Nondestructive inspection of control rods in nuclear power plants using an encircling-type magnetic camera

摘要

This paper proposes an encircling-type magnetic camera for inspecting the damage to control rods. The magnetic source and sensors in the magnetic camera were separated from each other, which minimized the electromagnetic interference of adjacent sensors and improved the spatial resolution. An induced current (IC) appears on a control rod whenever a timevarying current is applied to the magnetic source, i. e., an encircling coil. The IC flows along the circumferential direction (theta-direction) of the rod, and generates a time-varying magnetic field in the radial direction (r-direction) around the defect. Bobbin-type integrated Hall sensors are arrayed inside the encircling coil, and therefore the r-direction magnetic field around the defect is measured. Seventeen Hall sensors were arrayed at a 21. angle with respect to 1.85-mm resolution to measure the electromagnetic field distribution on a stainless-steel small bore piping system, which had an external diameter of 10 mm and a thickness of 0.5 mm. The 40% (1-mm diameter) and 20% (3-mm diameter) depths of the hole-type wears were inspected using the proposed system. In addition, slit-type cracks with a length of 3 mm, depth of 0.2 mm, and width of 0.1 mm in both axial and circumferential directions could be inspected.