Nuclear Science User Facilities 94 Figure 5. Small loops observed in HT9 and T91 following high temperature high dose irradiation. size of around 5 nm in the samples irradiated at highest temperature of 604°C and 3 dpa (see Figure 5). The abnormal defect structure is further illustrated by the multimodal size distribution of dislocation loops in samples irradiated for more than one cycle in the medium tempera- ture range of 400 to 500°C.These phenomena can be explained by the lack of temperature control in reactor irradiation experiments.The specimens were sealed in capsules, and the main heating mechanism was by prompt gamma heating.The irradiation temperature was not well controlled during the experiment. The samples may have experienced a fair amount of low-temperature irradiation, which leads to significant defect-cluster nucleation.These small clusters will then rapidly grow and develop into the scenario observed. The size of the dislocation loops is summarized in Figure 6 as a function of dose at 350°C.At low-temperatures, the loop sizes are similar in two mate- rials, while the loops are significantly larger inT91 at higher temperatures, suggesting faster loop growth. Radiation-induced segregation (RIS) STEM-EDS and APT were used to analyze radiation-induced segregation (RIS) and precipitation. Significant segregation of Cr, Si, Ni, Mn, P, and Figure 6. Dislocation loop size in HT9 and T91 irradiation at 350, 500 and 600°C.