Picosecond time-resolved investigation of NO₃˙ in PUREX solvent

The early phenomena induced by the irradiation of solvents used in the PUREX process are investigated using picosecond electron pulse radiolysis. Various concentrations of solvent components - namely nitric acid (HNO₃), tributyl phosphate (TBP), dodecane, and, to a lesser extent, water are studied, in absence of the spent nuclear fuel. The transient optical absorption spectra at 5 ps constitute of the spectra of solvated electron in TBP and NO₃˙ radical, characterized by absorption band with maxima at 640 nm and 670 nm, as well as a band at 440 nm. The yield of each species is determined. While the concentrations of HNO₃ and TBP of the environments exhibit opposing trends, the highest NO₃˙ yield is recorded in a solution of 1.9 M HNO₃/3.3 M TBP/0.8 M H₂O (G_Total(NO₃˙) = 3.2 × 10^-7 mol J^-1). This yield arises from both the direct radiolytic effect on HNO₃ and the ultrafast indirect effect of energy transfer-via excitation and ionization-from TBP and H₂O to HNO₃, which act as the primary precursors of NO₃˙. Replacement of part of TBP by dodecane has little influence because they behave similarly. Despite the complexity of the system, the partial dose absorbed by each component is used to unravel the mechanism of NO₃˙ formation and yields corresponding to direct and indirect effects across the range of concentrations studied. Interestingly, at highest HNO₃ concentration the absorbance and yield of NO₃˙ is lower. At longer time within a few hundred nanoseconds, the decay of the NO₃˙ radical follows pseudo-first-order kinetics and it is attributed to a hydrogen-atom transfer from TBP and dodecane to NO₃˙. But the rate constant of this reaction is very low (0.9 × 10⁶ L mol^-1 s^-1) and, if the spent fuel were present, the oxidation by NO₃˙ of uranium and plutonium ions would be expected to be predominant.