Résumé
Recycling americium (Am) alone from the spent fuel is an important option studied for the future nuclear cycle (Generation IV systems) since Am is one of the main contributor to the long-term radiotoxicity and heat power of ultimate waste. Since 2008, a liquid-liquid extraction process called EXAm has been developed by the CEA to allow the recovery of Am alone from a PUREX raffinate (already cleared from U, Np and Pu). A mixture of DMDOHEMA (N,N'-dimethyl-N,N'-dioctyl-2-(2-(hexyloxy)ethyl)-malonamide) and HDEHP (di-2-ethylhexylphosphoric acid) in TPH is used as the solvent and the Am/Cm selectivity is improved using TEDGA (tetraethyldiglycolamide) as a selective complexing agent to maintain Cm and heavier lanthanides in the acidic aqueous phase (HNO3 5M). Americium is then stripped selectively from light lanthanides at low acidity (pH 3) with a polyaminocarboxylic acid.The feasibility of sole Am recovery was already demonstrated during hot tests in ATALANTE facility and the EXAm process was adapted to a concentrated raffinate (HAC) to optimize the process compactness. The speciation of TEDGA complexes formed in the aqueous phase with Am, Cm and lanthanides was studied to better understand and model the behavior of TEDGA in the process. Some Ln-TEDGA species are extracted into the organic phase and this specific chemistry might play a role in the Am/Cm selectivity improvement. Hence the hydrophilicity-lipophilicity balance of the complexing agent is an important parameter. In this comprehensive study, new analogues of TEDGA were synthesized and tested in the EXAm process conditions to understand the relationship between their structure and selectivity. New derivatives of TEDGA with different N-alkyl chain lengths and ramifications were synthesized. The impact of lipophilicity and steric hindrance on ligand partitioning and Am/Cm selectivity was investigated. Ligands with different size of spacer between the two amide functional groups were also synthesized. These molecules show an inversion of Am/Cm selectivity with a better affinity for Am instead of Cm.