Development of Rod-embedded Fiber LDV to Measure Velocity in Fuel Rod Bundles

Abstract

Precise measurement of velocity in fuel bundles is required to improve the thermal hydraulic characteristics of Pressurerized Water Reactor (PWR) spacer grids. However, the use of conventional laser Doppler velocimetry (LDV) is restricted by the existence of invisible region in fuel rod bundles and insufficient spatial resolution for the narrow gaps in rod bundles. An LDV was therefore developed to overcome these problems. The new LDV is miniaturized with fiber optics embedded in a fuel cladding. The rod-embedded fiber LDV (rod LDV) can be inserted in an arbitrary grid cell instead of a fuel rod, and has the advantage of not disturbing the flow field since it is the same shape as a fuel rod. As there are a number of fuel rods in the flow field of a rod bundle, it is difficult to distinguish between Doppler signals and noise due to laser reflections from surrounding rods. To improve the data rate, a pedestal trigger was used to provide gate signals. It was shown that the rod LDV was capable of measuring the flow velocity in a rod bundle, and data for cross-flow and axial-flow velocity downstream of the spacer grid were obtained.