2018 | ANNUAL REPORT 93 Radiation-induced defect microstructures BothTEM and STEM were used to characterize irradiation-induced dislocation-loop structures inT91. MostTEM samples were prepared using an electropolishing technique. For the conditions where electropolished specimens were not available, the FIB lift-out technique was used to prepare the samples forTEM experiments. The damage microstructures inT91 under various irradiation conditions are shown in Figure 4.The defect evolution in HT9 is similar toT91. The sizes of the dislocation loops strongly depend on the irradia- tion temperature, where the largest dislocation loops were observed at the highest temperature. Under the similar dose level, at low temperature (around 350°C), damage was only visible above 1 dpa (except under special weak beam imaging condi- tions). At higher temperatures, visible loops were observed as early as 0.44 dpa. Considering the actual highest irradiation temperature is above 600°C, minimum irradiation damage was expected due to multiple factors including high-point defect recombination rate, low defect-cluster nucleation rate and thermal recovery above 600°C. On the contrary, signifi- cant irradiation damage was observed in this temperature regime.We also observed small loops with an average Figure 4. Effect of dose and temperature on the structure in T91.