Abstract: A reactor fuel system has fuel pins secured to respective carrier rings as an inner sub-assembly and an outer sub-assembly for mutual telescoping relation, with radial spacing thimbles interconnecting the two assemblies and also permitting interconnection to adjacent assemblies to ensure axial coincidence of cooling flow paths. In the case of booster fuel of annular section, which may be rod-mounted for ready insertion into and withdrawal from the reactor core, the center passages of respective pins are brought into axial alignment to foster effective coolant circulation therethrough.

Abstract: A core for a nuclear reactor of high specific power output is provided with an arrangement of fuel bundles in which at least some bundles incorporate tubular pencil elements mounted between end plates, having coolant flow restricting means secured to the bundle to control the balance of coolant flow between external flow along the outsides of the pencils and internal coolant flow within the hollow pencils.