Development of Pt-Mo alloy thermocouples for incore temperature measurement in HTGRs

The Pt-Mo alloy thermocouples with element wires of Pt-5%Mo and Pt-0.1%Mo were developed for incore temperature measurements at a maximum temperature of 1,200degC for a long term and of 1,350degC for a short term in VHTRs. The thermocouples were tested at high temperature conditions of 1,000-1,400degC in an outpile electric furnace and in an incore capsule in the JMTR. The total thermal neutron fluence irradiated was 8.1x1020 n/cm2. The changes in their electromotive forces due to those exposures to high temperatures and radiation in the reactor core were evaluated to be less than ±2%. This meets the specifications that is required for the incore temperature measurements in VHTRs. Hence, the developed thermocouples can be used also in the incore temperature measurement in advanced type HTGRs and in incore irradiation experiments in the HTTR, in future. (author)