This thesis subject is dedicated to the optimization of the uranium/plutonium separation process (PUREX) for spent nuclear fuels reprocessing. New extractant molecules will be studied for targeted elements separation by liquid-liquid extraction from nitric acid medium. They should be soluble in industrial aliphatic diluents, adaptable to extraction technologies actually used in terms of density, viscosity, solubility and extraction kinetics. In order to help in the target molecules selection, theoretical DFT calculations (Density Functional Theory) will be investigated to make a comparative estimate of their affinity towards U and Pu elements. Then the student will have to determine the feasibility of organic synthesis and to optimize synthesis pathways. Once the molecules will be purified and their structures and purity characterized, their separation performances will be assessed by liquid-liquid batch extraction tests involving radioelements. Finally, the structures of metallic complexes formed in organic phases with those new extractant molecules will be studied to better understand extraction mechanisms.