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A nuclear reactor fuel core assembly comprises an array of sub-assemblies comprising a central zone of fuel/breeder sub-assemblies (20F) surrounded by an annular zone of shielding sub-assemblies (20S). The fuel/breeder sub-assemblies (20F) are flexibly mounted from the reactor diagrid top plate 26 w

A nuclear reactor fuel core assembly comprises an array of sub-assemblies comprising a central zone of fuel/breeder sub-assemblies (20F) surrounded by an annular zone of shielding sub-assemblies (20S). The fuel/breeder sub-assemblies (20F) are flexibly mounted from the reactor diagrid top plate 26 while the shielding sub-assemblies (20S) are mounted as stiff cantilevers so as to provide resilient restraint against bow of the central sub-assemblies (20S) during reactor operation, the restraint being exerted via abutment pads (40) at the level of the tops of the sub-assemblies (20S, 20F). The central core sub-assemblies (20F) also have abutment pads (34) at a lower level to form a lower restraint plane at this level controlling sub-assembly bowing in the central core zone.

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A nuclear reactor core assembly comprising an array of core sub-assemblies of which a first group forms a central core zone and a second group forms an annular outer core zone surrounding the central core zone, each subassembly of both groups comprising an elongate tubular wrapper provided at its lo

A nuclear reactor core assembly comprising an array of core sub-assemblies of which a first group forms a central core zone and a second group forms an annular outer core zone surrounding the central core zone, each subassembly of both groups comprising an elongate tubular wrapper provided at its lower end with a spike formation which engages in a support structure for supporting the sub-assemblies of both groups in upright parallel relationship and at its upper end with first abutment surfaces for contact with the corresponding first abutment surfaces of neighbouring sub-assemblies so as to form an upper restraint plane for the sub-assemblies of both groups, characterised in that: at a level below the upper restraint plane, the wrappers of the first group of sub-assemblies, but not of the second group of sub-assemblies, are each provided with second abutment surfaces for contact with the corresponding second abutment surfaces of neighbouring sub-assemblies of the first group so as to form a lower restraint plane effective only for the sub-assemblies of the first group; each of the sub-assemblies of the first group has a connection between the wrapper and the respective spike formation allowing tilting of the wrapper relative to the spike formation; and each of the sub-assemblies of the second group has a connection between the wrapper and the respective spike formation which is stiffer than the connection aforesaid so that each second group sub-assembly, being subject by virtue of the stiffer connection to greater restraint against tilting of the wrapper relative to the spike formation, acts as a cantilever beam which, on take up of clearance between the first and second abutment surfaces, radially restrains the sub-assemblies inboard thereof, the radial restraint exerted by the second group sub-assemblies being effective for the first group subassemblies via the upper restraint plane.