Hydriding is a primary degradation mode of plutonium metal, and although it tends to be mitigated by the presence of the native surface oxide, the degree of protection depends heavily on the oxide chemistry. Here, we use hybrid density-functional theory calculations to evaluate the thermodynamics and kinetics of hydrogen in ${\mathrm{PuO}}_2$ as a function of environmental oxygen content. Under O-rich conditions, hydrogen will incorporate primarily through interstitial protons ${\mathrm{H}}_i^{+}$, which peak in concentration at ${10}^{16}$ ${\mathrm{cm}}^{-3}$. ${\mathrm{H}}_i^{+}$ has very small kinetic barriers ($<0.1\mathrm{eV}$), although its diffusion will be limited by its small concentrations. For more O-poor conditions, hydrogen prefers to substitute for oxygen as ${\mathrm{H}}_{\mathrm{O}}$; this species can reach concentrations exceeding ${10}^{20}$ ${\mathrm{cm}}^{-3}$. ${\mathrm{H}}_{\mathrm{O}}$ diffuses into neighboring oxygen vacancies $V_{\mathrm{O}}$ with barriers between 0.39–0.71 eV, depending on the charge state of $V_{\mathrm{O}}$. These barriers will be overcome readily at room temperature. In addition, as conditions become increasingly O-poor, electron polarons ${\eta }^{-}$ will form in higher concentrations, reflecting reduction from ${\mathrm{Pu}}^{4+}$ to ${\mathrm{Pu}}^{3+}$. These species will tend to reduce Coulombic repulsion between ${\mathrm{H}}_{\mathrm{O}}$ and $V_{\mathrm{O}}$, resulting in faster diffusion. Our results provide important insights into the properties of hydrogen in plutonium oxides, which dictate the critical process of hydriding.

Close

PrevNext

View figure in article

• Defects
• Hydrogen bonds
• Polarons
• Actinides
• Density functional theory