A three-dimensional computational analysis is employed to investigate the transient flow and heat transport within an annular-core High-Temperature Reactor pebble bed, the blind core, and the surrounding graphite structures including 36 embedded coolant feeding channels. Well established models of pressure loss in the pebble bed, heat transfer between helium and the pebbles, and the heat transport within the porous medium by conduction and radiation (using an effective conductivity) have been implemented into an extended version of CFX-4. The graphite structures of the reflector are simulated closely coupled to the flow simulation as heat conducting solids. The model has been verified for axisymmetric power distributions by comparison with known results from 2D analysis. Various 3D-power distributions are assumed to represent a possible loss of forced cooling incident after unusual plant operation such as asymmetric positioning of absorber rods. It is confirmed, that 2D analysis is not adequate, because the predicted maximum temperature of the fuel elements after fission shutdown may exceed the maximum temperature predicted by 2D analysis.

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