GB1126323A - Improvements in or relating to high temperature gas cooled reactors - Google Patents

Abstract

1,126,323. Fuel elements. UNITED KING- DOM ATOMIC ENERGY AUTHORITY. 7 Jan., 1966 [8 Jan., 1965], No. 921/65. Heading G6C. A nuclear reactor fuel assembly comprises a number of columnar blocks of moderating material, each of the blocks containing axially extending channels, at least some of which contain nuclear fuel particles having fission product retaining outer layers. In one embodiment, as shown in Fig. 1, a graphite block 1 of hexagonal cross-section is penetrated by a number of holes 2 which receive fuelled bodies formed with a series of holes, at least one of these holes serving as a coolant flowpath and others containing fission product retaining fuel, e.g. coated nuclear fuel particles dispersed in moderator material. A first form of fuelled body 3a comprises an open ended graphite cylinder 11 providing a central coolant channel 12 and frirmed with an annular row of holes 13 containing nuclear fuel 14. A second similar form of fuelled body 3b comprises an open ended cylinder 15 which is, however, centralized with respect to, and spaced from, the wall of a hole 16 by extension ribs 15a. Several outer coolant channels 17 are thus provided as well as an inner coolant channel 18. In a second embodi- ment, as shown in Fig. 2A, an hexagonal graphite block 30 is formed with an array of holes 31, 32 of four different diameters: the holes 31 of the largest diameter serving as coolant channels C penetrate the block completely, whilst the holes 32 of smaller diameters are blind and retain nuclear fuel bodies 33. The latter are cylindrical regions of loose or consolidated fuel particles coated with fission product retaining material. A coolant channel C is located at the centre of each group of holes 32. In both embodiments, the fuel may be placed in the holes by firstly filling these with coated fuel particles and then depositing carbon in the interstices between the particles, e.g. from the gas phase. In an alternative arrangement (Fig. 28, not shown), the graphite block 30 is of circular cross-section and is enclosed by a tubular sleeve of hexagonal cross-section. In any of the described fuel arrangements, the bores of the coolant channels may be coated with pyrolytic carbon or SiC to reduce the rate at which the hot coolant gas passing through the channels corrodes the matrix material. By distributing the fuel in lumped segregated zones spaced apart in a block of moderator, a low enriched fuel (about 5% by wt. U-235) can be substituted for the more highly enriched fuel normally associated with reactors employing fuel in a coated particle form.