A reactor core vessel of a pebble-bed nuclear reactor contains fuel pebbles and reflector pebbles fed into the vessel through respective conduits. A guide ring can be provided such that the reflector pebbles pass through the guide ring and form a reflector column, while the fuel pebbles pass outside the guide ring and form an annular fuel column surrounding the reflector column. The guide ring controls the size and shape of the reflector column and controls mixing of the two types of pebbles. Furthermore, the fuel pebbles can have a substantially larger diameter than the reflector pebbles. Accordingly, the fuel-pebble column will have more void space, and cooling gas will consequently flow preferentially through the fuel-pebble column.
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