Accurate Predictions of Volatile Plutonium Thermodynamic Properties

The ability to predict the nature and amounts of plutonium emissions in industrial accidents, such as in solvent fires at PUREX nuclear reprocessing facilities, is a key concern of nuclear safety agencies. In accident conditions and in the presence of oxygen and water vapor, plutonium is expected to form the three major volatile species PuO₂, PuO₃, and PuO₂(OH)₂, for which the thermodynamic data necessary for predictions (enthalpies of formation and heat capacities) presently show either large uncertainties or are lacking. In this work we aim to alleviate such shortcomings by obtaining the aforementioned data via relativistic correlated electronic structure calculations employing the multi-state complete active space with second-order perturbation theory (MS-CASPT2) with a state-interaction RASSI spin-orbit coupling approach, which is able to describe the multireference character of the ground-state wave functions of PuO₃ and PuO₂(OH₂). We benchmark this approach by comparing it to relativistic coupled cluster calculations for the ground, ionized, and excited states of PuO₂. Our results allow us to predict enthalpies of formation Δ_fH(298.15 K) of PuO₂, PuO₃, and PuO₂(OH)₂ to be -449.5 ± 8.8, -553.2 ± 27.5, and -1012.6 ± 38.1 kJ mol^-1, respectively, which confirm the predominance of plutonium dioxide but also reveal the existence of plutonium trioxide in the gaseous phase under oxidative conditions, though the partial pressures of PuO₃ and PuO₂(OH)₂ are nonetheless always rather low under a wet atmosphere. Our calculations also permit us to reassess prior results for PuO₂, establishing that the ground state of the PuO₂ molecule is mainly of ⁵Σ_g⁺ character, as well as to confirm the experimental value for the adiabatic ionization energy of PuO₂.