Patent Number: 050664530
Section: claims

1. In a nuclear fuel assembly for a fast nuclear reactor arranged to be cooled by liquid metal, the assembly comprising an elongate metal wrapper, and a plurality of nuclear fuel pins arranged lengthwise therein at a position intermediate the ends of the wrapper, and one of said ends comprising ann inlet for the flow of liquid metal through the wrapper, wherein the improvement comprises a variable flow restrictor for the liquid metal located at the inlet, the variable flow restrictor defining a plurality of ducts for the liquid metal aligned in parallel relationship longitudinally with respect tot he wrapper, means for plugging at least some of the ducts, the plugging means having an array of plugging members locatable in some of the ducts, the plugging members being of different lengths so that longitudinal displacement of the plugging means progressively opens or closes some of the ducts, responsive means disposed lengthwise in the wrapper and responsive to neutron radiation in a manner to cause a predetermined neutron-induced growth of the responsive means, and connecting means for connecting the responsive means to the plugging means so as to effect said longitudinal displacement of the plugging means in response to neutron-induced growth of the responsive means. 2. In a nuclear fuel assembly as claimed in claim 1 wherein at least some of the plugging members terminate in tongue portions thereof to maintain alignment of the plugging means relative to the variable flow restrictor as the plugging means is progressively withdrawn therefrom. 3. In a nuclear fuel assembly as claimed in claim 1, wherein the connecting means includes spring means for biasing the plugging means towards the variable flow restrictor. 4. Apparatus as claimed in claim 1, in which the neutron radiation responsive means comprises at least one elongate member supportable by the fuel assembly and arranged to support an elongate portion of the connecting means, the elongate member comprising at least one material having a greater neutron-induced growth property than that of the elongate portion so as to cause longitudinal displacement of the connecting means as the elongate member grows under neutron irradiation. 5. Apparatus as claimed in claim 4, in which the elongate member comprises a rod member extendable within a said elongate portion in tubular form. 6. Apparatus as claimed in claim 5, in which the rod member comprises a plurality of rod portions disposed one upon the other, at least some of the rod portions having different neutron-induced growth properties than other said rod portions so as to produce a required longitudinal growth of the rod member under the neutron radiation. 7. Apparatus as claimed inn claim 3, in which the rod member is arranged so that an initial longitudinal neutron-induced growth is necessary before the rod member supports the elongate portion. 8. Apparatus as claimed inn claim 3, in which some of the rod portions are mechanically treated to introduce a delay in the onset of growth in the rod portions during neutron irradiation. 9. Apparatus as claimed in claim 4, in which the elongate member is in tubular form and the elongate portion comprises a rod element extending within said tubular elongate member.