To study the behavior of the reactor that NAAREA is designing, codes and models have been developed during this master thesis through an internship. The XSMR in conception at NAAREA is a GEN IV reactor which requires the implementation of several calculation codes to simulate the overall behavior in nominal, incidental or accidental conditions. Hence, a first model that couples neutronics and thermal hydraulics has been developed in Python language. It has been adapted for the MSRE, a reactor built by the ORNL in the 60's, since data are widly available. Some accidental situations (reactivity insertion, pump coasts-down) have been simulated and compared to the experimental data and other results from litterature.