A passively-cooled spent nuclear fuel pool system comprising: a spent nuclear fuel pool comprising a body of liquid water having a surface level, at least one spent nuclear fuel rod submerged therein that heats the water; a lid covering the spent nuclear fuel pool to create a hermetically sealed vapor space between the surface level and the lid; and a passive heat exchange sub-system including a riser conduit comprising first and second riser inlet sections and a primary riser section that receives water vapor therefrom. Each riser inlet section has a respective inlet positioned in a respective section of the vapor space. A downcomer receives and condenses water vapor from the primary section forming condensed water vapor. A return conduit fluidly coupled to the downcomer and having an outlet located in the body of liquid water returns the condensed water vapor thereto.