Results show that PuO(2+x), a high-composition (x </= 0.27) phase containing Pu(VI), is the stable binary oxide in air. This nonstoichiometric oxide forms by reaction of dioxide with water and by water-catalyzed reaction of dioxide with oxygen. The PuO(2) + H(2)O reaction rate is 0.27 nanomoles per meter squared per hour at 25 degrees C; the activation energy at 25 degrees to 350 degrees C is 39 kilojoules per mole. Slow kinetics and a low lattice parameter-composition dependence for fluorite-related PuO(2+x) are consistent with a failure to observe the phase in earlier studies. Perplexing aspects of plutonium oxide chemistry can now be explained.

https://science.sciencemag.org/content/sci/287/5451/285.full.pdf

Reaction of Plutonium Dioxide with Water: Formation and Properties of PuO2+x

Thermodynamic modelling of advanced oxide and carbide nuclear fuels: Description of the U–Pu–O–C systems

The electronic structure of PuO2± x was studied using first-principles quantum mechanics, realized with the self-interaction corrected local spin density method. In the stoichiometric PuO2 compound, Pu occurs in the Pu(IV) oxidation state, corresponding to a localized f4 shell. If oxygen is introduced onto the octahedral interstitial site, the nearby Pu atoms turn into Pu(V) (f3) by transferring electrons to the oxygen. Oxygen vacancies cause Pu(III) (f5) to form by taking up electrons released by oxygen. At T = 0, the PuO2 compound is stable with respect to free oxygen, but the delicate energy balance suggests the possible deterioration of the material during long-term storage.

R.S. Street

Papers

The plutonium-oxygen phase diagram