In case of a Hypothetical Core Disruptive Accident (HCDA) in a Liquid Metal Reactor, the interaction between fuel and liquid sodium creates a high pressure gas bubble in the core. The violent expansion of this bubble loads the vessel and the internal structures, whose deformation is important. The experimental test MARS simulates a HCDA in a small scale mock-up containing all the significant internal components of a Fast Breeder Reactor. The test-facility is filled with water topped by an argon blanket and the explosion is generated by an explosive charge. This paper describes the MARS test-facility and presents two numerical modelings used to simulate the test with the CASTEM-PLEXUS code. In the first model, the main internal structures are described with a classical shell model. The second model represents, in addition, the heat exchangers and pumps by means of a porosity model homogenizing the components with the surrounding fluid.