GB1084999A - Improvements in or relating to nuclear reactor fuel elements - Google Patents

Abstract

1,084,999. Reactors. S. WRIGLEY. April 4, 1966 [April 5, 1965], No. 14420/65. Heading G6C. A spherical fuel element comprises a mass of clad nuclear fuel supported within a spherical shell having apertures to allow coolant free access to the clad fuel. The mass of nuclear fuel, which may be spherical and of metallic or ceramic form clad, e.g. in aluminium, zirconium, stainless steel or ceramic, is supported centrally within the spherical shell which may be of graphite, beryllium oxide, silicon carbide or zirconium. Voidage to accommodate fission product gases can be achieved either by using a porous low neutron absorption material as the kernel of the fuel mass or by using a hollow fuel mass. In one embodiment, a solid spherical fuel mass has cladding applied, e.g. by vacuum metallizing process and/or an electroplating process, and is supported in a shell comprising two hemispheres which lock together and which are provided with projections which bear against the clad fuel. In an alternative embodiment, the spherical fuel mass is hollow and has a preformed metal cladding from which radially extend three support wires which are clamped between the two hemispheres forming the shell. Such fuel elements may be used in a boiling heavy water reactor as shown in Fig. 3, the core 1 of which is located within a concrete structure 2 and sub-divided into five zones, a central zone 3 and four radial zones 4. The vertical sides 5 of the core 1 are of unperforated material to assist natural convective water flow, but the bottom 6, top 7 and internal core divisions 8 are of perforated material. Each zone is fitted with a charge mechanism 9 located on a small diameter tower 10 attached to the structure 2 and with a discharge pipe 11. The charge and discharge mechanisms each comprise a plunger 30 having a transverse bore 31 which accommodates one or more fuel elements and to unload fuel elements the bore 31 of a plunger 30 is brought below an outlet from the pressurized core 1 so that fuel elements fall into it. The plunger 30 is then moved to bring the bore 31 into alignment with a discharge orifice through which the fuel elements pass through pipes 11 to a skip 13. When sufficient fuel is in the skip 13 it is hoisted up a tube 15 to the used fuel store 17. All the fuel handling takes place under heavy water and the fuel is always adequately cooled. The fuel passes from the store 17 through one or more grading stations 18 to the graded fuel store 19. New fuel can be introduced by means of an additional station (not shown). From the fuel stations, the fuel is selected according to the fuel cycle and is passed to five stations 20 which feed the five core zones by means of the charging mechanisms 9. Flux measurements can be carried out readily by passing small in-pile ion chambers through small bore tubes of low neutron absorbing material passing through the core. The design of the spherical fuel elements also permits the use of light water/steam as the coolant, the resultant reactor being an epithermal reactor. Neutron absorber in the form of spherical elements of similar construction to the fuel elements but with a neutron absorbing mass in place of the fuel mass can be added to the fuel elements before loading into the core, and the large negative temperature coefficient makes it self-correcting. However, the large negative temperature coefficient dictates a large positive reactivity in the cold condition, and conventional methods of control are needed for start up and for large changes of power.