This study is part of the development of simulation tools for ultrasonic telemetry. Telemetry is a technique chosen for monitoring sodium-cooled fast reactors, which consists in locating various reactor structures using an ultrasonic inspection performed by immersion. In order to model the interaction between the acoustic wave and the immersed structures, classical diffraction models have been firstly evaluated for rigid structures, including the geometrical theory of diffraction (GTD) and the Kirchhoff approximation (KA). These two approaches appear to be complementary. Combining them so as to retain only their advantages, we have developed the so-called refined KA based on the physical theory of diffraction (PTD). The refined KA provides an improvement of the prediction in the near field of a rigid scatterer. To deal with the scattering from a finite impedance target more representative of the reactor structures, the initial (non refined) KA model is then extended. The obtained model, the so-called “general” KA model, is compared to a reference model and provides a satisfactory solution for the application to telemetry. Finally, a complete simulation tool for telemetry is built by coupling this general KA diffraction model with a stochastic model developed for wave propagation in inhomogeneous media as sodium.