Patent GB-1084463-A (GB)
Title: Nuclear fuel assembly
Inventor: ASHCROFT DAVID JOHN
IPC: G08B5/00
Filed: 1964-02-27
Abstract: 1,084,463. Fuel elements. UNITED KING- DOM ATOMIC ENERGY AUTHORITY. Feb. 15, 1965 [Feb. 27, 1964], No. 8294/64. Heading G6C. A nuclear fuel assembly comprises a cluster of fuel pins and control pins spaced laterally by at least one cellular spacer grid which is axially located within the fuel assembly by engagement with at least one control pin. An integral pressurized water reactor incorporating fuel assemblies according to the invention includes a core in the form of a cylindrical frame 12 housing a plurality of elongate fuel assemblies 17 of hexagonal section. The frame 12 is watertight apart from circular apertures 15 and slots 16 in the end plates 11, 14. The fuel assemblies 17 are housed within hexagonal tubular shrouds 18 which at their lower ends have caps 23, each with a hollow stub 24 carrying external piston rings 25 received in one of the bottom plate apertures 15 with which it makes a watertight joint. At the upper end of each shroud 18 is a cap 26 with a hollow stub 27 which carries an interrupted external thread. A nut 28 with a co-operating interrupted internal thread is screwed on to the stub 27 which projects through a top plate aperture 15. The fuel assemblies 17 are cooled by water flowing through the shrouds 18 from the hollow stubs 24 to the hollow stubs 27. Control blades 31 of neutron absorbing material are suspended from rods 32 so as to be movable between the fuel assemblies 17, entering the core through the top plate slots 16. Each rod 32 carries three blades 31 which form a Y-section control member. These control members are cooled by a flow of water through the slots 16. In a typical pin distribution (Fig. 3, not shown), the fuel pins are arranged on a triangular lattice with two burnable poison pins situated at each corner of the hexagonal section. These poison pins are free to slide in a number of spacer grids 22 disposed along the fuel assembly, as are the fuel pins. In addition, there is a fixed burnable poison pin adjacent each side of the hexagonal section. The fuel pins comprise uranium oxide clad in stainless steel and the control pins burnable poison in the form of zirconium boride clad in a zirconium alloy. The spacer grids 22 each comprise a plurality of stainless steel strips secured together to define hexagonal cells 33 penetrated by the fuel and poison pins with resilient ring spacers 34 carried by the grids engaging the pins and centralizing them in the cells. Marginal spacers are secured to the shroud 18 to replace the ring spacers at the periphery of the assembly. Each fixed poison pin 21c (Fig. 4) penetrates one cell 33 of the grid and is positioned by three ring spacers 34 (of which one is shown in broken outline). A ferrule 35 is welded to the cladding 36 of the pin 21c and a disc 37 welded to one end of the ferrule. When the grid is positioned so that it bears against the disc 37, a second disc 38 is slipped over the pin 21c and welded to the ferrule 35. The ferrule 35, discs 37, 38 and cladding 36 are all of zirconium alloy; no welding to the stainless steel grid is required. In order to space adjacent fuel assemblies apart laterally, ribs of zirconium alloy are welded to selected corners of the shrouds 18 which are also of zirconium alloy, these being alternate corners in the case of the fuel assemblies other than those at the periphery of the core. Engagement of the ribs of adjacent fuel assemblies supplements the locating action of the fuel assembly end fittings and also serves to counter rotation of the fuel assemblies and to maintain the water-filled interspaces between adjacent fuel assemblies. The blades 31 of the Y-section control members are slidable in these interspaces and alignment of these blades is assisted by bearing pads of zirconium alloy in the form of rivets located in stainless steel tips to the blades (Fig. 3, not shown). These pads bear against guide strips provided by extensions of the ribs where required. A further function of the ribs is to restrict lateral fluid flow from the interspaces, particularly lateral fluid flow into a core peripheral reflector.