## 3 Results  
### Phases Observed in U-Mo Prior to Irradiation  
Understanding of the microstructure in terms of phases present, impurities, chemical composition of phases, and uniformity in enrichment is essential to understand the irradiation behavior of the fuel particles. Scanning electron microscopy (SEM) images shown in Figure 1 provide an overview of the fuel particles dispersed in the aluminum fuel plate matrix (Figure 1a) and the particle microstructure and coating (Figure 1b) showing lamellar-like features (Figure 1c) distributed non-uniformly across the sample. Previous analysis [34] of particles produced by the same atomization process indicates that the particles are composed primarily of the body-centered cubic \(\gamma\)-U-Mo phase. Tomograms from the current APT analysis for an analyzed volume of 50 \(\times\) 50 \(\times\) 5 nm are shown in Figure 2a, along with the binomial distribution of 235U and Mo confirming uniform elemental distribution in Figure 2b. The measured composition inside the \(\gamma\)-U-Mo phase is shown in Table 1, which matched well with the expected composition of 15.8 at.% Mo (7 wt.% Mo). The 235U enrichment value of 19.8 measured by APT by the isotopic ratio method compares well with expected values, as shown in our previous work [30]. \begin{table}
\begin{tabular}{c c c} \hline
**Elements** & **at.\%** & **2 Sigma** \\ \hline U & 84.07 & 0.57 \\ \hline Mo & 15.83 & 0.20 \\ \hline Fe & 0.02 & 0.01 \\ \hline N & 0.01 & 0.01 \\ \hline Cr & 0.01 & 0.01 \\ \hline C & 0.01 & 0.01 \\ \hline O & 0.01 & 0.01 \\ \hline \end{tabular}
\end{table} 
Table 1: Measured composition from fresh U-7 wt.% Mo (15.8 at.% Mo). Figure 1: SEM images at different magnification of (**a**) as-fabricated U-7 wt.% Mo particles dispersed in Al matrix coated with ZrN, shown with dotted lines in (**b**,**c**). APT analysis was conducted from the specimen prepared from the lamellar microstructure-like features identified using electron microscopy shown in Figure 1c. The thickness of the lamellar microstructural features varied from 5 to 15 nm, as shown in Figure 3a,b, with the iso-concentration surface and the 1D plot taken across one of the lamellar-like structures indicated in Figure 3a. Concentration profiles indicate that these features contain nearly 100 at.% U, consistent with previous identification, as \(\alpha\)-U while adjacent to these lamellar structures consists of Mo-enriched \(\gamma\)-U-Mo [35; 36]. Note that these decompositions of \(\gamma\)-U-Mo into \(\alpha\)-U did not affect enrichment levels, as expected. Figure 3: (**a**) 3D distribution of network of \(\alpha\)-U (grey color) shown with 100 at.% iso-concentration surfaces, (**b**) 1D profile taken across the lamellar structure by placing a cylinder (shown in (**a**)) across the interface. (blue: enrichment, red: Mo, grey: U). Figure 2: (**a**) 50 \(\times\) 5 nm cuboid showing distribution of U, Mo, and 235U isotope. (**b**) Observed and binomial distribution of Mo and 235U isotope in analyzed volume obtained using APT from as-received U-7 wt.% Mo. Specimens prepared from the network of lamellar structures also revealed multiple interfaces in a single tip, as shown in Figure 4a-d. The cylinders highlighted on the left of the subfigures show where a 1D profile is taken along different interfaces. The analysis revealed that the presence of Mo enriched \(\gamma\)'-U-Mo, \(\alpha\)-U, \(\gamma\)'-U\({}_{2}\)Mo, and grain/cell boundaries, showing segregation of Si. Note that APT analysis for these samples could not provide crystallographic orientation across the boundary, hence it is not possible to make a distinction between grain and cell boundaries. They have been demonstrated to almost always coincide in fresh fuel. The presence of a grain boundary in the analyzed volume suggests precipitation in the form of a lamellar-like microstructure consisting of different phases of U and Mo.