Safety has always played a key role in the development of nuclear technologies, and, after the Fukushima accident in 2011, a huge effort is put in the study and development of passive safety systems in new designs of nuclear reactors, such as the one modelled in this work. The aim of this study is to develop a SMR modeling with the code CATHARE3, in the framework of the ELSMOR European project. A description of the modelling process is given in detail and an analysis of the Station Black Out accident is carried out focusing on the evolution of the pressure and temperature in different components during the short, medium and long term.