Classically, positioning and sizing of cracks detected by ultrasonic techniques using tip diffraction echoes is based on both the position in space (scanning position) and time of flight of the echoes. Immersion ultrasonic techniques are used to test the reactor pressure vessel of French nuclear plants. The presence of a cladding on the internal wall has a strong influence on echo positioning: the irregular geometry of the cladding/water interface disturbs beam refraction. Moreover, the anisotropic nature of the cladding increases such perturbations. These effects must be taken into account to accurately characterize discontinuities. Consequently, a model based technique has been developed with the aim of accurately locating a linelike scatterer simulating a crack edge. It uses simulations of the forward problem based on modeling tools developed at the French Atomic Energy Commission. First, the actual surface of the vessel is analyzed via onsite ultrasonic measurements and accounted for in further simulations. Second, a two step inversion scheme is developed. In a first stage, a set of possible positions is determined. In a second stage, the most probable position is deduced from direct comparisons between simulated and experimental B-scans. The technique is validated on linelike scatterers on several mockups and on actual onsite test measurements. The accuracy of the technique complies with industrial requirements motivating this study.