Interactions of Plutonium Dioxide with Water and Oxygen-Hydrogen Mixtures

摘要

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.