Design of encircling remote field eddy-current probe

This paper explains the progress in designing a shielded encircling remote-field eddy-current (RFEC) probe to inspect nuclear fuel rods and investigates resulting signal characteristics. To force electromagnetic energy from the encircling exciter coil to penetrate into the rod, the exciter coil is shielded outside. The effects of the shielding and operating frequency are studied by finite-element analysis and the location for the sensor coil is decided. The sensor coil is also shielded to prevent direct influence from exciter fields. The totally shielded probe dramatically improves signal characteristics. Numerical modeling with the designed probe shows that the phase signal is almost equally sensitive to inner- and outer-diameter defects and there is a linear relationship between phase signal strength and defect depth, which are typical characteristics of RFEC signal.