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The ongoing decommissioning of the Fukushima Daiichi (1F) nuclear power plant requires the inspection of the inside of containment vessels that have been submerged in water. These inspections must locate leaks and map the distribution of fuel debris in water with very low visibility. This paper reports the design and testing of an echo-PIV system that uses a single divergent signal wave and delay-and-sum processing to efficiently map the interior and fluid flow within a submerged vessel. The diverging wave and delay-and-sum processing improve upon the performance of conventional ultrasound PIV methods specifically to meet the demands of containment-vessel inspections. The imaging method uses an ultrasonic linear sensor array that emits a diverging wave that covers a wide angle with a single transmission. The delay-and-sum algorithm combines echo signals recorded by each element of the sensor array. We optimized the design of an echo-PIV system in laboratory-scale tests, and then tested the prototype with a mockup of a containment vessel in a water tank. The small-scale prototype successfully located a mock leak and mapped the surface of a piece of mock debris. This prototype can be scaled up readily for inspections at the Fukushima Daiichi plant.

Measurement accuracy of flow velocity by phased array UVP method was confirmed in order to develop a basic telemetry system for detections of leakage points. The basic telemetry system which consists of a robot arm for moving ultrasonic sensor and a phased array sensor for wide range area measurement of flow velocity was developed. Additionally, flow mapping of water flow around imitated leakage was conducted by the developed system and PIV method. As a result, the velocities by both methods were in agreement with 20 %. The possibility of measuring two-dimensional velocity of flow near the leakage point by the developed telemetry system was verified. Consequently, the basic telemetry system for the detection of the leakage was established.

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