Interactions of Plutonium Dioxide with Water and Oxygen-Hydrogen Mixtures

Pressure-volume-temperature data and mass spectrometric results obtained during exposure of PuO{sub 2} to D{sub 2}O show that the dioxide reacts with water at room temperature to produce a higher oxide (PuO{sub 2+x})and H{sub 2}. Results demonstrate that PuO{sub 2+x} is the thermodynamically stable oxide in air. The absence of O{sub 2} at detectable levels in the gas phase implies that radiolytic decomposition of water to the elements is not a significant reaction. The rate of the PuO{sub 2}+H{sub 2}O reaction is 6{+-}4 nmol H{sub 2}/m{sup 2} day, a value that is independent of the H{sub 2}O concentration on the oxide over a range that extends from fractional monolayer coverage to saturation by liquid water. Evaluation of literature data shows that oxide compositions in excess of PuO{sub 2.25} are attained, but the maximum value of x is unknown. During exposure of PuO{sub 2} to a 2:1 D{sub 2}:O{sub 2} mixture at room temperature, the elements combine by a process consistent with a surface-catalyzed reaction. Water is simultaneously formed by the H{sub 2}+O{sub 2} reaction and consumed by the PuO{sub 2} + H{sub 2}O reaction and accumulates until the opposing rates are equal. Thereafter, PuO{sub 2+x} is formed at a constant rate by the water-catalyzed PuO{sub 2} + O{sub 2} reaction. The failure of earlier attempts to prepare higher oxides of plutonium is discussed and the catalytic cycle that promotes the reaction of PuO{sub 2} with O{sub 2} is described. Implications of the results for extended storage and environmental chemistry of oxide are examined. Moisture-catalyzed oxidation of PuO{sub 2} accounts for observation of both pressure increases and decreases in oxide storage containers with air atmospheres. Application of the experimental rate results indicates that the reaction of a typical oxide with 0.5 mass % of adsorbed water maybe complete after 25 to 50 years at room temperature