[en] When forced cooling is switched off or fails, after the gas paths to the steam generators have been shut down, an intensive natural convection is observed in the core of a pebble-bed reactor. This convection must be investigated at higher temperatures because of the fission product transport it effects and the cooling effect it has in the center of the core. This report describes an appropriate model, the experimental confirmation of this model and its application to the pebble-bed reactor. The expected cooling effect becomes evident in a limitation of the maximum core temperatures. Natural convection in the core is triggered by the rapidly increasing temperature differences between the center of the core and the heat conductive walls resulting from a failure in forced cooling. Figure 1 illustrates this fact with the fictitious temperature change without convective heat transport in the core of the 500 MW/sub th/ pebble-bed reactor PR 500