Stabilization of Nuclear Thermocouples: A Materials Science Approach: A Materials Science Approach

Stable, accurate, and precise temperature measurements are critical for efficient reactor operation, reactor lifetime extensions, and for the advancement or reliable modeling and simulation routines. In addition, traditionally implemented thermocouples experience decalibration and drift when they are exposed to high temperatures and neutron fluence, common to the next generation nuclear reactor designs. Accordingly, the INL has recently developed high temperature irradiation resistant thermocouples (HTIR-TCs) composed of phosphorus-doped niobium and lanthana-doped molybdenum thermoelements, an alumina insulation, and a niobium sheath. In order to stabilize the signal generated from the dissimilar metal junction during extended operation and use, the thermocouples must undergo a preliminary heat treatment above its maximum service temperature. In this research, we have taken a material science approach to understand the mechanistic contributions to the signal stabilization, which is found to exist in three primary forms - defects from cold-working, chemical diffusion, and grain restructuring. In this study, we have evaluated the impact of the stabilization heat treatment process (two primary methods were used - resistive heating the thermocouples and furnace heat treatments) on the microstructure, chemical stability, and electrical properties. Accordingly, recrystallization, alumina diffusion, and Nb3P precipitates were observed in the doped niobium thermoelement during the stabilization heat treatment leading to destabilization in operation. The doped molybdenum thermoelement was largely unaffected by the high temperature exposures. The impact of the stabilization heat treatment on the Seebeck coefficient and resistivity will be discussed.