  Tributyl-phosphate (TBP), a ligand used in the PUREX liquid-liquid separation
process of spent nuclear fuel, can form explosive mixture in contact with
nitric acid, that might lead to violent explosive thermal runaway. In the
context of safety of a nuclear reprocessing plant facility, it is crucial to
predict the stability of TBP at elevated temperatures. So far, only the
enthalpies of formation of TBP is available in the literature with a rather
large uncertainties, while those of its degradation products, di-(HDBP) and
mono-(H$_2$MBP}) are unknown. In this goal, we have used state-of-the art
quantum chemical methods to compute the formation enthalpies and entropies of
TBP and its degradation products di-(HDBP), mono-(H$_2$MBP) in gas and liquid
phases. Comparisons of levels of quantum chemical theory revealed that there
are significant effects of correlation on their electronic structures, pushing
for the need of not only high level of electronic correlation treatment, namely
local coupled cluster with single and double excitation operators and
perturbative treatment of triple excitations [LCCSD(T)], but also
extrapolations to the complete basis to produce reliable and accurate
thermodynamics data. Solvation enthalpies were computed with the conductor like
screening model for real solvents [COSMO-RS], for which we observe errors not
exceeding 22 kJ mol$^{-1}$. We thus propose with final uncertainty of about 20
kJ mol$^{-1}$ standard enthalpies of formation of TBP, HDBP, and H$_2$MBP which
amounts to -1281.7$\pm$24.4, -1229.4$\pm$19.6 and -1176.7$\pm$14.8 kJ
mol$^{-1}$, respectively, in the gas phase. In the liquid phase, the predicted
values are -1367.3$\pm$24.4, -1348.7$\pm$19.6 and -1323.8$\pm$14.8 kJ
mol$^{-1}$, to which we may add about -22 kJ mol$^{-1}$ error from the COSMO-RS
solvent model. From these data, we predict the complete hydrolysis of TBP to be
nearly thermoneutral.
