Effect of Gadolinium Doping on the Air Oxidation of Uranium Dioxide
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Effect of Gadolinium Doping on the Air Oxidation of Uranium Dioxide Randall D. Scheele, Brady D. Hanson, Stephen E. Cumblidge, Evan D. Jenson, Anne E. Kozelisky, Rachel L. Sell, Paul J. MacFarlan, and Lanee A. Snow Environmental Technology Directorate, Pacific Northwest National Laboratory Richland, WA 99352, USA, [email protected] ABSTRACT Researchers at the Pacific Northwest National Laboratory (PNNL) investigated the effects of gadolinium oxide concentration on the air oxidization of gadolinium oxide-doped uranium dioxide using thermogravimetry and differential scanning calorimetry to determine if such doping could improve uranium dioxide’s stability as a nuclear fuel during potential accident scenarios in a nuclear reactor or during long-term disposal. We undertook this study to determine whether the resistance of the uranium dioxide to oxidation to the orthorhombic U3O8 with its attendant crystal expansion could be prevented by addition of gadolinium oxide. Our studies found that gadolinium has little effect on the thermal initiation of the first step of the reported two-step air oxidation of UO2; however, increasing gadolinium oxide content does stabilize the initial tetragonal or cubic product allowing significant oxidation before the second expansive step to U3O8 begins. INTRODUCTION Because of UO2’s importance as a nuclear fuel, its resistance to oxidation the recognized pathway for oxidation of cubic UO2 to orthorhombic U3O8 (O:U = 2.67) is a two-step process [1] with the first step being the formation of either a cubic U4O9+y (O:U=2.25 to 2.4) or tetragonal U3O7 (O:U=2.33) intermediate product. McEachern and Taylor, [1] in their review of past studies, report that the first reaction is diffusion-controlled displaying parabolic reaction kinetics while the second reaction step producing U3O8 follows a nucleation and growth mechanism displaying sigmoidal reaction kinetics. Often this second reaction proceeds concurrently with the first while U4O9/U3O7 continues to form by diffusion of oxygen into the lattice. While the oxidation to the orthorhombic U3O8 causes significant crystal structure expansion, the initial oxidation to the tetragonal or cubic oxides does not. It would thus be desirable for the fuel to resist expansion when exposed to air either in-reactor or later during disposal in a geologic repository to prevent significant breaching of the cladding. Building on past studies [1,2,3,4,5,6] which show increased resistance for the oxidation from U4O9/U3O7 to U3O8 with increasing burn-up (increased fission product content) and increased lanthanide- or actinide-content, we investigated doping UO2 with transition and lanthanide metal oxides in an effort to develop an improved fuel with greater second stage oxidation resistance. In this paper, we present results of our heat-ramp thermoanalytical studies of air oxidation of Gd2O3-doped UO2. EXPERIMENTAL DETAILS To investigate the stabilization effects of gadolinia doped into UO2, we used two thermoanalytical methods, thermogravimetric analysi
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