铈是核反应的重要裂变元素,其氧化物CeO2容易与UO2燃料形成U1?xCexO2固溶体.铀铈固溶体会因为U4+发生氧化而生成(U1?xCex)3O8?δ混合氧化物.Ce在U3O8中的存在会改变U3O8母体的局域结构,影响着(U1?xCex)3O8?δ产物的物理化学性质.研究铀铈混合氧化物的局域结构,可以帮助理解处于苛刻环境中乏燃料的性状.使用共沉淀方法制备了铈掺杂的U3O8混合氧化物样品,即(U1?yCey)3O8?(δ y=0,0.05,0.20),利用粉末X射线衍射(X-ray Diffraction,XRD)获取其物相信息,借助同步辐射X射线吸收精细结构(X-ray Absorption Fine Structure,XAFS)技术来获取(U1?yCey)3O8?δ中铀和铈的局域结构.结果表明:与掺杂前的U3O8具有C2mm空间群不同,掺入Ce后的(U1?yCey)3O8?δ转变成了与U3O8的空间群一样的晶型,Ce的存在还导致铀的平均价态升高,层内最近邻Ce-U(Ce)的原子距离减小.%[Background] Cerium is one of the important products of the mixed oxide (MOX) reaction, and CeO2 forms U1?xCexO2 solid solution with UO2. In the post-treatment of spent fuel, UO2 may be oxidized to U3O8 resulting in an increase in spent fuel volume. The structure and volume of spent fuel are concerned with the solubility, valence and electronic structure of uranium-cerium solid solutions. [Purpose] This paper aims to study the local structure of Ce-substituted U3O8 by X-ray absorption fine structure (XAFS) technique. [Methods] First of all, (U1?yCey)3O8?δ (y=0, 0.05, 0.20) mixed oxides were prepared by coprecipitation method. Then, powder X-ray diffraction (XRD) and X-ray absorption fine spectroscopy (XAFS) characterizations at U L3 edge and Ce K edge have been undertaken to study the chemical changes of uranium-cerium oxides. [Results] The XRD data shows that the Ce-substituted U3O8 has the space group which is different with α-U3O8. The U L3-edge X-ray absorption near edge structure (XANES) data of Ce-substituted U3O8 shows a slight increase in the uranium average valance. The X-ray absorption near edge structure (EXAFS) data shows that the neighboring atoms of Ce cations are far away from those of U cations. [Conclusions] The Ce cations replaced the U site in U3O8, and the U3O8 sample was changed from the original C2mm to space group. The lattice volume was increased and the local structure around U cation was changed by adjusting the atomic distance and increasing the valence of U.
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