Development of a new extraction system for selective americium separation from spent nuclear fuels

Recycling americium from spent nuclear fuels is considered as an important option for a future sustainable nuclear fuel cycle. Indeed, americium is the main contributor to the long-term radiotoxicity and heat-load of the remaining waste. To reduce the footprint and lifetime of a final disposal facility it is possible to separate americium and then transmute it into shorter lived radionuclides (which requires high flux neutron irradiation). The PUREX solvent extraction process allows to separate uranium and plutonium from a solution of spent fuel dissolved in nitric acid. In a second step, it would be desirable to separate americium from fission products, and especially lanthanides, but also from curium. As those elements have very close physicochemical properties, their mutual separation remains a scientific challenge. Separation processes based on solvent extraction have already been developed to achieve this arduous separation like EXAm developed at CEA, EURO-EXAm and AMSEL processes, both developed within European collaboration projects (SACSESS and GENIORS). New selective molecules (ADAAM) were also designed at JAEA showing high Am/Cm selectivity. All processes developed in the past show some limitations (high number of extraction stages, low solubility of the complexing agent, non-CHON molecules, low resistance of the complexing agent towards radiolysis, etc…). The objective of this new PhD subject is to develop a new extracting system based on a diglycolamide extractant to co-extract An(III) and Ln(III) in a first extraction step selectively from fission products present in the PUREX raffinate. For the second step, the selective stripping of Am from a loaded diglycolamide-based organic phase, new aqueous soluble complexants will be designed, tested, and a new extraction system based on this molecule will be elaborated. The new molecule(s) should be CHON compliant to ensure facile waste evacuation, and will impose sufficient selectivity to reduce the number of required separation stages. Furthermore, the impact of radiation on the performance of the new solvent extraction system will be investigated. Several new complexing agents will be synthesized (by an external partner/company) and tested for their selectivity towards americium. Also its stability in acid conditions and solubility in suitable diluents will be evaluated. After selection of the complexing agent by screening different candidates, batch extraction data will be acquired to assess the most important parameters and their influence on the separation process (concentrations of extractant, complexing agent, nitric acid concentration, temperature, etc…). Molecular screening based on spiked extraction tests with Am-241, Cm-244 and Eu-152 radionuclides as well as Am macro-concentration experiments will be performed. Speciation techniques such as mass spectrometry (ESI-MS) and UV-VIS, NMR and XAS spectroscopies can be used to better characterize speciation in both phases and take it into account in the model. Then extraction isotherms will be obtained to develop a thermodynamical model and propose a flowsheet of the process. Spiked test in centrifugal contactors can then be conducted to demonstrate the feasibility of Am separation with this new system. In parallel, the radiation resistance of the complexing agent, and the extraction solvent as a whole, will be evaluated using calibrated gamma irradiation tests (BRIGITTE facility at BR2). This PhD project concerns a collaboration between SCK CEN, KU Leuven and CEA (“Commissariat à l'énergie atomique et aux énergies alternatives“). Research stays at the ATALANTE facility of CEA are envisaged to perform experiments and analysis complementary to those at SCK CEN.