Analysis of defect detection in steam generator tubes of FBR, under support plates and in the presence of sodium using multi-frequency eddy current algorithms

Mihalache, O. ; Ueda, Masashi ; Yamaguchi, Toshihiko ; Yamashita, Takuya

In the present paper we estimate the effect of sodium in the in-service inspection of non-magnetic steam generators tubes using eddy current technique and eddy current probes based on a differential double bobbin coil configuration. Experimental measurements of defects signals in steam generator tubes of fast breeder reactor are compared with simulations results of a two-dimensional axisymmetric finite element code to validate a reliable electromagnetic model of the system (eddy current coils, steam generator tube, defect) when there is no sodium on the outer steam generator tube surface. The electromagnetic code is used to evaluate the sodium band and sodium layer signals when a defect is located under steam generator support plate. Using a multi-frequency algorithm, its parameters are determined in the "no sodium condition" (there is no sodium on the outer steam generator tube surface), defects signal is enhanced during eddy currents inspection, even when the defects are located under support plate in the presence of sodium layers with thickness ranging from 10 to 300m and sodium circumferential bands 5 mm wide and 2.5 mm high near support plate. The paper presents a full detailed analysis for detection of defects in various configurations, using eddy current technique, and tests the robustness of the multi-frequency approach to suppress support plate signal of non-magnetic SG tubes of fast breeder reactors.