Patent GB-1002244-A (GB)
Title: Improvements in or relating to fuel elements for nuclear reactors
IPC: G21C3/04
Filed: 1964-02-03
Abstract: 1,002,244. Nuclear fuel elements. GENERAL DYNAMICS CORPORATION. Feb. 3, 1964 [Feb. 12, 1963], No. 4529/64. Heading G6C. A self-purging fuel element for a gas-cooled nuclear reactor comprises an elongated coolant gas-permeable body 36 having one or more fuelcontaining chambers 34, a bore 32 extending through the body 36, and a restriction 38 within the bore 32, the chambers 34 being in fluid communication with the bore 32 adjacent the restriction 38 by means of at least one passageway 40. In the embodiment described, the reactor is helium cooled and the body 36 is of graphite having a helium permeability of 10<SP>-2</SP> to 10<SP>-3</SP> cm.2/sec., and a pore structure which resists the back diffusion of the fission products. As shown, the restriction 38 is the throat of a Venturi plug 42 formed with an enlarged end portion 52 which co-operates with a counterbore at the lower end of the body 36, the chambers 34 communicating with one another via an annular recess 44 formed on the upper surface of the end portion 52. A filter ring 54, e.g. a porous carbon ring, is disposed in the recess 44. A cylindrical graphite end piece 56 is connected to the plug 42, itself of graphite, and is bevelled so as to be received in a corresponding recess in a cylindrical cup 60 for mounting in the grid plate 62 of the reactor. An upper cylindrical graphite end-piece 64 having a vertically extending bore 66 is connected to the upper end of the body 36. In operation, helium is caused to pass upwardly through the grid plate 62, two-thirds passing along the exterior of the fuel elements and one-third passing upwardly through apertures 58, 63 in the end piece 56 and the cup 60, respectively, into the bore 46 of the plug 42. This latter portion of the coolant gas passes through the restriction 38 resulting in a pressure drop across the restriction, the minimum static pressure being at the point of maximum restriction. This minimum pressure results in a reduced pressure in the chambers 34 and coolant gas is induced to pass through the walls of the graphite body 36 into the chambers 34. In the greater length of the chambers 34 is packed a particulate fuel, e.g. UC 2  and/or ThC 2 , which may be uncoated or formed with either a single layer fission products retaining coating of, e.g. pyrolytic carbon, SiC or ZrC, or with a multiple layer fission products retaining coating comprising an inner layer of pyrolytic carbon, an intermediate layer of a metallic carbide, e.g. SiC or ZrC, and an outer layer of pyrolytic carbon. Alternatively, a mixture of carbon-coated UC 2  and ThC 2  particles dispersed in a graphite matrix may be used. Directly below the fuel particles is a layer of graphite particles which acts as a moderator and below this is a layer of a suitable trapping reagent for the fission products scavenged from the fuel particles by the coolant gas. A suitable reagent is a metalcoated charcoal, such as silver-coated charcoal. This removes all fission products except the noble gases which are removed by external trapping systems. Instead of using the Venturi plug 42, the pressure differential may be produced by an apertured plate, baffle, or other restricted passage.