganic Chemistry of Rez: calixarenes, cavitands, tripodal compounds, beads or dendrimers bearing different ligating sites like soft and hard donor groups, CMPO, picolinamide, malonamide, glycolamide, COSAN, TTFA. The extraction properties of the compounds synthesised were tested at CEA Cadarache, ECPM and NRI. Promising compounds were sent to CIEMAT for stability tests towards hydrolysis and radiolysis. NMR spectroscopy was used in Liege to elucidate the structures of extracted species as well as luminescence studies to determine the hydration state of complexed ions. Some key factors involved in the efficient and selective extraction of cations by suitable ligands, were studied by ULP using computer simulations. The most interesting results achieved during the present project can be summarized in the following items: - Preliminary results revealed a very promising actinide/lanthanide separation of a simple tripodal system containing tri-CMP ligating sites. Nevertheless, a third phase appears during extraction preventing from an possible use of this compound for industrial liquid-liquid separation. Effort should be done to improve this compound and to find the good system (solvent-extractant). - Picolinamide derivatives show interesting Am/Eu selectivity but an efficiency, which is highly pH, dependent. In fact, whereas distribution coefficients are quite high at [HNO3] less or equal than 10{-3}M, they decrease drastically for higher concentration. Combined with Cosan, these derivatives could however exhibit promising characteristics. - Wide rim tetra CMPO calyx [4]arenes present good extraction abilities and also rather good selectivities at high acidity. But, the stability towards hydrolysis is not very good. N-methylated compounds exhibit low extraction efficiency but a higher stability towards hydrolysis when they are in contact with concentrated nitric acid (3M). - Tetra CMPO cavitand presents nearly the same extraction properties of wide rim tetra CMPO calix[4]arene but a slightly higher selectivity. Attempts on extending the chain length between the moieties and the platform were not successful. - Among functionalised calix[6]arene, narrow rim hexa CMPO calix[6]arenes are very promising, having very high distribution coefficients, especially at high acidities, and also remarkable selectivities - Extractions by using dendrimer or magnetic particle supported polypropylenimine bearing CMPO moieties are of particular interest because of the simplified process used (liquid extraction and filtration or magnetic separation). Moreover, this kind of approach does not need the use of an organic phase, which reduces considerably the volume of wastes. - Wide rim tri CMPO-monoacetamide calix[4]arene was found to be a very promising selective extractant. But, the tests performed on simulated outlets from the purex process were not very conclusive because of the relatively low solubility of the molecule in the organic solvent used. Anyway, this compound was send to CIEMAT for stability studies. - Selective extraction with magnetic and non magnetic particles bearing CMPO calix[4]arenes on the surface was particularly good and it seems that it is clearly advantageous to pre-organise the CMPO functions on a common platform before attachment on particles. Furthermore, short spacers between the platform and the CMPO moieties were found to be more efficient. - The covalent attachement of dendrimers with a high number of terminal amino groups on the surface of porous silica particles allowed the binding of selective chelators for radionuclides in a high density. The introduction of the dendrimer spacer leads to a 50 to 400fold increase of the distribution coefficients for the Eu and Am extraction in comparison to the reference particles without the dendrimer. - Calix[4]pyrazolone extracting agent forms a highly unusual electrically neutral complex in which a lanthanide ion and a sodium ion are encapsulated close to each other in a helical arrangement. Distribution studies and quantum modelling optimisations show that the coordination of sodium is highly specific and favors the extraction of the lanthanide ions. - Adding BrCosan in the organic phase enhances the extraction ability at low pH but has no effect on selectivity. Calix[4]arene bearing Cosan exhibits higher extraction ability than its synergic mixture. - The aggregation state of extracted metal complexes has been investigated by the nuclear magnetic relaxation dispersion technique (NMRD). The formation of aggregates is the first step in the appearance of a third phase during extraction processes and is thus of practical interest. Calix[4]arenes substituted on the wide rim easily form supramolecular entities while their narrow rim analogues have a monomeric behavior. Contrary to expectation, calixarene dendrimers are an exception to this rule and forms polymetallic complexes of well-defined stoichiometry.

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