Pacific Northwest National Laboratory, Richland, Washington 99352, United States.
Stanford Synchrotron Radiation Lightsource, Menlo Park, California 94025, United States.
Langmuir. 2017 Nov 21;33(46):13189-13196. doi: 10.1021/acs.langmuir.7b02800. Epub 2017 Nov 9.
Uranium oxide is central to every stage of the nuclear fuel cycle, from mining through fuel fabrication and use, to waste disposal and environmental cleanup. Its chemical and mechanical stability are intricately linked to the concentration of interstitial O atoms within the structure and the oxidation state of U. We have previously shown that, during corrosion of the UO (111) surface under either 1 atm of O gas or oxygenated water at room temperature, oxygen interstitials diffuse into the substrate to form a superlattice with three-layer periodicity. In the current study, we present results from surface X-ray scattering that reveal the structure of the oxygen diffusion profile beneath the (001) surface. The first few layers below the surface oscillate strongly in their surface-normal lattice parameters, suggesting preferential interstitial occupation of every other layer below the surface, which is geometrically consistent with the interstitial network that forms below the oxidized (111) surface. Deeper layers are heavily contracted and indicate that the oxidation front penetrates ∼52 Å below the (001) surface after 21 days of dry O gas exposure at ambient pressure and temperature. X-ray photoelectron spectroscopy indicates U is present as U(IV), U(V), and U(VI).
氧化铀是核燃料循环各个阶段的核心，从采矿到燃料制造和使用，再到废物处置和环境清理。其化学和机械稳定性与结构内间隙 O 原子的浓度和 U 的氧化态密切相关。我们之前已经表明，在 1 个大气压的 O 气体或室温下含氧水中 UO(111)表面腐蚀期间，氧间隙扩散到基体中，形成具有三层周期性的超晶格。在当前的研究中，我们展示了来自表面 X 射线散射的结果，揭示了（001）表面下氧扩散轮廓的结构。表面以下的最初几层在其表面法向晶格参数中强烈振荡，表明表面以下的每一层都优先占据间隙，这与在氧化（111）表面下形成的间隙网络在几何上是一致的。更深的层被强烈收缩，表明在环境压力和温度下干燥 O 气体暴露 21 天后，氧化前沿穿透到（001）表面以下约 52 Å。X 射线光电子能谱表明 U 以 U(IV)、U(V)和 U(VI)的形式存在。
