As an alternative fuel form, the annular metallic fuel design eliminates the liquid sodium bond between the fuel and the cladding, providing back-end fuel cycle and other benefits. The fuel-cladding chemical interaction (FCCI) of annular fuel also presents new features. Here, state-of-the-art electron microscopy and spectroscopy techniques were used to study the FCCI of a prototype annular U-10wt%Zr (U-10Zr) fuel with ferritic/martensitic HT-9 cladding irradiated to 3.3% fission per initial heavy atom. Compared with sodium-bonded solid fuels, negligible amounts of lanthanides were found in the FCCI layer in the investigated helium-bonded annular fuel. Instead, most lanthanides were retained in the newly formed UZr₂ phase in the fuel center region. The interdiffusion of iron and uranium resulted in tetragonal (U,Zr)₆Fe phase (space group I4/mcm) and cubic (U,Zr)(Fe,Cr)₂ phase (space group Fd$\overline{3}$m). The (U,Zr)(Fe,Cr)₂phase contains a high density of voids and intergranular uranium monocarbides of NaCl-type crystal structure (space group Fm$\overline{3}$m). At the interdiffusion zone and inner cladding interface, a porous lamellar structure composed of alternating Cr-rich layers and U-rich layers was observed. Next to the lamellar region, the unexpected phase transformation from body-centered cubic ferrite (α-Fe) to tetragonal binary Fe-Cr σ phase (space group P4₂/mnm) occurred, and tetragonal Fe-Cr-U-Si phase (space group I4/mmm) was identified. Due to the diffusion of carbon into the interdiffusion zone, carbon depletion inside the HT-9 led to the disappearance of the martensite lath structure, and intergranular U-rich carbides formed as a result of the diffusion of uranium into the cladding. These detailed new findings reveal the unique features of the FCCI behavior of annular U-Zr fuels, which could be a promising alternative fuel form for high burnup fast reactor applications.

作为替代燃料形式，环形金属燃料设计消除了燃料和覆层之间的液态钠键，从而提供了后端燃料循环和其他好处。环形燃料的燃料包壳化学相互作用（FCCI）也呈现出新特征。在这里，最新的电子显微镜和光谱技术用于研究原型环状U-10wt％Zr（U-10Zr）燃料的FCCI，该燃料具有铁素体/马氏体HT-9包层，每次初始裂变辐照度为3.3％重原子。与钠键合固体燃料相比，在研究的氦键合环形燃料中，FCCI层中的镧系元素含量可忽略不计。相反，大多数镧系元素保留在燃料中心区域中新形成的UZr ₂相中。铁和铀的相互扩散导致四方（（U，Zr）₆ Fe相（空间群I4 / mcm）和立方（U，Zr）（Fe，Cr）₂相（空间群Fd$\overline{3}$m）。（U，Zr）（Fe，Cr）₂相包含高密度的空隙和NaCl型晶体结构（空间群Fm$\overline{3}$m）。在互扩散区和内包层界面处，观察到了由交替的富Cr层和富U层组成的多孔层状结构。在层状区域附近，从体心立方铁素体（α-Fe）到四方二元Fe-Crσ相的意外相变（空间群P4 ₂/ mnm），并鉴定出四方的Fe-Cr-U-Si相（空间群I4 / mmm）。由于碳扩散到互扩散区内，HT-9内部的碳耗尽导致了马氏体板条结构的消失，并且由于铀扩散到覆层中而形成了晶间富铀的碳化物。这些详细的新发现揭示了环形U-Zr燃料FCCI行为的独特特征，对于高燃耗快堆应用而言，这可能是一种有前途的替代燃料形式。