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Sur des solutions contenant en plus de l'uranium des ions sodium, marquées au 22Na euou du plutonium 239, l'extraction de l'uranium reste optimale. Sur des échantillons d'urine, après minéralisation, environ 100 % de l'uranium sont extraits par le calixarène. Ces premiers résultats montrent que le calixarène choisi présente une structure adéquate vis-à-vis de la structure de coordination de l'ion uranyle, permettant ainsi une extraction sélective de l'uranium. ABSTRACTSelective extraction of actinides: Application to radiotoxicological analysis. The extraction efficiency of uranium has heen studied in a two phases solventextraction system with a macrocyciic molecule, a calix[6]arene, bearing carboxy groups. The aqueous phase contained uranium in presence of sodium ions and the caiixarene was dissolved in benzene. The study of the extraction parameters allowed to propose an extraction mechanism. The extraction efficiency was found to increase with pH, with an optimal value around 100%. On synthetic solutions containing in addition to uranium sodium ions (spiked with 22Na) a n d or no decrease of uranium extraction was ohserved. When applied to urine samples, after organic digestion, about 100% of uranium have been extracted hy the caiixarene at pm3.7. These first results have shown that the caüxarene which has been selecteà has an adequate structure relative to the uranyl ion co-ordination structure, which permits a selective extraction of uranium.* service de dosimétrie, IPSN, B .P. no 6,