| Abstract Scope |
Advanced nuclear energy systems are uniquely suited to meet the world’s increasing energy demands, particularly through designs operating under extreme in-core conditions—high temperature (>300 °C), high stress (>100 MPa), high dose (>25 dpa), and corrosive environments. These extremes impose significant demands on the irradiation creep resistance of structural materials. To investigate material behavior under such conditions, ion-thin tapered tensile specimens were developed to introduce a constant load stress gradient for accelerated irradiation creep testing. The effect of this gradient on the multi-length-scale microstructural evolution of 316L stainless steel (316L SS) was examined under both stressed thermal and proton irradiation conditions. Experiments were conducted at 450 °C and 550 °C with stress gradients of 79–158 MPa and 158–316 MPa, reaching doses of 0 and 0.2 dpa over 150 loading hours. A key objective was to evaluate the suitability of tapered geometries as surrogates for uniform-stress specimens by comparing local strain rates, dislocation microstructures, and microchemical features. While dislocation microstructures were relatively consistent, radiation-induced segregation (RIS) behavior diverged. At 158 MPa, uniform specimens showed enhanced chromium enrichment and iron depletion along edge-on grain boundaries aligned with the stress, relative to tapered specimens. At 316 MPa, a shift from nickel depletion to enrichment occurred along grain boundaries and dislocations, underscoring the complex influence of high temperatures and stress on RIS phenomena. Understanding the coupling of constant load-induced stress gradients, temperature gradients, and chemical gradients on the evolution of structural materials is critical to defining the lifetime of new nuclear reactors. |