Mechanical and Chemical Performance of High Temperature Irradiation Resistant Thermocouples

Instrumentation capable of obtaining accurate, real-time data is essential for accelerating innovations in nuclear energy. In pile temperature measurement technology is currently limited as standard devices lose reliability after long durations at high temperatures. High temperature irradiation resistant thermocouples (HTIR-TCs) offer a potential solution to this problem and are of interest for instrumenting commercial and research nuclear reactor fuel cores. Grain growth due to time at elevated temperatures causes signal drift and reduced ductility. By annealing HTIR-TCs above operating temperatures, Idaho National Laboratory can increase the stability of these devices at a cost to their ductility, which is important for deployment. Differences in grain size and chemical composition were quantified in heat-treated HTIR-TCs using electron backscatter diffraction (EBSD) and energy dispersive x-ray spectroscopy (EDS), then correlated to the mechanical behavior of each. The grain growth was found to be inhibited in niobium HTIR-TCs containing 1% zirconium when compared to pure niobium HTIR-TCs exposed to the same conditions. Bend testing of the HTIR-TCs was conducted to determine the effects of annealingand sheath composition on the flexural strength of the thermocouples. Understanding the grain growth, chemical, and mechanical behavior of HTIR-TCs will guide the future design of thermocouples for nuclear applications