The deployment of the sodium fast reactor (SFR) is currently limited in large part by the availability of materials which can sustain both radiation damage at high levels of displacements per atom (dpa) and corrosion by liquid sodium to meet the necessary safety and economic criteria for licensing and commercialization. Fast reactor cladding and duct material must 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 at high dpa, the focus for cladding and duct applications shifted to ferritic-martensitic (F-M) steels.
This project explores 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 informed the selection HEA families, CrFeMnNi and NbTaTiV. Conventional alloys consist of one or two principal elements and minor concentrations of alloying constituents. In contrast, HEAs are composed of four or more metallic elements with concentration <35 at. % mixed in a single-phase solid solution. Some HEA are theorized to resist degradation by radiation. Low dpa in-situ heavy-ion irradiation under transmission electron microscope (TEM) is intended to illuminate hypothetical radiation damage mechanisms, while high dpa studies simulate long-term microstructural, micromechanical, and microchemical radiation effects in a nuclear reactor environment.
The nature of HEA as a class of alloy presents the unique challenge of selecting suitable HEA compositions for nuclear applications from a large compositional range and thus unlimited number of elemental combinations. Thus high-throughput experimental methods and modeling techniques are critical for alloy development on reasonable timescales. Compositionally graded combinatorial thin films are being employed along with CALPHAD thermodynamic calculations to rapidly fabricate slices of HEA phase diagrams and test predictions of the stable phases at relevant temperatures using X-ray diffraction (XRD). This technique for exploration of HEA compositions naturally precedes the development of high-throughput irradiation experiments, which are being planned with the UW Ion Bean Lab.