Patent Number: 041982720
Section: summary

BACKGROUND OF THE INVENTION This invention relates to fuel sub-assemblies for liquid metal cooled fast breeder nuclear reactors. A fuel assembly for a liquid metal cooled fast breeder nuclear reactor comprises a bundle of spaced fuel pins within a tubular wrapper or sleeve through which liquid metal coolant can be flowed in heat exchange with the fuel pins. The wrapper is extended at one end by a tubular neutron shield of massive steel and the other end, hereinafter referred to as the lower end, has a spike extension whereby the sub-assembly can be located by plugging into a support structure and arranged with other sub-assemblies to upstand in side-by-side array to form a fuel assembly. Hitherto, it has been proposed, see for example U.S. Pat. No. 3,383,287, to arrange the fuel assembly of a liquid metal cooled fast breeder nuclear reactor in a plurality of groups of sub-assemblies and for the sub-assemblies of each group to be biassed inwardly towards the centre of the group so that the sub-assemblies of the group lean on each other or on a central support member. Whilst such a system has the advantage of elimination of vibrational movement of the sub-assemblies due to coolant flow it is now recommended that the fuel assembly of a liquid metal cooled fast breeder nuclear reactor should be restrained as a whole from the periphery, that is, the fuel sub-assemblies of the fuel assembly should be centripetally urged together in parallel vertical array. For a fuel assembly of this kind some provision should be made to allow lateral displacement of the wrappers of the sub-assemblies relative to the spike extension in order to relieve stresses throughout the fuel assembly. Accordingly it is an object of the present invention to provide a construction of fuel sub-assembly which has sufficient flexibility to enable some lateral movement of the wrapper relative to the spike extension. SUMMARY OF THE INVENTION According to the present invention in a fuel sub-assembly for a liquid metal cooled fast breeder nuclear reactor comprising a bundle of spaced fuel pins within a tubular wrapper having a spike extension for plugging into fuel assembly support structure the wrapper is pivotally connected to the spike extension. The invention also resides in a liquid metal cooled fast breeder nuclear reactor having a fuel assembly comprising a plurality of closely packed fuel sub-assemblies upstanding in side-by-side array on a support structure and wherein each sub-assembly comprises a bundle of spaced fuel pins within a tubular wrapper having a spike extension for plugging into the support structure and wherein the wrapper is pivotally connected to the spike extension. The invention provides that lateral displacement of individual fuel pin containing wrappers to accommodate dimensional changes within the fuel assembly is effected by movement of each wrapper relative to its spike extension. The spike extension of a fuel sub-assembly may conveniently comprise a sleeve providing axially spaced spigot surfaces for engaging vertically spaced sockets in a fuel assembly support structure the wrapper being tied to the sleeve by a resilient tie member extending through the sleeve and having ends encastered, that is fixed to the wrapper and the sleeve. In a preferred construcion of fuel sub-assembly the end of the tubular wrapper which is connected to the sleeve is arranged to enclose an end region of the sleeve with annular clearance therebetween and the sleeve carries resilient split rings for sealing against liquid metal coolant flow between the wrapper and the sleeve.