This report describes the assessment of the RELAP5 code capability for HTR and VHTR design and safety analysis performed within the framework of the ENEA/MSE research program. To this purpose some tests in steady state and transient conditions were conducted in the HE-FUS3 loop (CR BRASIMONE) to provide an experimental data base characterizing the loop thermal-hydraulics and its behavior in safety relevant transient conditions. The RELAP5 model of the loop was developed taking advantage of the results of previous assessment activities and already available experimental data. By means of a pre-test analysis a set of transients representative of operational and accident conditions was defined: the plant start-up by steps, 2 Loss of Flow Accidents with a different dynamic of the accidental event, 2 Loss of Coolant Accidents at different loop pressure. The post-test analysis was carried out to assess that the RELAPS code is able to correctly reproduce the gas system thermal-hydraulics and dynamics, as well as to provide recommendations on gas cooled system modelling for the development of consistent numerical models for the HTR and VHTR accident analysis. Moreover, highlighting the weaknesses present in some aspects of the simulations performed it has been possible to suggest the needs of the future code developments.