[en] Small-diameter sheathed thermocouples are placed within nuclear fuel rod simulators to measure high temperature (up to 1100⁰C) and severe thermal transients (up to 100⁰C/s) encountered during simulated nuclear reactor accidents in thermal-hydraulic test facilities. These extreme requirements, in addition to operational life requirements of up to 1000 h, place severe demands on the thermocouples. Examination of thermally cycled thermocouples along with investigations of their time/temperature behavior yielded three criteria for improvement of thermocouple life-time. Methods to eliminate thermocouple prestrain, reduce impurity content, and refine thermoelement grain size were developed and introduced into the thermocouple fabrication process