The objective of this project is to evaluate the mechanism of irradiation-induced solute clustering in the ferritic-martensitic alloy T91 and confirm experimental temperature shift estimates when using charged-particles to emulate neutron irradiation. Ferritic/martensitic (F/M) alloys are leading candidates for structural and fuel cladding applications for advanced nuclear reactor designs due to their high strength and thermal conductivity. Furthermore, their high sink strengths provide resistance to irradiation-induced swelling and embrittlement. As such, it is critical to have a clear understanding of how the microstructures of these alloys will evolve with long-term irradiation. To date, studies evaluating nanocluster evolution in the commercial F/M alloys HCM12A and HT9 have exhibited variable nanocluster evolution of Cu-rich, Si-Mn-Ni-rich and Cr-rich nanoclusters after irradiation with protons, Fe2+ ions, or neutrons to otherwise common conditions of ~3dpa at 500°C.



A calculation method is developed to predict evolution of nanocluster radius over irradiation time based on the theory of Nelson, Hudson, and Mazey. It has been demonstrated mathematically that this calculation model may be used to isolate the clustering behavior for each solute species. Previous solute calculations (for HCM12A and HT9) have shown a strong correlation between solute dissolution efficiency and known literature values of displacement energy for various solutes. With this experiment, we will be able to test the capability of the NHM-based model as a predictive tool for solute cluster evolution, informing future F/M and nanofeatured alloy development. Additionally, the NHM-based model has enabled evaluation of the temperature shift requirements for higher dose irradiations through study of temperature sensitivity on the clustering evolution of several species of solutes upon various irradiation conditions. This experiment enables evaluation if temperature shift requirements on an additional alloy to inform future charged particle irradiation experiments for fast neutron emulation.



This project focuses on the commercial F/M alloy T91, irradiated separately to the following conditions: a) 2 MeV protons to 2.4 dpa at 500°C, b) 5 MeV Fe2+ ions to 3 dpa at 500°C, and c) 5 MeV Fe2+ ions to 100 dpa at 500°C. Characterization of samples irradiated to ~3 dpa enables direct comparison with previously analyzed specimens irradiated with fast neutrons (RTE 13-419) at the same dose and temperature. Subsequent calculations using the NHM model will validate the solute cluster evolution model and confirm the predicted temperature shift requirements to emulate nanocluster evolution in F/M alloys using higher dose rate irradiations. Finally, characterization of the specimen irradiated to 100 dpa (Fe2+ ions) will verify if solute redistribution at high dose is consistent across multiple F/M alloys with common solute species.



The project will accomplish two major goals: a) validation of our solute specific model of cluster evolution in irradiated b.c.c. Fe-based alloys, and b) confirm the temperature shift requirements for using higher dose irradiations. Each will provide valuable information for nanofeatured alloy development and charged particle irradiation experimentation, both of which are of growing interest to the Department of Energy Office of Nuclear Energy.

Most broadly, this project aims to further understanding of irradiation-induced solute clustering behavior in cladding and structural alloys. Light water reactor life extensions and the promise of advanced reactor and small modular reactor designs are accompanied by the challenge of finding suitable structural and cladding materials that will withstand the harsh in-reactor operating conditions. Ensuring the integrity of these materials under high temperatures, corrosive environments, cyclic loading, and high irradiation damage, is paramount to the DOE-NE mission. The alloy proposed for this study is a representative b.c.c. Fe-based alloy and candidate for use in future nuclear reactors, thus directly addressing DOE-NE base programs on Advanced Reactor Technologies (ART) and Advanced Small Modular Reactors (aSMR). Solute clustering is also a concern in pressure vessel steels, so the results of this project are also relevant to the DOE-NE base program on Light Water Reactor Sustainability (LWRS).



The outcome of this project is an experimentally-validated model of solute clustering evolution under irradiation in irradiated b.c.c. Fe-based alloys. This will be a versatile and valuable tool to assist current and future nanofeatured alloy development. Success of this project will have broad implications. First, results of this project will be relevant to all F/M and other nanocrystalline alloys based on the b.c.c. Fe-Cr matrix. This is particularly noteworthy since nanofeatured alloys are increasingly important in nuclear power applications, due to their high irradiation tolerance. Ongoing efforts aimed at developing nanofeatured alloys are also supported by the Nuclear Energy Enabling Technologies (NEET) program, while modeling of advanced cladding and structural materials are supported by the Nuclear Energy Advanced Modeling and Simulation (NEAMS) program. Second, this project will confirm the temperature shift requirements for using higher dose irradiations, informing subsequent charged particle irradiation experiments designed to emulate nanocluster evolution in a fast neutron irradiation environment.



The results of this project will help evaluate the long-term microstructural integrity of F/M alloys in fission reactors, and the model can easily be adapted to other alloy systems. Ensuring the integrity of the microstructure of engineering alloys throughout their service lifetime will ensure success of multiple DOE-NE programs, including ART, LWRS, and aSMR. Therefore, this project fulfills the DOE-NE mission of meeting the country’s energy, environmental, and security needs with nuclear power.