An external water wall type containment cooling system is one of the passive containment cooling systems that use no active components and are intended for system simplification in the next generation power reactors. The core decay heat during a postulated loss-of-coolant accident is accumulated in the suppression pool (SP) and transferred to the outer pool, which is a cooling pool located outside and adjacent to the SP, by only natural phenomena such as natural convection, heat conduction, and evaporation. The temperature profiles and the convection heat transfer coefficients in the pools were measured using a 5-m height apparatus. The formation of a thermal stratification boundary at the vent outlets, which restricts the effective heat transfer area between pools, was clarified, and a correlation for natural convection heat transfer coefficients was obtained. Condensation heat transfer coefficients on the containment vessel wall and evaporation heat transfer coefficients on the SP surface under a noncondensable gas presence, which strongly affected the heat removal from the wet well, were evaluated based on the test results, and the correlations were obtained. The heat removal evaluation models, which analyze the trends of the temperatures and pressure, were developed and verified with system tests. As for the improvement of heat removal capability, two methods were proposed. One is a baffle plate to mitigate thermal stratification in the SP and enlarge the effective heat transfer area between pools. The second method is a divided wet well to avoid noncondensable gas effects. The thermal-hydraulic behavior in the SP with a baffle plate was clarified by three-dimensional analysis, and the effectiveness of these methods was experimentally confirmed.