# Accurate Predictions of Volatile Plutonium Thermodynamic Properties

## TL;DR

This paper uses advanced electronic structure calculations to accurately determine thermodynamic properties of volatile plutonium oxides, improving safety predictions in nuclear accident scenarios.

## Contribution

It introduces a relativistic multi-reference computational approach to predict thermodynamic data for plutonium oxides, addressing previous uncertainties and lacking data.

## Key findings

- Predicted enthalpies of formation for PuO2, PuO3, and PuO2(OH)2 with quantified uncertainties.
- Confirmed the predominance of PuO2 in gaseous phase under oxidative conditions.
- Reassessed the electronic ground state and ionization energy of PuO2.

## Abstract

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 $\rm{PuO_2}$, $\rm{PuO_3}$, and $\rm{PuO_2(OH)_2}$, 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 a two-step multi-reference approach (MS-CASPT2 with SO-RASSI), which is able to describe the multireference character of the ground-state wave functions of $\rm{PuO_3}$ and $\rm{PuO_2(OH)_2}$. We benchmark this approach by comparing it to relativistic coupled cluster calculations for the ground, ionized, and excited states of $\rm{PuO_2}$. Our results allow us to predict enthalpies of formation $\Delta_fH^\ominus(\rm{298.15~K})$ of $\rm{PuO_2}$, $\rm{PuO_3}$ and $\rm{PuO_2(OH)_2}$ to be $\rm{-449.5\pm8.8}$, $\rm{-553.2\pm27.5}$, and $\rm{-1012.6\pm38.1~kJ\;mol^{-1}}$, 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 $\rm{PuO_3}$ and $\rm{PuO_2(OH)_2}$ are nonetheless always rather low under a wet atmosphere. Our calculations also permit us to reassess prior results for $\rm{PuO_2}$, establishing that the ground state of the $\rm{PuO_2}$ molecule is mainly of $\rm{^{5}\Sigma_{g}^+}$ character, as well as to confirm the experimental value for the adiabatic ionization energy of $\rm{PuO_2}$.