The compatibility between thermocouple wires and sheath alloy for small diameter mineral-insulated metal-sheathed (MIMS) thermocouples in long-term aging

摘要

In this paper, the influence of long-term aging on the electromotive force (EMF) drift of small diameter mineral-insulated metal-sheathed thermocouples has been investigated on the two typical commercial thermocouple alloys for the nuclear reactor application. The results show that the stability of the de2-316L thermocouples is superior to that of de1 thermocouples. The drift is attributed to inhomogeneous oxidation, especially the KN thermoelement. The alloying elements diffuse towards the surface of thermocouple wire causing inhomogeneous composition even in the un-oxidized area. However, a premature failure is shown in the de2-Inconel thermocouples. This is because of the precipitation of Cr and Fe elements, which reduces the concentration of Cr and Fe in the Ni matrix. Fe generates a larger distortion of the crystal lattice and increases the instability of the solid solution. The deep understanding of the compatibility between the thermocouple alloys and sheath alloys is of crucial important in terms of the manufactures of thermocouple sensors and the safety operation systems in the nuclear reactor. (C) 2018 Published by Elsevier Ltd.