High-Temperature Characteristics of Pt-Mo Alloy Thermocouple for In-Core Temperature Measurements in Very High Temperature Gas-Cooled Reactor

摘要

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, 0001, 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 thermocouples were fabricated in trial and tested, expecting that the Ta sheath would work not only as a protector but also as a getter for impurity gases existing around the element wires. They showed very stable characteristics under a high-temperature condition for a long term with encouraging the future work in development of highlyreliable, long-life thermocouples for actual in-core services in VHTR.