Patent Number: 046631178
Section: summary

BACKGROUND OF THE INVENTION This invention relates to nuclear reactor fuel assemblies and is concerned mainly with multi-pin fuel assemblies with the pins contained within a hexagonal tubular wrapper and stabilised by a series of spaced grids. Such assemblies find one use in liquid metal cooled fast reactors. Conventionally, assemblies having a hexagonal wrapper do not have a fuel pin at the corners of the hexagonal shape. This is brought about as either cooling is difficult at this locality or the locality is required for structural members such as corner posts to hold the grids apart (see for example GB Pat. No. 2,079,033B, item 17 FIG. 2). There is a preference to have a fuel pin at this locality in order to maximise power output and the present invention aims to meet this preference. Further, it is now being observed that a wrapper which serves only a shrouding function with a minimum load-bearing function can have advantages as it allows a wider choice of materials for the wrapper and in particular allows the choice of materials which do not retain good structural characteristics under irradiation. Such materials, for example those that embrittle seriously with irradiation, would previously have been rejected for use as they would not tolerate welding or drilling for joining with other members (such as grids). It is known, in water-cooled reactors, to employ longitudinal support members for the fuel pin spacing members (see British Pat. No. 1137621) but these are located at the corners of the wrapper or outer casing and would be unacceptable in the kind of fuel assembly with which the present invention is concerned because coolant flow at the corners of a hexagonal array of fuel pins would be unduly restricted. FEATURES AND ASPECTS OF THE INVENTION The present invention seeks to provide a fuel assembly of the kind initially referred to with an improved pin support structure which permits the introduction of corner pins and enables the use of materials for the wrapper which would otherwise be problematic in situations requiring welding or mechanical fastening of grids to the wrapper at locations where neutron flux levels are high. According to the present invention there is provided a nuclear reactor fuel assembly of the kind comprising multi-pins located in a wrapper of hexagonal cross-section and stabilised by a series of spaced grids of cellular structure, characterised in that a plurality of elements extend lengthwise of the wrapper at locations between the vertices of the wrapper and are secured to the grids to form with the grids a cage structure within the wrapper. This arrangement involves no large cross-sectional area intrusions into the areas occupied by the grids and hence it becomes possible to have the full areas of the grids uniformly penetrated by fuel pins. In particular, the location of the elements between the vertices allows the use of corner pins without impeding coolant flow in the corner regions. The cage construction allows design creations in which access of welding electrodes for welding grid parts to cage is not problematical. The cage imposes only a very small and acceptable resistance to cooled flow through the assembly and very low turbulence. Weld designs can be chosen to have a high tolerance to tensile loads. The wrapper can be of a material having a low load bearing capacity. The invention is of particular value with assemblies having grids with unit cells at the peripheries of the grid as these cells can be fabricated to have integral tabs or exposed other integral parts for welding to the upturned parts of the cage.