The systematic presence of the ternary phases U6Mo4Al43 and UMo2Al20 is reported in a U–Mo/Al interaction layer grown by thermal annealing. This work shows, therefore, the low Mo solubility in UAl3 and UAl4 binary phases; it contradicts the hypothesis of the formation of (U,Mo)Al3 and (U,Mo)Al4 solid solutions often admitted in the literature. Using µ‐XAS (micro X‐ray absorption spectroscopy) at the Mo K edge and µ‐XRD (micro X‐ray diffraction), the heterogeneity of the interaction layer obtained on a γ‐U0.85Mo0.15/Al diffusion couple has been precisely investigated. The UMo2Al20 phase has been identified at the closest location from the Al side. Moreover, µ‐XRD mapping performed on an annealed fuel plate enabled the characterization of the four phases resulting from the γ‐U0.85Mo0.15/Al and (U2Mo + α‐U)/Al interactions. A strong correlation between the concentrations of UAl4 and UMo2Al20 and those of UAl3 and U6Mo4Al43 has been shown.

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