Interinstitutional Study of the New EURO-GANEX Process Resistance by Gamma Irradiation Test Loops

Future advanced nuclear fuel cycles are currently under development around the world with the purpose of increasing nuclear fuel cycle sustainability and reducing the long-term radiotoxicity and heat load of nuclear waste by means of separation and transmutation of the transuranic elements (TRU), specifically the minor actinides (MA: Np, Am, Cm), that together with Pu, have the highest contribution to the long-term radiotoxicity of spent nuclear fuel. With this aim, new separation processes based on solvent extraction for the recovery of TRU are being developed globally, which could follow the heterogeneous or homogeneous strategy for the actinides recycling. As part of the homogeneous actinide recycling strategy, the EURO-GANEX process is one of the most promising options to achieve the goal of minor actinides recovery. Improvements made to EURO-GANEX system have resulted in the emergence of the so-called New EURO-GANEX process, where the composition of the solvent has been modified by replacing TODGA and DMDOHEMA with cis-mTDDGA in the organic phase and SO₃-Ph-BTP with PyTri-Diol in the aqueous phase in order to resolve important issues such as the use of compounds that do not comply with the CHON principle. The objective of this work is 2-fold: evaluate the gamma radiolytic resistance of the new EURO-GANEX process by dynamic irradiation conditions simulating the three main steps of the process and validate the design of CIEMAT Nayade, CEA Marcel, and INL irradiation loop devices since each of them mimics different aspects of the real process. For that reason, this work is involved as part of an inter-institutional collaboration between CEA (Commissariat a l'energie atomique et aux energies alternatives) in France, INL (Idaho National Laboratory) in the United States, and CIEMAT (Centro de Investigaciones Energeticas, Medioambientales y Tecnologicas) in Spain. For this collaboration, each research centre employed its irradiation loop device to evaluate the resistance of the New EURO-GANEX process to gamma radiolytic degradation using similar conditions and same solutions. In particular, the Nayade and INL loops could irradiate the organic and aqueous phases together, whereas in the CEA loop, the irradiated solvent is recycled continuously inside a platform with several stages of mixer-settlers containing aqueous flows simulating the three main steps of the process. The extraction performances and changes in the composition of the solvent have been analyzed during the irradiation experiment by different techniques: gamma spectrometry and ICP-MS/OES for cations or radioactive tracer extraction and HPLC-MS to identify and quantify the degradation compounds. Despite some differences between the three irradiation facilities, this interinstitutional study shows that these three comparative tools provide similar trends in the radiolytic stability of a liquid-liquid extraction system. Favorable extraction results for the different steps are obtained according to the static irradiation studies found in literature. However, the degradation of cis-mTDDGA is appreciable leading to degradation compounds, some of which form precipitates and produce substantial changes in viscosity, important aspects that must be addressed prior to the successful industrial application of the new EURO-GANEX process. (authors)