Fuel-Cladding Chemical Interaction of a Prototype Annular U-10Zr Fuel with Fe-12Cr Ferritic/Martensitic Cladding

As an alternative fuel form, annular metallic fuel design eliminated 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 (FIMA). Compared to sodium-bonded solid fuels, negligible amounts of lanthanides were found in the FCCI layer in the investigated helium bonded annular fuel. Instead, most of the lanthanides were retained in the newly formed UZr2 phase at the fuel center region. The interdiffusion of iron and uranium resulted in tetragonal (U,Zr)6Fe phase (space group I4/mcm) and cubic (U,Mo)(Fe,Cr)2 phase (space group Fd3 m). The (U,Mo)(Fe,Cr)2 phase contains a high density of voids and intergranular uranium monocarbides of NaCl-type crystal structure (space group Fm3 m). At the interface of the interdiffusion zone and inner cladding, 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 (BCC) 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.