In Fukushima Daiichi Nuclear Power Plant (FDNPP) accident, the fuel debris formed in the Reactor Pressure Vessel (RPV) and Primary Containment Vessel (PCV) at Unit 1？-？3. To accelerate and decide further decommissioning steps of the FDNPP, it is crucial to obtain realistic information of the debris and localize contaminated water leakage from PCV. Due to high radiation？and dark environment inside the PCV, investigating instruments and？techniques should necessarily to meet specification of radiation resistance,？waterproofness, dust resistance and so on. This study focuses on development of ultrasonic measurement system using a couple of sectorial array sensors to？localize contaminated water leakage and visualize shape of object that？repre-？senting fuel debris, simultaneously. In this study, Total Focusing Method？(TFM) and Ultrasonic Velocity Profiler (UVP) methods are considered to visualize object shape and flow pattern around it, respectively. To demonstrate applicability and reliability of developed measurement system with sectorial array sensors, a mock-up experiment result？of simulated water leakage and fuel debris shape were discussed in this paper.
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