Thermal Transformations of Radicals Derived from Nuclear Waste Separation Ligands Exposed to Radiation

The thermal evolution of electron-beam-induced radicals generated in neat di-n-butyl N,N-diethylcarbamoylphosphonate (DBDECP), N,N-bis(2-ethylhexyl)butanamide (DEHBA), dioctyl phosphate (DOP), tributyl phosphate (TBP), and N,N,N',N'-tetraoctyl diglycolamide (TODGA), and TBP/dodecane extraction system was investigated using electron paramagnetic resonance (EPR) spectroscopy over the temperature range of 108-220 K. Density functional theory (DFT) calculations were employed to support the experimental results, clarifying radical structures and energetics of their thermal transformations. The stability of radicals as temperature increased varied significantly depending on the ligand structure. The diglycol- or amide-centered radicals, such as those found in TODGA and DEHBA, exhibited higher thermal stability than the alkyl radicals. This increased stability is attributed to the greater steric hindrance in the latter species. Calculated energy barriers (∼20 kcal/mol) suggested that thermal evolution predominantly involves conformational rearrangements of the radicals, rather than transformations of the radical structure itself. Additionally, trace amounts of oxygen led to the conversion of carbon-centered radicals into peroxide radicals during thermal annealing. The peroxide radicals formed decomposed more rapidly at elevated temperatures. This study provides important mechanistic insights into the radiolytic degradation of nuclear waste extraction ligands, helping to advance the development of more radiation-resistant solvents for nuclear waste separation processes.