Probe Design for the Eddy Current Inspection of Cooling Tubes in the Blanket of a Fusion DEMO Reactor

This study investigated the applicability of eddy current testing (ECT) to the non-destructive inspection of cooling tubes in the blanket of a fusion DEMO reactor. Pipes made of F82H steel with inner and outer diameters of 9.0 and 11.0 mm, respectively, were prepared, and slits imitating cracks were fabricated on the pipe surfaces. ECT was performed using a differential type bobbin probe having one exciting and two detecting coils designed in this study. The results of the inspections and subsequent three-dimensional finite element simulations revealed that a bobbin probe is effective in detecting cracks appearing on the inner surface of a pipe. Moreover, the detectability does not deteriorate significantly when cracks oriented in the circumferential directions are targeted,unlike in the case of ECT of the heat exchanger tubes of the steam generators of the pressurized water reactors. This indicates that a probe with a more complicated structure, such as a plus-point probe, would be unnecessary to detect flaws on the inner surface of a pipe. In contrast, the ECT signals from a non-penetrating slit on the outer surface were buried in noise even though the slit was as deep as 0.9 mm.