2016 | ANNUAL REPORT 83 Project Description A recent Nuclear Science User Facilities (NSUF) funded irradiation in the AdvancedTest Reactor included four reactor pressure vessel steels with the same nominal bulk Cu contents (≈0.4 wt.%) but varying Ni levels (0.01, 0.18, 0.84, and 1.25 wt.%) to isolate this element’s effect on irradiation hardening.The samples were irradiated to 1.7 dpa at ≈ 310°C. Microhardness on the irradiated samples showed that the steels with lower Ni contents (0.01-0.18) had much less hardening than those with higher Ni contents (0.84-1.25). Previous atom probe tomography (APT) studies of the higher Ni samples showed that the microstructure was dominated by a very high density of nm-scale Cu-Mn-Ni-Si precipitates with very few dislocation loops observed. In addition, post irradiation annealing studies showed that the hardening features in the lower Ni content steels were more stable than those in the higher Ni steels at temperatures up to 500°C.Thus, the purpose of this experiment was to use APT and transmission electron microscopy (TEM) to investigate the microstructure of both the as-irradiated and the irradiated and annealed low Ni alloys to better measure both the precipitates and the dislocation loops. Accomplishments Atom probe tomography and transmission electron microscopy were both completed on the two low Ni steels.The atom probe showed large dislocation loops ( ≈ 10 nm) in all samples that were enriched predominantly in Si, but also in Mn and Ni, shown in Figure 1. In addition, the density of Cu-rich clusters in the low Ni alloys was significantly lower than in the higher Ni alloys that were previously studied. Line profiles through the loops in the lowest Ni content (0.01 wt.%) steel, Annealing and atom probe tomography characterization of these RPV steels provides a quantitative assessment of the various hardening features formed under irradiation, supporting model development at extended life LWR fluences. — Nathan Almirall, Graduate Student The insight gained from atom probe tomography on this series of irradiated alloys will enable us to make better, physically based predictive models for extended life LWR operation.