Patent Number: 040574687
Section: summary

BACKGROUND OF THE INVENTION This invention relates to liquid metal cooled fast breeder nuclear reactors and to fuel element sub-assemblies therefor. In fast breeder nuclear reactors it is common to divide the fuel assembly into replaceable fuel element sub-assemblies each comprising a bundle of fuel pins contained within a wrapper or shroud through which liquid metal is flowed in heat exchange with the fuel pins. In one known construction of nuclear reactor the fuel element sub-assemblies are upstanding from a carrier associated with a core supporting diagrid and are arranged in groups and urged into leaning abutment with a central support member for each group of sub-assemblies. The upper regions of the sub-assemblies contain massive steel shielding to form, in combination, an upper shield for the core. Thus with the weight mass being disposed at the upper free end of each sub-assembly there is a tendency for vibration to be set up by coolant flow through the sub-assembly. It is expected that in one envisaged construction of reactor core the amplitude of vibration of the tip of a sub-assembly could be 1.0 mm and, apart from the deleterious effect on the mechanical reliability of the reactor core, such amplitude of vibration could create excessive reactivity noise, that is, it could create slight, variable reactivity changes. SUMMARY OF THE INVENTION According to the invention in a liquid metal cooled fast breeder nuclear reactor having a core comprising a plurality of elongate fuel element sub-assemblies upstanding from core support means, each sub-assembly has inertia damping means at a free upper end region. In a preferred construction of fuel element sub-assembly for a nuclear reactor according to the invention the inertia damping means comprises upper and lower sleeves mounted on a tubular spine, the lower sleeve being rigidly attached to the spine and secured atop a fuel containing tubular wrapper of the sub-assembly whilst the upper sleeve is resiliently attached to the lower sleeve and radially spaced from the spine there being duct means for enabling liquid metal to flood the radial spacing of the upper sleeve and spine when the sub-assembly is submerged in reactor coolant. In use, the device operates on a tuned inertia damping principle using liquid metal as the damping medium; when the upper sleeve vibrates the liquid metal is forced from one side of the spacing to the other and its resistance to this motion provides a damping force. For the optimum damping effect the mass of the upper sleeve needs to be as large as possible in relation to the mass of the lower region of the sub-assembly and from this aspect it is convenient to comprise the upper sleeve of massive steel neutron shielding. Preferably, the resilient attachment of the upper and lower sleeves comprises a bellows unit which is easily constructed and unlikely to break into small pieces should it fail through fatigue.