High-temperature characteristics of Pt-Mo alloy thermocouple for in-core temperature measurements in very high temperature gas-cooled reactor.

Abstract

As an initial part of development of Pt-Mo alloy thermocouples for applying them to in-core outlet-gas temperature measurement in a very high temperature gas-cooled reactor (VHTR), bare element wires of Pt-5%Mo/Pt-0.1%Mo thermocouple were tested in a high-temperature Ar atmosphere of 1,000∼1,400°C for 30 h and in vacuum at 1,200°C for 3,000 h, and stabilities of their electromotive force and mechanical strength were examined as well as observations of their metallographical changes. In the high-temperature Ar atmosphere, selective reaction of Mo with impurity gases contained in the Ar took place very actively in the element of Pt-5%Mo, and its mechanical strength decreased considerably with losing its luster. In the high-temperature vacuum, however, the element wires showed very stable electrical, mechanical and metallographical characteristics for a long-term of 3,000 h. This suggested the importance of clean environmental atmosphere around the element wires. Based on the result, Ta-sheathed thermocouple...