A pulse radiolysis investigation of the reactions of tributyl phosphate with the radical products of aqueous nitric acid irradiation

Tributyl phosphate (TBP) is the most common organic compound used in liquid-liquid separations for the recovery of uranium, neptunium, and plutonium from acidic nuclear fuel dissolutions. The goal of these processes is to extract the actinides while leaving fission products in the acidic, aqueous phase. However, the radiolytic degradation of TBP has been shown to reduce separation factors of the actinides from fission products and to impede the back-extraction of the actinides during stripping. As most previous investigations of the radiation chemistry of TBP have focused on steady state radiolysis and stable product identification, with dibutylphosphoric acid (HDBP) invariably being the major product, here we have determined room temperature rate constants for the reactions of TBP and HDBP with the hydroxyl radical [(5.00 ± 0.05) × 10⁹, (4.40 ± 0.13) × 10⁹ M ^-1 s^-1], hydrogen atom [(1.8 ± 0.2) × 10 ⁸, (1.1 ± 0.1) × 10⁸ M^-1 s ^-1], nitrate radical [(4.3 ± 0.7) × 10⁶, (2.9 ± 0.2) × 10⁶ M^-1 s¹], and nitrite radical (<2 x 105, <2 x 105 M-1 s_1), respectively. These data are used to discuss the mechanism of TBP radical-induced degradation.