Uranium and plutonium can already be industrially separated from spent nuclear fuels by the PUREX solvent extraction process. By recovering americium from a PUREX raffinate, the capacity of a deep geological repository can be increased by a factor of up to seven. This separation became feasible by ingeniously combining the selectivity of a suitable extracting agent (TODGA) and a water-soluble complexing agent (PrOH-BPTD). The former co-extracts americium, curium, and lanthanides into the organic phase, rejecting other fission products (FP). The development of this process, called AmSel, was already initiated during previous European projects but the selectivity could be further improved, especially the Cm/Am separation factor. In order to separate those elements, which have very close physico-chemical properties, both the lipophilic extractant molecule in the organic phase and the complexing agent in nitric acid medium should be optimized. Batch extraction tests will be performed in glove boxes in ATALANTE facility at CEA Marcoule with radionuclides of interest (241Am, 244Cm, 152Eu). The behavior of relevant fission products (e. g. Tc, Pd, Zr, Mo, Ru, Sr) both in extraction and stripping conditions will also be evaluated. Experiments using a simulated feed solution containing all elements (including americium) in nominal concentrations will validate the loading capacity and separation performances. The resistance towards radiolysis of the selective ligand used as Am stripping agent in the aqueous phase will be evaluated by in situ alpha irradiations with 241Am in nominal concentration. Degradation will be evaluated by ESI-MS measurements coupled with HPLC to both identify and eventually quantify degradation products and complexes formed with those compounds.