University of Illinois Urbana-Champaign<\/a> details how technology is allowing us to \u2018look inside\u2019 irradiated steels to determine the durability of a nuclear structure.<\/h2>\nOne of the founding principles in materials science is that the internal microstructure of a material controls the material performance. Because of this, there has always been an intense interest in studying and classifying the internal structures in materials as a means of understanding and predicting their performance. Most of this work has been aided by the continuing and rapidly advancing development of microstructural analysis techniques to probe smaller and smaller regions of the material structure, down to the atomistic level.<\/p>\n
The atomistic level information is critical to understand the internal changes in the material. However, the major challenge is to link that understanding to the performance of realistic-scale materials structures. We have been interested in building, on the atomistic level, information to understand how actual structures behave under real-life application conditions \u2013 can we understand the durability of a structure from our atomistic-level materials characterisation?<\/p>\n
Irradiated steel in nuclear applications<\/h3>\n
Irradiated steel structures have long been used for building nuclear systems and are a high priority for applications in the next generation of advanced nuclear reactor systems. For advanced nuclear systems, steels with compositions of Fe-9 to 12Cr have attracted the most interest and the largest levels of experimental and modelling activities. These types of irradiated steel have been used in advanced nuclear systems and are of high interest for future systems because they are resistant to the internal damage caused by irradiation. Exposure to intense irradiation fields inside a nuclear reactor can drastically change the material\u2019s mechanical properties and can alter the material\u2019s physical dimensions. Finding materials that can resist these changes is essential for reactor life cycles and safety.<\/p>\n
Here, we describe experimental discoveries on Fe-9 to 12 Cr model and commercial steel alloys to show what happens inside irradiated steel when it is subjected to intense irradiation fields at elevated temperatures inside an operating nuclear reactor. Our materials were irradiated in the Advanced Test Reactor (ATR) at Idaho National Laboratory (INL). They were subjected to neutron irradiations up to 10 displacements per atom (dpa). The figure of merit for radiation damage, dpa, indicates how many times on average each atom in the material has been knocked out of its normal position into another location in the material. 10 dpa indicates that every atom in the material has been knocked or displaced from its starting position to another location ten times during the irradiation exposure. So, on average, all of the atoms in the material are not in the same positions where they started. Many of them fall back into normal positions in the material\u2019s crystal lattice, but some move around and combine with other \u2018displaced\u2019 atoms to form \u2018defects\u2019 or clusters that are different from the starting structure.<\/p>\n
Investigating irradiation damage<\/h3>\n
It is possible to watch the evolution of defect clusters in irradiated steel. Using high-energy ion beams directed into a transmission electron microscope stimulates the damage found in nuclear reactors. This technique provides a clear picture of the ways in which small defect clusters form and grow with irradiation damage. The small \u2018black\u2019 dots that form with irradiation exposure, shown in Fig. 1, are small clusters of atoms that have been \u2018displaced\u2019 and moved together to form small clusters. These clusters act as a strengthening agent to make the material stronger, but they also tend to make the material more brittle.<\/p>\n