The present paper focus on the multifrequency and a novel window multifrequency ECT algorithm applied specifically to the magnetic SG tubes of a Fast Breeder Reactor (FBR). The algorithms parameters are determined and validated using 3D numerical Finite Element Simulations of the RF-ECT signal based on codes previously developed in-house by authors. The algorithm robustness is checked and validated using first numerical simulations and then experimental measurements conducted with a small mock-up test tank with SG tubes and full circumferential grooves or partial defects located on the external SG tube surface near tube support plate.