Facing the challenge of predicting the standard formation enthalpies of n-butyl-phosphate species with ab initio methods

摘要

Tributyl-phosphate (TBP), a ligand used in the PUREX liquid-liquid separationprocess of spent nuclear fuel, can form explosive mixture in contact withnitric acid, that might lead to violent explosive thermal runaway. In thecontext of safety of a nuclear reprocessing plant facility, it is crucial topredict the stability of TBP at elevated temperatures. So far, only theenthalpies of formation of TBP is available in the literature with a ratherlarge uncertainties, while those of its degradation products, di-(HDBP) andmono-(H$_2$MBP}) are unknown. In this goal, we have used state-of-the artquantum chemical methods to compute the formation enthalpies and entropies ofTBP and its degradation products di-(HDBP), mono-(H$_2$MBP) in gas and liquidphases. Comparisons of levels of quantum chemical theory revealed that thereare significant effects of correlation on their electronic structures, pushingfor the need of not only high level of electronic correlation treatment, namelylocal coupled cluster with single and double excitation operators andperturbative treatment of triple excitations [LCCSD(T)], but alsoextrapolations to the complete basis to produce reliable and accuratethermodynamics data. Solvation enthalpies were computed with the conductor likescreening model for real solvents [COSMO-RS], for which we observe errors notexceeding 22 kJ mol$^{-1}$. We thus propose with final uncertainty of about 20kJ mol$^{-1}$ standard enthalpies of formation of TBP, HDBP, and H$_2$MBP whichamounts to -1281.7$\pm$24.4, -1229.4$\pm$19.6 and -1176.7$\pm$14.8 kJmol$^{-1}$, respectively, in the gas phase. In the liquid phase, the predictedvalues are -1367.3$\pm$24.4, -1348.7$\pm$19.6 and -1323.8$\pm$14.8 kJmol$^{-1}$, to which we may add about -22 kJ mol$^{-1}$ error from the COSMO-RSsolvent model. From these data, we predict the complete hydrolysis of TBP to benearly thermoneutral.