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Hydrometallurgical reprocessing of Spent Nuclear Fuel (SNF) relies on liquid–liquid extraction systems whose safety and sustainability are largely dictated by the organic diluent. While significant efforts have focused on ligand design, the diluent matrix remains a critical bottleneck due to toxicity concerns and incomplete compliance with the CHON principle. In recent years, research has aimed at moving beyond the separation effectiveness achieved with the conventional PUREX process, exploring advanced strategies capable of improving the overall sustainability. Innovative options are being developed that involve the co-extraction of Minor Actinides (MAs) together with Lanthanides (Lns), followed by their subsequent selective separation 1. Tetraoctyl Diglycolamide (TODGA), a benchmark ligand for the co-extraction of MAs and Lns from PUREX raffinate [2], is conventionally dissolved in a 95% hydrocarbon–5% 1-octanol mixture. Despite its widespread use, these formulations pose environmental and safety concerns, motivating the search for greener and more sustainable alternatives.
In this work, two strategies are proposed to redesign the organic phase based on TODGA without compromising the solvent performances. The first approach would bring circular economy principle into SNF advanced recycling, introducing waste cooking oil (WCO) as a renewable modifier instead of 1-octanol. The second strategy explores a paradigm shift through the complete substitution of the diluent with Deep Eutectic Solvents (DESs). Several DES formulations, obtained by tuning different components in various molar ratios, were screened to identify compositions effective in dissolving TODGA while ensuring long-term stability. The newly-proposed organic phases, i.e. 0.1-0.2 mol/L TODGA in kerosene + 5% WCO or in DES-based diluents, were comprehensively investigated by spiked liquid-liquid extraction tests and NMR investigations, to assess ligand solubility, acid compatibility, extraction efficiency under experimental conditions simulating the process environment (up to 3 mol/L nitric acid) and resistance to radiolytic and hydrolytic degradation (up to 200 kGy by gamma radiation, in contact with acid). Compared to the traditional solvent consisting of 0.1 mol/L TODGA in kerosene + 5% 1-octanol, both strategies led to new formulations with comparable extraction efficiencies, combined with remarkable stability under gamma irradiation and prolonged acid contact (Figure 1).
The integration of renewable feedstocks and the implementation of DES-based systems demonstrate that solvent engineering can significantly enhance the sustainability of SNF recycling without sacrificing performance, thus supporting the transition toward safer, greener, and more resilient nuclear fuel cycle technologies.
1 R. Taylor et al., Progress in Nuc. En. Volume 164, 2023, DOI: https://doi.org/10.1016/j.pnucene.2023.104837
[2] G. Modolo et al., Reprocessing and Recycling of Spent Nuclear Fuel, 2015, DOI: https://doi.org/10.1016/B978-1-78242-212-9.00010-1