Experimental measurements and simulations of ect signal for ferromagnetic SG tubes covered by a sodium layer

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

For a Fast Breeder Reactor (FBR) cooled by liquid sodium, the wall of steam generator tubes (SG) is the only barrier between sodium in the tubes vessel and water inside of tubes. By using eddy currents, the SG tube integrity is usually assured during the In-Service Inspection (ISI). During this procedure, the 500 degrees sodium is drained and then SG tubes cooled down slowly in 2 weeks to room temperature. After sodium draining, sodium adheres to SG tubes and other structures in the SG vessel in thin layers. Experimental data shows that sodium drops also appear in specific areas along tubes or tube support plates. The high electrical conductivity of sodium adhering to the outer SG tube surface influences the ECT signal according to sodium layer thickness or sodium deposits located on the outer SG tube surface. The sodium adhering properties depends on several factors among them being the surface roughness of tube material, sodium temperature and wetting time. The ECT effect of sodium adhering to the outer SG tube surface was measured using a small mock-up tank in which were introduced SG tubes similar with the ones in FBR (2.25Cr-1Mo alloy). Defects and support plates were added to tubes and ECT signal was measured before and after sodium draining. Variations in the sodium layer thickness and consequently its ECT effect were measured by filling and draining the tank three times in order to recreate each time new layers of sodium. The paper describes the experimental conditions for defects, SG support plates signal before and after draining of sodium. Additionally, sodium structures were visualized using a CCD camera, confirming the recorded ECT signals. The paper presents details about sodium layer measurements for several parts of the SG tubes by scratching and collecting the sodium on small areas 10x10 square mm. Numerical 3D simulations are performed using a 3D FEM code developed in our laboratory in order to validate the performance of the ECT sensors detection.