Molecular Dynamics Mechanisms of Effect of TBP-n-dodecane Radiolysis on Viscosity

To elucidate the molecular mechanism by which radiolysis affects the viscosity of solvents in the Purex process, this study employed molecular dynamics simulations on model systems of long-chain organophosphates with varying degrees of polymerization within TBP-n-dodecane systems, and conducted ⁶⁰Co gamma irradiation experiments on a 30vol% TBP-n-dodecane mixture solvent. The viscosities of NPT ensembles with different proportions of tributyl phosphate (TBP), n-dodecane, and their long-chain compounds were systematically simulated. According to the theoretical study, the increase in the content of long-chain organophosphate ester degradation products will drive viscosity growth through enhanced molecular entanglement, expanded van der Waals contact area, and directional dipole interactions of P=O bonds. The viscosity shows a linear growth trend (1.86-3.44 mPa·s). The molecular topology exhibits non-monotonic regulation on viscosity: The pure-phase viscosity of the T-C-T structure jumps significantly to 10.06 mPa·s due to the doubling of polar sites; The “lubricant” effect of flexible alkyl chains in the T-C-T-C structure reduces the pure-phase viscosity to 7.74 mPa·s; The viscosity of the T-C-T-C-T pure-phase system rises to 12.78 mPa·s because more TBP groups are locked with each other. This confirms that the increase in the number of TBP groups dominates intermolecular locking and sliding inhibition, and thus determines viscosity behavior. Combined with P element radial distribution function analysis, it is demonstrated that the increase in TBP groups within the molecular topology induces the non-monotonic change in viscosity via an enthalpy-driven mechanism. Furthermore, ⁶⁰Co irradiation experiments reveal that high-dose irradiation forms highly polar topological structures, leading to entropy-enthalpy imbalance, reducing contribution of alkyl chain flexibility, and consequently accelerating viscosity increase. Based on the scientific findings, the engineering risk of pipeline blockage originates from the formation of persistent three-dimensional crosslinked networks during continuous solvent radiolysis via metal coordination, P-O-P crosslinking, and micellar self-assembly. This network formation triggers moisture adsorption and the generation of crud-like precipitates, ultimately obstructing pipelines. Consequently, effective risk mitigation necessitates an integrated approach combining strategies of irradiation dose control, rheological monitoring, and staged chemical treatment (initial washing with 5% NaOH followed by deep treatment with concentrated acid/oxidant).