A 2200°C Fuel Centerline Johnson Noise Power Thermometer

A Johnson Noise Power Thermometer (JNPT) for temperature measurements to 2200°C in reactor fuel rods has been fabricated. The instrument is designed for use in LOFT reactor safety tests conducted under the direction of the Nuclear Regulatory Commission. Preparations are underway to evaluate the instrument in a reactor environment. The JNPT determines temperature by measuring the thermal noise power spontaneously generated in a sensing resistor coil. Because the noise power is not a function of the coil resistance, the thermometer output remains unaffected by radiation induced transmutation in the sensor and can provide constant "in-situ" temperature calibration. The design also has the capability of operating simultaneously as a thermocouple. The primary obstacle in fabricating a 2200°C JNPT is the construction of a localized high electrical resistance sensor at the tip of the 0.062-in OD sheath probe. At 2200°C, the sensor must possess a "loop resistance" greater than 50 ohms, while maintaining an insulation resistance greater than 100 ohms. The sensing resistor consists of a 0.003-in diameter Re wire closely wound in a coil with an outside diameter of 0.010-in. The coil is placed inside of a two hole hard fired HfO2 insulator, and the coil leads are laser welded to 0.010-in diameter connecting wires. These wires are also insulated with HfO2. The four foot long W/Re probe is hermetically sealed with a ceramic-to-metal seal capable of surviving thermal transients to 350°C/min. Preliminary JNPTs have been fabricated and tested to 2200°C.