Traditional methods for measuring in-pile temperatures degrade above 1100°C. Hence, the Idaho National Laboratory (INL) initiated a project to explore the use of specialized thermocouples for high temperature in-pile applications. 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 resist material interactions and remain ductile at high temperatures. In addition, results indicate that the candidate thermoelements have a thermoelectric response that is single-valued and repeatable with acceptable resolution. The selection of the thermocouple materials depends on desired peak temperature and accuracy requirements. For applications at or above 1600°C, tests indicate that thermocouples having doped molybdenum and Nb-1%Zr thermoelement wires, HfO2 insulation, and a Nb-1%Zr sheath could be used.

INL has worked to optimize this thermocouple's stability. With appropriate heat treatment and fabrication approaches, results indicate that thermal cycling effects on this thermocouple's calibration is minimized. INL initiated a series of high-temperature (1200 to 1800°C) long-duration (up to 6 months) tests to assess the long-term stability of these thermocouples. Initial results indicate that the INL-developed thermocouple's thermoelectric response is very stable. Typically, <20°C drift was observed in a 4000-h test at 1200°C. In comparison, commercially available types K and N thermocouples included in these 1200°C tests experienced drifts up to 110°C.