Abstract: The invention relates to a pebble bed reactor comprising a core cavity filled with spherical fuel elements and bounded by reflector material. Spherical fuel elements pass through the core cavity under the effect of gravity. Coolant gas flows through the core cavity in a descending or ascending stream. The coolant-gas lines provided for the purpose discharge laterally above the base of the cavity bottom. In the reactor according to the invention, coolant gas can be conveyed through the pile of fuel elements without problems over long operating times.

Abstract: A ball-bed (pebble-bed) nuclear reactor, instead of having fuel elements more or less continuously withdrawn and new or reconstituted fuel elements more or less continuously reintroduced, is initially partly filled with fuel balls of which two-thirds have a fissionable material content 12% below and the upper third 24% higher than the average content. This filling meets the requirements of criticality in order to begin operation. Thereafter, fuel balls are added slowly, a few hundred per day, having 150 to 250% of the average fissionable material content of the initial loading thus preserving the criticality requirements, while keeping the temperature within safe limits until the reactor cavern is filled. Thereafter the reactor is shut down, cooled off, pressure relieved and emptied, the last step typically from above. An ordered array of the fuel balls in regular layers avoids excess pressure loads on the reflector over the life time of the filling.

Abstract: A ball-bed (pebble-bed) nuclear reactor, instead of having fuel elements e or less continuously withdrawn and new or reconstituted fuel elements more or less continuously reintroducted, is initially partly filled with fuel balls of which two-thirds have a fissionable material content 12% below and the upper third 24% higher than the average content. This filling meets the requirements of criticality in order to begin operation. Thereafter, fuel balls are added slowly, a few hundred per day, having 150 to 250% of the average fissionable material content of the initial loading, thus preserving the criticality requirments, while keeping the temperature within safe limits until the reactor cavern is filled. Thereafter, the reactor is shut down, cooled off, pressure relieved and emptied, the last step typically from above. An ordered array of the fuel balls in regular layers avoids excess pressure loads on the reflector over the life time of the filling.