Reactions of plutonium dioxide with water and hydrogen-oxygen mixtures: Mechanisms for corrosion of uranium and plutonium

Investigation shows that plutonium dioxide interacts chemically with water and catalytically with oxygen-hydrogen mixtures to form water. Water adsorbs strongly on the oxide below 120 degreesC and desorbs as the temperature is increased to 200 degreesC. Hydroxide formed by dissociative adsorption of water promotes formation of the higher oxide (PuO2+x) plus H-2, and in the presence of O-2, drives a water-catalyzed cycle that accelerates formation of PuO2+x by the PuO2 + O-2 reaction. Results are consistent with kinetic control of plutonium corrosion by the adherent oxide layer on the metal and imply that moisture-enhanced oxidation is driven by the water-catalyzed cycle. Evaluation of experimental results and literature data for U and Pu lead to a comprehensive corrosion mechanism applicable to both metals in dry air. water vapor, and moist air. In all cases, corrosion proceeds by diffusion of oxide ions through the oxide product, not by transport of hydrogen or hydroxide. Rates vary as changes in the concentration of adsorbed water determine the rate of O2- formation and the gradient in oxygen concentration across the oxide diffusion barrier. Results account for a sharp decrease in the corrosion rate of Pu by water and moist air as the temperature approaches 200 degreesC. (C) 2001 Elsevier Science B.V. All rights reserved.