The velocity and temperature fields of natural convective flows in a water pool simulating a simple passive safety system are experimentally investigated using particle image velocimetry and laser-induced fluorescence. Typical natural convective flows, large-scale circulation (LSC), and a vertical thermal plume are indicated by the mean velocity fields. The initial developing height of the vortex flows in a confined space corresponds to that of the LSC center in a broad space. The mean temperature clearly represents the thermal stratification. The high-temperature zone well matches the high-velocity region below the LSC center, where no significant energy dissipation occurs. The local Grashof number and Richardson number are examined. The results indicate that shear and inertia play similar roles to that of buoyancy, except with regard to the mean flow near the heater in a confined space. The regions of significant energy transfer due to shear and heat are near the free surface and middle layer, respectively. To investigate the energy transfer between the mean flow and turbulence, the turbulent energy production is determined. The results show that energy dissipated from the thermal plume to the turbulence drives the LSC and vortex flows. (C) 2019 Elsevier Ltd. All rights reserved.