Author(s)

NARUKI SHOJI, HIDEHARU TAKAHASHI, HIROSHIGE KIKURA

Abstract

In March 2011, the severe accident of the Fukushima Dai-ichi (1F) nuclear power plant was happened by the earth quack and massive tsunami in Tohoku, Japan. And then, fuel debris was generated within the primary containment vessels (PCVs) of units 1, 2, and 3, respectively. Recently, the decommissioning of 1F is underway to remove the fuel debris, and the inside inspection with robots was conducted so far. Optical techniques have been applied for inspecting the PCVs, but information of the contaminated water leakage has been not unveiled due to non-clear water causing poor visibility of the camera. Therefore, non-optical techniques are required to unveil the leakage location, and we focused on the ultrasonic measurement technique. Ultrasonic measurement can be applied to opaque liquid, high-radioactive, and dark environments. In this study, we have developed the ultrasonic velocity profiler (UVP) system for the investigation of leaking locations. The UVP is based on the pulsed Doppler method, and it can measure instantaneous velocity profile along an ultrasonic beam path. In the original UVP principle, it can measure only one-dimensional velocity measurement. Therefore, we extended the UVP to multi-dimensional measurement. To achieve this, a multi-element sensor was used and an algorithm of three-dimensional (3D) velocity vector reconstruction was developed. The 3D vector measurement is realized by simultaneous receiving Doppler signal at each ultrasonic element. The measurement performance was evaluated by the rigid rotating flow measurement, and the relative error of velocity magnitude and vector angle were minimum –1.6% and 11.2% respectively. In addition, we checked the validity of the system for leakage detection by measuring the simulated leakage flow.

Keywords

ultrasound, velocity profile, vector measurement, array transducer, multi-dimensional