Nuclear Science User Facilities 96 G-phase is a Ni-rich silicide frequently observed in irradiated steels due to the radiation-induced segregation of Ni, Mn and Si.The spatial distribution of G-phase particles is visualized in Figure 8 using APT Ni mapping.We analyzed the particles using Ni, Si, Mn isoconcentration surfaces.The concen- tration evolution with irradiation is illustrated in Figure 9.The Ni, Si ratio is close to 1 for low-dose specimens, suggesting similar RIS for these two elements.The ratios then converge to the G-phase ratio, confirming that the particles are G-phase. Cr-rich α' precipitates were also observed in HT9 andT91 under certain conditions.The precipitation of α' agrees well with the previously published results. In the 12Cr steel HT9, α' was observed in both low and medium temperature ranges, while in 9Cr steel T91, it was only observed in the low temperature range.The APT images and the size, density and volume fraction of α' are shown in Figure 9. A Cu-rich phase was observed inT91 (Figure 11). Even though RIS of Cu was also observed in HT9, no clearly defined clusters of Cu were found in HT9. InT91 irradiated at high temperature to 3.23 dpa, Cu-rich particles were the only small-scale precipitates observed in APT. In the medium temperature range, G-phase and Cu-rich phase coexist and appear to have spatial correlations (See Figure 10). Nitrides are important precipi- tates in this type of steel.The most prominent type of nitrides in HT9 andT91 is vanadium nitride (VN). The thermodynamic calculation shows thatVN is not a stable phase. Instead CrVN (Z phase) is the stable phase in high Cr steels.We observed the RIS of Cr on theVN phase boundary (Figure 12) and the modification of such phase. Cr2N was also observed at both materials in low temperatures after long-term irradiation, up to 10 dpa. Figure 8. Ni atom maps showing the size and distribution of G-phase in irradiated HT9. Figure 9. The distribution of α' visualized by 20% Cr isoconcentration surfaces. The size, density and volume fraction are also shown.