The paper deals with the assessment of the pressure suppression system of the international thermonuclear experimental reactor (ITER). An extensive experimental research programme performed in a small-scale test facility analysed the steam direct condensation at sub-atmospheric pressure (about 400 tests). A similitude analysis was elaborated for extrapolating the results to the ITER full scale system. The derived scale laws were applied to analyse the performances of the pressure suppression system in order to manage a large loss of coolant accident (LOCA) scenario postulated in the vacuum vessel of ITER. Experimental tests, performed in a large-scale test facility built at the University of Pisa (geometrical scale about 1 and 1/10 power scale), analysing the first part of the large LOCA event, assessed the scale laws emphasizing some discrepancies determined in the extrapolation of reduced scale results to the actual system. Introducing a justified correction factor, the presented similitude analysis assured a suitable and reliable temperature and pressure increase prediction of full-scale systems, even in scaled configuration affected by relevant thermal stratification.