Behavior of Uranium Dioxide: Chemistry and Catalysis in the UO 2 -water System
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Behavior of Uranium Dioxide: Chemistry and Catalysis in the UO2-water System Jérôme Devoy1, John Haschke2, Daqing Cui1 and Kastriot Spahiu3 1 Studsvik Nuclear AB, Nyköping, Sweden. 2 Actinide Sciences Consulting, Waco, TX, USA. 3 SKB, Stockholm, Sweden.
ABSTRACT Interactions during extended exposure of UO2 to 2:1 H2+O2 mixtures at room temperature and 0.13 bar pressure are investigated in an effort to describe chemical and kinetic behavior of spent fuel following contact with groundwater. Oxidation of UO2 to UO2+x by O2 occurs initially when oxide is directly exposed to the gas mixture or submerged in water, but immersion is accompanied by a 25-fold reduction in the rate. The initial rate is proportional to [O2]2 for gasphase oxidation and to [O2]1.5 for the submerged oxidation. Continued measurement during direct oxide-gas contact indicates sequential reactions in which the UO2+x product is further oxidized by H2O and ultimately reacts with H2 to form an oxide hydride. INTRODUCTION Near-field hydrogen, temperature, and radiolytic products are considered important in determining the dissolution behavior of spent fuel in groundwater after containment failure in a repository. Temperatures in the planned Swedish repository may extend to 70°C and H2 pressures produced by corrosion of the iron container may reach 50 bar. Disruption of the equilibrium state for oxide dissolution is possible if chemical properties of uranium oxide are altered by reaction with species formed by radiolysis of water. These products include H2, O2, H2O2, and radical species. Hydrogen at this temperature range is considered inert, while oxidizing radiolytic species are expected to react readily with the uranium dioxide fuel matrix. The observed consumption of the radiolytic oxidants in the presence of hydrogen with no apparent increase of the uranium concentrations [1] may be due to hydrogen activation by radiation or by the UO2(s) surface. A competing pathway for oxygen removal is reformation of water by the oxide-catalyzed hydrogen-oxygen reaction, a known process for PuO2 [2]. This study departs from prior investigations that focus primarily on the chemistry of equilibrium solutions. Interactions of UO2 with components of water are of interest because behavior may be significantly altered by chemical and catalytic properties of the oxide, as well as by radiolysis. The ultimate objective is to establish a sound technical basis for predicting the behavior of spent fuel after exposure to water.
EXPERIMENTAL DETAILS 235
Measurements were performed with commercial (Westinghouse) uranium oxide (0.63% U) after reduction to UO2.0018 with H2 (5% H2 + 95% Ar) at 800°C. The oxide composition
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was determined using a kinetic phosphorescence analyzer (KPA-11, Chemcheck Instruments) and the specific area (1.25 m2/g) was measured using a BET/N2 method. Although the product was not stoichiometric dioxide and its composition may have increased during handling in air, the UO2 formula is used in this report. Interaction of UO2 with radiolysis products at
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