Development of a multipoint liquid film sensor using an optical waveguide film on simulated fuel rod of BWRs

Measurement techniques for distribution of liquid film thickness (LFT) with high spatial and high temporal resolutions are necessary for evaluation of thermal margins in boiling water reactors. To experimentally investigate high-speed phenomena of the liquid film on a simulated fuel rod, we developed an LFT sensor using an optical waveguide film (OWF) with 42 mu m spatial resolution and 1 MHz time resolution. The OWF is flexibly embedded in the surface of the simulated fuel rod and includes multiple light paths. Micro-mirrors located at ends of the light paths function as sensors and intensity of light reflected at the liquid film surface is detected. First, we determined the relationship between the LFT and the light intensity in a static liquid film. Second, signal processing for the OWF was shown by simulating random waves to extract influences of the surface angle on the output signals. Finally, we demonstrated measurement of the LFT in vertical annular flows with two sensors of the OWF. Mean LFT measured by the OWF gave good agreement with results from visualization within -20 to +10%. Furthermore, evaluation of the wave velocities for disturbance waves and ripples showed them to correspond to the visualization results.