Sensitivity of UO 2 Stability in a Reducing Environment on Radiolysis Model Parameters
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Sensitivity of UO2 Stability in a Reducing Environment on Radiolysis Model Parameters Richard S. Wittman and Edgar C. Buck Energy and Environment Division, Pacific Northwest National Laboratory, Richland, WA, 99352, U.S.A. ABSTRACT Results for a radiolysis model sensitivity study of radiolytically produced H2O2 are presented as they relate to Spent (or Used) Light Water Reactor uranium oxide (UO2) nuclear fuel (UNF) oxidation in a low oxygen environment. The model builds on previous reaction kinetic studies to represent the radiolytic processes occurring at the nuclear fuel surface. Hydrogen peroxide (H2O2) is the dominant oxidant for spent nuclear fuel in an O2-depleted water environment. The most sensitive parameters have been identified with respect to predictions under typical conditions. As compared with the full model with about 100 reactions, it was found that only 30 to 40 of the reactions are required to determine [H2O2] to one part in 10–5 and to preserve most of the predictions for major species. This allows a systematic approach for model simplification and offers guidance in designing experiments for validation. INTRODUCTION Assessing the performance of Spent (or Used) Nuclear Fuel (UNF) in geological repository requires quantification of time-dependent phenomena that may influence its behavior on a time-scale up to millions of years. A high-level waste repository environment will be a dynamic redox system because of the generation of radiolytic oxidants and reductants and the corrosion of Fe-bearing canister materials. One major difference between used fuel and natural analogues, including unirradiated UO2, is the intense radiolytic field. The radiation emitted by used fuel can produce radiolysis products in the presence of water vapor or a thin-film of water (including hydroxide (OH•) and hydrogen (H•) radicals, oxygen ion (O2-), aqueous electron (eaq), hydrogen peroxide (H2O2), hydrogen gas (H2), and the secondary radiolysis product, oxygen (O2)) that may increase the waste form degradation rate and change radionuclide behavior. As a base model we consider the combined reaction kinetics published by Pastina and LaVerne[1] and Christensen and Sunder [2] applied to a system that assumes a local dose at a solid-aqueous boundary. Our purpose is to provide a basis for validation and a stronger scientific basis for the UNF performance assessment model. Of course, uncertainty in model parameters and reaction mechanisms results in uncertain predictions. We consider a limited analysis to quantify the sensitivity of H2O2 generation to model parameters. Results identify where the model can be simplified and where uncertainty can be reduced to have greatest benefit to model predictability. Additionally, the results should help identify experiments that can best reduce the driving model uncertainties. RADIOLYSIS MODEL DEFINITION Along with ionization, the interaction of energetic radiation with water molecules can generate very short-lived (10–15 s) electronic excitations that favorably de-excite through intermed
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