Johnson Noise Thermometry For Space Reactor Temperature Measurement

Abstract

A primary difference between terrestrial and remotely located reactors is the ability to periodically recalibrate and replace the instrumentation. Because of this, space reactors place a premium on self‐calibrating, long‐term reliable instrumentation. The primary temperature measurements for the SP‐100 reactor were to be made using W/W‐Re thermocouples. However, the large gamma and neutron dose expected at the coolant outlet (>1 MGy γ; 3×1015 fast neutron fluence) combined with the high temperature (1375 K nominal; 1650 K maximum) meant that the thermocouples would drift significantly over the lifetime of the reactor. A combined Johnson noise resistance thermometer capable of performing under these extreme conditions was developed by ORNL (Carroll, 1994). Johnson noise is a fundamental representation of temperature—it is the vibration of the electronic field surrounding atoms as they thermally vibrate. Johnson noise, however, is fundamentally a small signal (∼4×10−7 Vrms for a 100 Ω resistor at 300 K, using a 100 kHz bandwidth) spread throughout the frequency spectrum. Creating the electronics and signal processing required to effectively measure and interpret the noise signal remains challenging. ORNL has recently developed closely related Johnson Noise Thermometry (JNT) electronics and signal processing capabilities under a DOE International Nuclear Energy Research Initiative Project with the Korean Atomic Energy Research Institute (U.S. DOE, 2002). An overview of the application of JNT to space nuclear power and the current status of the ORNL JNT capabilities is the subject of this paper.