Radiation Resistant High-Entropy Alloys for Sodium Fast Reactor Fuel Cladding Applications

The deployment of SFR as a Gen IV reactor is currently limited in large part by the development of materials which can sustain both radiation damage at high levels of dpa and corrosion to meet the necessary safety and economic criteria for licensing and commercialization. For instance, the Terrapower® TWR design will only be successful if cladding and duct materials with radiation resistance up to 600 dpa can be designed. Fast reactor cladding and ducts require alloy(s) that also maintain adequate creep strength up to 650°C and fracture toughness at 320°C or lower and exhibit high levels of corrosion resistance in liquid sodium or liquid lead-alloy coolants. In the United States, the original material of choice for SFR structural components was 316 austenitic stainless steel. However due to unacceptable levels of void swelling in this steel, the focus for cladding and duct applications shifted to ferritic-martensitic (F-M) steels. The majority of research on F-M steels has focused on 9–12Cr steels. Based on corrosion resistance, fracture toughness, and void swelling considerations, the 9Cr steel was chosen over HT9 for the next generation cladding and duct application for fast reactors. Further optimization led to the consideration of compositionally optimized austenitic alloys such as D9 as well as G92 F-M steels. D9 on account of its austenitic structure is not expected to have adequate swelling resistance at damage levels of hundreds of dpa while HT9 and G92 have not been tested in excess of 200 dpa and have shown dramatic decrease in mechanical properties after testing in liquid Na at 650°C. In addition, HT9 has limited resistance to creep and creep fatigue above 550°C. Oxide dispersion strengthened (ODS) ferritic-martensitic steels show promise, but fabrication of claddings of this material at reasonable cost in sufficient volumes, and with predictable and reproducible properties continues to be a significant challenge.

It is therefore necessary to explore new alloy compositional designs outside the paradigms of ferritic and austenitic steels for the SFR cladding and in-core applications requirements. Recent research in on high entropy alloys (HEA) has allowed us to select two High Entropy Alloy (HEA) family for this study, CrFeMnNi and NbTaTiV. HEAs represent a radical departure from conventional alloys, which consist of one or two principal elements and minor concentrations of alloying constituents. In contrast, HEAs are composed of four or more metallic elements mixed in equimolar ratio in a single phase solid solution. The focus is on radiation damage effects on the microstructure in this class of alloys at high temperatures and dpa (displacements per atom) levels, but this project also includes mechanical property measurements of irradiated layers (up to operational temperatures) and corrosion performance of the alloys in high temperature liquid sodium environment.