Development proposals for large pebble-bed cores - equilization of the pebble flow and modified shutdown system

The paper deals with two proposals to solve specific pebble-bed problems. In order to obtain a constant radial and azimuthal velocity profile of the spherical fuel elements, the Gesellschaft fuer Hochtemperaturrekator-Technik (GHT) is testing and developing a cone of rhombical cross-section, which will be placed inside the funnel-shaped core bottom directly above the pebble extraction duct. This so-called ''Fliesskegel'' (pebble flow distributor) construction is meant to be an alternative to the multidischarge-hole core concept. It was possible to show the superiority of this device in obtaining a flat velocity profile even for a core with seven discharge outlets. To determine the changes in the properties of large pebble-bed cores compared to the THTR size, GHT proposes to develop a helical absorber which has to be screwed into the pebble-bed employing a certain slip. The advantage of this alternative to the absorber rods is a drastic decrease of the load on the fuel elements as well as on the core bottom. In addition, the number of required absorber elements can be reduced by half with respect to the already known ''turning rod absorber''. (author)