## 3 Results
### Microstructure and Chemistry of U-Mo after Irradiation  
Figure 6 provides a comparison of mass spectra obtained by APT for the three U-Mo samples (fresh, intermediate burnup--M11, and high burnup--M14). The spectra are divided into 9 segments over the range of 28 to 137 Daltons (Da) to ensure that comparison is feasible between all 3 samples. Background noise in all three cases was less than 30 ppm, which is key to ensure all peaks are accounted for in the quantification and for revealing the chemistry of the fission products labeled on top of all subfigures. Note that small traces of signal observed before 28 Da arising from H, C, N, O, OH, and OH2 are attributed to impurities arising from sample preparation and handling or from residual gases present in the analysis chamber. The combined contribution of these impurities was equal to or less than 0.3 at.% for each of the three samples. Peaks in the irradiated fuel spectra are assigned based on comparison to the mass spectrum to fresh U-Mo and by their relative isotopic abundance. For instance, signals appearing at 46, 47, 48, 49, and 50 Da were assigned as Mo\({}^{2+}\) (shown in Figure 7b) since it matched the expected natural abundance. Additional peaks in the range of 44 to 50 Da were assigned to be fission products, as follows: 44 Da-88 Sr\({}^{2+}\), 44.5 Da-89Y\({}^{2+}\), 45 Da-90 Zr\({}^{2+}\), 46-138 Ba\({}^{3+}\)/92 Zr\({}^{2+}\), 46.33 Da-139 La\({}^{3+}\), 46.5 Da-93 Nb\({}^{2+}\), 46.67 Da-140 Ce\({}^{3+}\), 47.33 Da-142 Ce\({}^{3+}\), and/or 147 Nd\({}^{3+}\) and 49.5 Da-99 Tc\({}^{2+}\) and/or 99 Ru\({}^{2+}\). A similar approach was followed for assigning chemical identity to all fission products for both irradiated U-Mo samples. Measured compositions of U, Mo (added as an alloy and fission product Mo), other fission products, and impurities from M11 and M14 samples are provided in Table 2 in at.%. The term impurities refer to C, N, Si, O, Cr, and Fe appearing in a sample due to the fabrication process and/or due to its introduction into the specimen during the milling process in FIB. 3D distribution of fission products from both irradiated samples is shown in Figure 7 by placing a cube of dimension 18 \(\times\) 18 \(\times\) 2 nm\({}^{3}\) in an analyzed volume. It shows clear fission products' enrichment into spherical clusters distributed uniformly over the analyzed volume for M11 (Figure 7a), whereas M14 (Figure 7b) showed a non-uniform distribution. Previous work with TEM already reported on the presence of a bubble superlattice structure with similar dimensions and also showed differences in distributions of fission products for both M11 and M14 samples [12]. Note that the analysis of fission products (size, shape, and spatial distribution) in APT is subject to local magnification artifacts [37]. This effect arises from differences in the evaporation field values for the different components present on the apex of the tip [38; 39; 40]. The actual location of U, Mo, and fission products can be discerned due to possible artifact overlap of trajectories of ions during the field evaporation process. The predominant presence of a local magnification effect is expected from the current system, in which clusters of fission products (radius of -2 nm) are uniformly distributed in the U-Mo metallic matrix. Studies are planned to use advanced simulation methods to investigate  
Figure 5: (**a**) 3D distribution of U-C shown with iso-concentration surfaces of C (shown in brown) and \(\alpha\)-U (shown in grey) along the grain boundary, and 90\({}^{\circ}\) rotated view of analyzed volume shown in **(a)** showing platelet-shaped carbides along the grain boundary. (**b,c**) 1D profile taken across the grain boundary by placing a cylinder along carbide precipitates and without precipitate, respectively. (brown: C, blue: enrichment, grey: U, red: Mo). how field evaporation affects the size, shape, and spatial distribution of fuel components (U-Mo) and fission products (solids and gases).