Effects of Shield Impedance, Connector Resistance, and Coaxial Baluns on Ground Noise Interference in Nuclear Reactor Instrumentation Systems

Abstract

Electrical noise interference in low-level (approximately 50 ..mu..V), wide band (approximately 15 MHz) flux monitoring systems applied to nuclear reactor control causes safety and reliability problems. Others have shown that one predominant source of noise interference is conduction of currents in instrument cable shields and building conduits. Since these currents produce noise that is similar to signals produced by nuclear detectors, such noise interference reduces the ability of a reactor instrumentation system to determine the condition of a reactor. Model equations of ground noise interference were derived for a system model consisting of a sensor, coaxial cable, and an amplifier. These equations describe the effect of ground impedance, sensor impedance, transfer impedance, and connector resistance on both low-frequency (less than 100 kHz) and high-frequency (greater than 100 kHz) ground noise interference. Other model equations were derived for a system with a coaxial balun (a flexible coaxial cable wound around a ferrite core) added between the sensor and the sensor amplifier input. Analysis of the model equations reveals the effects of ground noise currents on instrumentation systems and the conditions for minimizing ground interference.