During the period 1993 -2004, D’Auria had the opportunity to cooperate with the University of Genova (noticeably Prof. Misale). This cooperation brought to a valuable contribution to the scientific community based on the characterization of the single phase instabilities in a simple atmospheric pressure ‘glass’ loop previously available by UNIGE and upgraded within the framework of the cooperation with UNIPI. The characterization of the loop performance and the analysis of performed experiments were performed by the help of system thermal-hydraulic codes; those analyses confirmed some early investigations results by Velander in 1968. The present paper deals with the application of RELAP to the analysis of unstable experimental data in the UNIGE loop.