Molecular Insights into the Extraction and Stripping Mechanisms of UO ₂ ²⁺ and Th ⁴⁺ by the Externally α-Irradiated Tri- iso -pentyl Pyrophosphate

Understanding the extraction and stripping behavior of UO 2 2+ and Th 4+ with α-irradiated extractants is critical for their applications in spent nuclear fuel (SNF) reprocessing. In this work, the effect of external α-radiolysis on the extraction and stripping of UO 2 2+ and Th 4+ by the TiAP extraction system was investigated via experimental and theoretical approaches. Experiments indicated that α-radiolysis enhanced the extraction of UO 2 2+ and Th 4+ but suppressed their stripping. Mass spectrometry (MS) and molecular dynamics (MD) simulations demonstrated that strong electrostatic interactions between the representative α-radiolysis products, di- iso -amyl phosphate (DiAP – ) and tri -iso -pentyl pyrophosphate (TriPP – ), and the metal ions led to the formation of UO 2 (DiAP/TriPP) 2 (TiAP) 2 and Th(DiAP/TriPP) 4 (TiAP) complexes, thereby enhancing extraction. Furthermore, the adsorption of stable UO 2 (DiAP/TriPP) 2 (H 2 O) 3 and Th(DiAP/TriPP) 4 (H 2 O) 5 complexes at the liquid–liquid interface hindered stripping and may promote third-phase formation. Moreover, the minor acidic pyrophosphate ester compounds were found to be more detrimental to the TiAP extractant system. This work elucidates the effects of α-radiolysis on UO 2 2+ and Th 4+ extraction in the 36% TiAP- n DD/HOK–HNO 3 system, providing crucial insights for the subsequent application of TiAP in SNF reprocessing.