Abstract Tritium behavior in a DEMO reactor is a key design issue because of its self-sufficiency and impact on safety. Considering the difficulty in handling tritium, and that the transport, diffusion and permeation phenomena involve a large number of physical properties and parameters, it is intended to prepare a simulation tool to predict its behavior. A preliminary model for tritium transport at system level has been developed for the HCPB (Helium Cooled Pebble Bed) breeder concept (DEMO2014), focusing on the multi-physics of the release, diffusion, permeation, recombination processes. The numerical technique presented here is based on EcosimPro simulation tool, a program with an object-oriented nature which offers the possibility of mixing various disciplines by robust equation-solving algorithms. The study presents the results obtained in a reference case in which the most generic parameters provided by the HCPB designers have been used. Together with the breeder blanket concept, a tritium extraction system (TES) and a coolant purification system (CPS) have been included into the gases circuits. The results of tritium permeation to the coolant provide relatively low values (around 0.2 g/d), that will be improved in the future by a more detailed system description.