In nuclear fuel reprocessing, after separation of uranium and plutonium using the PUREX process, minor actinides (americium and curium) are the main contributors to the long term radiotoxicity and heat load of the spent fuel. Therefore, new processes have been developed to separate americium and curium using polyaminocarboxylic acids as selective complexant. Due to the presence of radioactive elements, these organic ligands in solution are subjected to radiolysis, thus leading to a possible impact on the process efficiency. Understanding the behavior of polyaminocarboxylic acids when exposed to ionizing radiation is important for further development of minor actinides separation processes. The aim of this work is to study the radiolytic stability of polyaminocarboxylic acids and determine if their stability is different when the ligand is free in solution or involved in a complex with a metallic cation.
In this study, the cyclic polyaminocarboxylic acid, DOTA (1,4,7,10-tetraazacyclo-dodecantetraacetic acid) ligand was chosen as a reference molecule, due to its interesting properties regarding the complexation of actinides (An) +III and +IV. DOTA¬An complexes with a 1:1 stoichiometry are very stable. The preliminary experiments consist in external radiolysis on a cyclotron facility or a gamma irradiator of DOTA free in solution or DOTA involved in a complex with a lanthanide (non-radioactive metal). Then the nature and the energy of the ionizing radiation, the composition of the aqueous solutions (pure water or nitric acid) and the effect of the complexation will be examined on all these systems.
More precisely, the degradation of free DOTA and DOTA involved in a complex with neodymium (DOTA-Nd) in aqueous solutions was investigated by external radiolysis on a cyclotron facility. The DOTA solutions were irradiated by alpha particles (4He2+) with energies about 9.4 MeV and 60.7 MeV at a maximum deposed dose of 18kGy. Yield formation of gases like H2 and CO2 as well as yield formation of products in solution (H2O2 and/or HNO2) were determined for different concentrations of DOTA. Moreover, nuclear magnetic resonance (NMR) and electrospray ionization mass spectrometry (ESI-MS) analysis on irradiated DOTA solutions enabled the identification of some DOTA’s degradation products which are mainly due to the decarboxylation of the molecule or the opening of the cycle.
 M. Audras et al., Inorg. Chem., 2017, 56 (20), 12248-12259.
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Atomic Energy Commission – Marcoule Center – Nuclear Energy Division, Research Department on Mining and fuel Recycling Processes – BP 17171 – 30207 BAGNOLS-SUR-CEZE CEDEX – FRANCE