[en] The 15 MW experimental nuclear power Station with a helium-cooled high-temperature reactor and spherical fuel elements was constructed in the period from 1959 to 1966 and handed over to the Operator in 1969. Full-power and experimental Operation, which was originally only intended to last a few years, was extended until the end of 1988. During this time, approx. 1,670 GWh of electric power was generated. The aim of this large-scale experiment was to test the concept as well as the components, especially the fuel elements, which were continuously further developed and perfected during the Operation period. The time utilization factor during the entire Operation time was 67.2 %. The best value of 91.9 % was achieved in 1976. It must be remembered that the extensive experimental programme required numerous downtimes. The present report lists in detail all the shutdowns and power decreases, their causes as well as major malfunctions. All malfunctions (a turbine oil fire, an ingress of acid into the steam generator, steam generator damage leading to the ingress of considerable water volumes into the primary circuit, as well as bearing damage at one of the coolant gas circulators) were completely eliminated without impairing further Operation. The behaviour of all essential components and the water-steam circuit are described. The reactor physics, control of the coolant gas impurities, the final experimental programme as well as radiation protection are dealt with. The report on the more than 21-year Operation of the plant and on the Performance of numerous experiments shows that the helium-cooled high-temperature reactor with spherical fuel elements can be successfully built and operated. The System has excellent safety features and is not only suitable for electricity generation but also for the production of process heat at a high temperature level (e.g. for the gasification of coal) and as a heat source for helium turbine power plants. (author)