Tritium behavior in a breeding blanket is a key design issue because of its impact on safety and fuel self-sufficiency best performance. Considering the difficulty in handling and its high cost, it is intended to prepare a simulation tool for tritium transport in the HCPB (Helium Cooled Pebble Bed) breeder blanket unit concept. The objective of this work is to evaluate tritium inventory inside several components of the breeder unit (pebble bed and purge gas) and its permeation into the coolant. Some simplified assumptions have been adopted and the results compared with others studies obtained by different simulation tools. Finally an example in which different experimental values of tritium residence time in ceramic breeder is presented with the purpose to observe the capability of the program to be integrated with experimental campaigns.