Alternative Ceramic Insulators for Nuclear Thermocouples

The drive for more energy efficient nuclear reactors has spurred the need for advanced in-core temperature sensors to regulate reactor environments and fuel-cycle development. The Idaho National Laboratory (INL) has developed high-temperature irradiation-resistant thermocouples (HTIR-TCs) to enhance temperature control and measurement stability in nuclear reactor cores. These thermocouples are composed of commercially available phosphorus-doped niobium (Nb-P) and lanthana-doped molybdenum (Mo-LaO) thermoelements, alumina (Al2O3) insulation, and a sheath made of either niobium or its alloys. However, the maximum service temperature of the HTIR-TCs is limited to 1700 °C due to the instability of alumina above this temperature. This study evaluated and characterized the stability of alternative refractory ceramic insulators with HTIR-TC relevant materials for application above 1700 °C. The thermodynamic stabilities of ceria (CeO₂), lanthana (La₂O₃), and praseodymia (Pr₂O₃) were compared to alumina because of their superior thermal properties. To assess the chemical stability of the down selected ceramics with niobium, diffusion couples between ceria, lanthana, and praseodymia with niobium foils were exposed to 1700 °C for six hours. A saturation region of approximately 75 micrometers formed at the lanthana-niobium interface, a lack of interaction between praseodymia and niobium was observed, and ceria will be further discussed in the poster.