System experiments were conducted at the ROSA-V Large Scale Test Facility (LSTF) for investigation of new safety systems to mitigate consequences of postulated accidents in pressurized water rectors (PWRs). Tested systems included a steam generator (SG) secondary-side automatic depressurization system (SADS) and gravity-driven injection system (GDIS), which are candidates of safety systems for some next-generation PWR designs. The experimental results showed several thermal–hydraulic behaviors typical of these safety systems, including the primary depressurization due to natural circulation cooling, a nonuniform flow behavior among SG U-tubes, an accumulation of the non-condensable gas originally contained in the injected water, liquid holdup in U-tubes due to the countercurrent flow limiting, and long-term passive core cooling with the GDIS injection. From the assessment of the RELAP5/MOD3 code using the present data, it was found that the inability of the code to predict the U-tube nonuniform flow behavior resulted in overprediction of the primary depressurization rate at a pressure less than 1 MPa, and exaggerated oscillation of the natural circulation flow rate in the primary loop.