Patent Number: 041815718
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT The bracing grid shown in FIG. 1 has a right hexagonal boundary and comprises 325 unit cells 1 arranged in honeycomb pattern. The cells are of generally hexagonal shape all except the outer row of cells being right hexagonal. The cell at each corner of the complex is adapted to receive a corner post 6 of a sub-assembly to be described hereinafter. Each unit cell 1 is formed from strip material the edge-to-edge ends being spaced apart. In the outer row of the unit cells designated 1b in FIG. 1 the edge-to-edge ends of the strips occur in the middle of one side as shown in FIG. 2 whilst the inner unit cells designated 1a in FIG. 1 have the edge-to-edge ends at a corner of the hexagonal cell as shown in FIG. 3. Three alternate sides of each cell have a rectangular window 3 whilst the remaining sides each have an elongate embossment or dimple forming a guide pad for a fuel pin. In an alternative construction the windows in the outermost sides of the outermost cells are omitted. There is a pair of smaller embossments 5 disposed one beyond each end of each of the elongate embossments to form linear groups extending parallel to the longitudinal axis of the cells. The smaller embossments provide additional bracing pads or backstops for a fuel pin in the event that bowing of the pin occurs during irradiation of the fuel sub-assembly. Each pair of abutting sides of the cells are secured together by a pair of spot welds disposed in opposed end regions of the sides. The grid has across flats dimension 134.6 mm the cells 1 being formed from stainless steel strip 25.5 mm wide by 0.20 mm thick with windows 12.5 mm long and extending laterally acorss the full width of the side. The embossments 4 define a cell diameter nominally of 5.8 mm whilst the cell diameter bounded by the embossments or pads 5 is 6.1 mm. The fuel sub-assembly shown in FIG. 3 comprises a central fuel section 11, a lower end locating section 12 and an upper end neutron shielding section 13. The fuel section 11 comprises a bundle of spaced elongate fuel pins 14 enclosed within a tubular wrapper 15. The pins are supported at their lower ends by a grid 16 and are braced intermediate their lengths by cellular grids 1 of honeycomb form. The grids 1 are disposed at intervals along the wrapper being secured thereto by engagement of the corner cells with notched posts 6 (FIG. 1) secured in the corners of the wrapper. The lower end locating section comprises a spike 18 for engaging a socket in a fuel assembly support structure 37 and has apertures 19 through which coolant can flow from within the diagrid. A conical mesh filter 20 and gag means 21 are provided for the coolant between the spike 18 and the bundle of fuel pins 14. The gag means 21 comprises a plurality of apertured plates 22 spaced apart by woven wire mesh discs 23. The upper section 13 is of massive steel and comprises a massive steel tubular member 24 which has an internal lip 25 for engagement by lifting means. On assembly the bundle of fuel pins is threaded through successive grids 17 the grids being sufficiently compliant to enable the pins to deflect the unit cell structures to allow the pins to penetrate successive unit cells without causing scoring of the pins. After irradiation the fuel sub-assembly may be readily dismantled for reprocessing by withdrawing the bundle of fuel pins from the bracing grids and wrapper 15 combination thereby reducing the contaminated waste to be processed. The process of withdrawal of the pins is facilitated because of the compliancy of the grid and the reduced bowing of the fuel pins. By constructing the grids of unit cells manufacture is facilitated because the strips can be preformed by a jig to provide a multiplicity of identical cells which can then be assembled in honeycomb array on a second jig to maintain the cells in accurate geometrical relationship and they can be finally joined together by edge welds applied easily from each face of the grid. A plurality of sub-assemblies is used to form a fuel assembly 31 shown in the reactor construction of FIG. 5. The fuel assembly 31 forming the reactor core is submerged in a pool 32 of liquid sodium coolant in a primary vessel 33. The primary vessel is suspended from the roof of a containment vault 34 and there is provided a plurality of coolant pumps 35 and heat exchangers 36 only one each of the pumps and heat exchangers being shown. The fuel assembly 31 mounted on a structure 37 is housed with the heat exchangers in a core tank 38 whilst the pumps 35, which deliver coolant to the diagrid, are disposed outside of the core tank. The core or fuel assembly 31 comprises a plurality of the described fuel sub-assemblies which upstand from the diagrid 37 in closely spaced side-by-side array. Control rods 39 and instrumentation 40 penetrate the roof of the vault of the core tank. In operation of the nuclear reactor coolant is flowed from the pump 35 to the fuel assembly by way of the diagrid 37 which distributes the coolant flow throughout the fuel assembly. Flow is upwardly through the fuel sub-assemblies by way of the tubular wrappers 25 and in heat exchange with the fuel pins. Flow is thence from the upper region of the core tank 38 back to the outer region of the pool by way of the heat exchangers 36.