Unrestricted core heatup analysis based on recent experimental investigations of heat, gas, and fission product transport mechanism under ultimate accident conditions proves for the example of the 500-MW(thermal) plant the inherent safety potential of the pebble-bed high-temperature reactor. Under the hypothetical assumptions of depressurization of the primary circuit through a large hole within the prestressed concrete reactor vessel top region and the failure of all heat sinks, including the liner cooling system, the core structure, fuel elements, and consequently the essential fission product barriers remain undamaged. The integrated 137Cs contamination of the containment after depressurization rises only to 1 Ci after 100 h.