Many advanced nuclear reactor designs require new fuel, cladding, and structural materials. Data are needed to characterize the performance of these new materials in high temperature, oxidizing, and radiation conditions. To obtain this data, robust instrumentation is needed that can survive proposed test conditions. Standard thermocuoples for measuring temperature in-pile degrade at temperatures above 1100°C. Hence, INL initiated a project to develop specialized thermocouples for high temperature in-pile applications. Results from efforts to develop, fabricate, and evaluate specialized high-temperature thermocouples for in-pile applications suggest that several material combinations are viable. Tests show that several low neutron cross-section candidate materials are resistant to material interactions and remain ductile at high temperatures. In addition, results indicate that the thermoelectric response is single-valued and repeatable with acceptable resolution for the candidate thermoelements considered. The final selection of the thermocouple materials will depend on the desired peak temperature and accuracy requirements. If thermocouples are needed that measure temperatures at 1600°C or higher, the doped Mo/Nb-1%Zr and Mo-1.6% Nb/Nb-1%Zr thermoelement wire combinations are recommended with HfO_2 insulation, and a Nb-1%Zr sheath. Additional evaluations are underway to characterize the performance of this proposed thermocouple design. INL has worked to optimize this thermocouple's stability. With appropriate heat treatment and fabrication approaches, results indicate that the effects of thermal cycling on the calibration of the proposed thermocouple design can be minimized. INL has initiated a series of high temperature (from 1200 to 1800°C) long duration (up to six months) tests. Initial results indicate the INL-developed thermocouple's thermoelectric response is stable with less than 15°C drift observed in over 3500 hours of the planned 4000 hours of tests at 1200°C. In comparison, commercially-available Type K and N thermocouples included in these 1200°C tests have experienced drifts up to of over 100°C.
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