Ab initio thermometry for long-term unattended space reactor operation

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 non-drifting, long-term reliable instrumentation. Two widely recognized temperature measurement techniques rely directly on fundamental phenomena. Radiation thermometry (RT) is based upon the variation of the emission of light from a surface with changes in its temperature. The origin of this surface radiance is the acceleration (oscillation) of the electrical charges within the material. Johnson noise thermometry (JNT), correspondingly, is based on electrically measuring the random vibrations of the charges in a resistor. Since temperature is defined as the mean translational kinetic energy of an atomic ensemble both measurement techniques are, in pure form, ab initio. Daunting technical challenges must be overcome to apply either of these techniques to space reactors. Both techniques rely upon precise measurement electronics that must be implemented in a radiation-tolerant form. Further, RT relies upon both invariance in the optical path between the measured surface and the condition of that surface. Consequently, both must be controlled throughout the mission for successful fundamental RT implementation. Johnson noise is a small signal, wide-band phenomenon, which must be distinguished from competing mechanical vibrations and external electromagnetic noises. In addition, the capacitance of the signal cable between the resistive element and measurement electronics and the input electronic circuitry itself spectrally distorts the Johnson noise, which limits the allowable separation between the delicate measurement electronics and the reactor. This paper provides an overview and analysis of possible RT and JNT implementations for space nuclear power reactors.