AbstractThe analyses performed on the rod bundles of the CEA/ENEA poloidal Breeder In Tube (BIT) blanket have shown the necessity to investigate experimentally the helium coolant thermal-hydraulic behaviour. The paper describes the design of the experiment splitted into test facility and test section design. In the frame of the latter item, a method has been developed in order to reduce the geometrical independent variables of the bundle. The method is based on the minimization of the bundle transversal thermal gradient. If adopted as design rule, it will allow the minimization of the bundle hot-spot factor.