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 (GTotal(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.