The objective of this work is to study the coolability limits of stationary porous structures and loose particle accumulations that might occur in liquid-metal fast breeder reactor subassemblies. Due to the simple geometry of the test sections, it was possible to produce a motion picture and coordinate it with the mass flow and temperature measurements. This approach has been shown to provide an adequate picture of the cooling mechanisms, especially at sodium boiling. A remarkable difference in boiling behavior in stationary porous structures and movable particle accumulations has been observed. Stationary porous structures are very sensitive to non-rewetting hot spot formation and dryout, whereas particle accumulations tend to form fluidized bed structures at sodium boiling. Dryout heat flux correlations for both the stationary porous structure and the movable particle accumulation have been developed from the experimental results.