A severe accident at TEPCO’s Fukushima Dai-ichi Nuclear Power Plant (FDNPP) occurred due to a strong earthquake and massive tsunami that struck Japan’s eastern region on 11 March 2011. During this accident, the reactor core of Units 1, 2, and 3 of the 1F melted and fuel debris formed in the reactor pressure vessel (RPV) and primary containment vessel (PCV). The Japanese Government has drawn a mid- and long-term roadmap towards 1F decommissioning. However, three reactors’ decommissioning work has faced difficulties due to the lack of realistic information about the damaged cores such as the distribution of fuel debris. Since the radiation levels inside the reactor buildings have been too high for human access, the robots and cameras have been using to investigate the internal status of PCV. To accelerate the decommissioning process of 1F, determining of fuel debris information and contaminated water leakage is crucial to decide further decommissioning steps and strategies. Therefore, in this study, we are developing an ultrasonic measurement system that can obtain object’s shape and the velocity around it. Because ultrasonic transducers are robust against high radiation dose and applicable in opaque liquids. The ultrasonic system used in the present study consists of two sectorial array transducers, which can measure object shape and 2D vector flow field. To measure the object’s velocity and shape that represents fuel debris, Ultrasonic Velocity Profiler (UVP) and Total Focusing Methods (TFM) are employed, respectively. Based on measurement results of the 2D vector field and shape of the object, we can predict contaminated water leakage and distribution of object at the bottom of the PCV in 1F.