A supercritical recompression CO2 power cycle has been simulated using the system code RELAP5-3D. This code is being developed by INL and has traditionally been used in the simulation of operational and accidental transients in fission nuclear plants. The aim of the work presented here, developed within the framework of the Spanish Fusion Technology Program Consolider TECNO_FUS, is to take advantage of the simulation capabilities of RELAP5-3D in a field where little if any experience exists in the use of the code; i.e., the simulation of the heat fluxes and the thermodynamic cycle that, in a fusion power plant, will convert thermal power from plasma into mechanical power as a previous step to electricity generation. Code capabilities that make it suitable for this purpose are, for instance, the compressor model and the libraries of fluid properties (among them CO2 and LiPb).; The reference plant for the simulation is the one being designed under TECNO_FUS, which is the Spanish proposal for DEMO. The model of the plant includes the primary coolant systems, i.e. helium and LiPb in the Spanish dual coolant modular design (doble refrigerante modular, DRM), compressors, turbine and heat exchangers (Printed Circuit type).; After the model has been set-up, several steady-state calculations have been run to test the performance of the model. After designing some minimal control features and adjusting their parameters, a few transient calculations have been run in order to demonstrate the capabilities of code and model. Finally, strengths and weaknesses of code and model are highlighted, along with some conclusions on their suitability for fusion technology applications.