A technique for measuring fluid mixing in water models of nuclear reactor components using fibre optics

An improved multichannel dye measuring system has been developed for monitoring fluid mixing in water models of nuclear reactor components. The system uses the technique of injecting dye as a flow tracer and measuring the absorption and scattering of light through the fluid at downstream probe locations, to determine the attenuated dye concentration. Visible light from a light emitting diode is transmitted along a fibre optic cable to the measuring cell where it crosses a small gap containing the fluid. A further fibre optic cable then returns the attenuated signal to a photodiode,. For the particular application, 72 pairs of fibres are encapsulated in thick multihole orifice plates representing tube plates. Up to 56 simultaneous records of dye distribution within the flow are available for analysis. The signals from the photodiodes are conditioned by further electronic circuitry to give a linear relationship between dye concentration and voltage output. Signal processing of the multichannel signals is achieved by interfacing with a high speed data logger. The results of the sensitivity, accuracy, and response of the system are presented and an in-situ calibration technique described. (author)