GB1421987A - Fuel element for high temperature reactors - Google Patents

Abstract

1421987 Fuel elements NUKEM GmbH 16 Feb 1973 [25 Feb 1972] 7751/73 Heading G6C A fuel element for a high temperature reactor comprises a graphite block formed with bores within which fuel inserts fit without intervening gaps. The fuel inserts each consist of coated fuel particles within a graphite matrix. They are produced by pressing from the coated fuel particles and a matrix powder mix and, in one method, the green pressings thus obtained are introduced into the bores in the graphite block. During the subsequent coking and heat-treatment operations, these inserts expand and fit tightly without gaps into the bores. The composition of the pressing powder is such that the resultant matrix has as high a thermal conductivity as the graphite block and that no gap is formed between the insert and the block during reactor operation. This is achieved by virtue of the fact that, from the beginning of fuel exposure onwards, the matrix expands more than the block, contracts less than the block or expands while the block contracts, with the result that at no time during operation of the reactor is a gap formed. The matrix consists of natural graphite alone or with at most 15%, preferably less than 10%, by weight of binder. With such a binder content, expansion of the matrix occurs on coking. In a further embodiment, the graphite matrix also contains up to 25% by weight electrographite and up to 25% by weight binder (the remainder of the matrix being natural graphite). The binder used for the matrix preferably consists of a readily graphitized material such as coal-tar pitch. In an alternative method, the fuel inserts are coked and calcined on their own and only thereafter introduced into the block. The matrix in this case consists of natural graphite with less than 5% by weight of binder. The fuel inserts can be additionally provided with a thin fuel-free shell. Because the matrix is selected to undergo greater thermal expansion than the structural graphite, the fuel inserts fit tightly in the bores without any gaps when the fuel element is heated to its working temperature in the reactor. Any stresses in the matrix are quickly dissipated by creeping so that they never become critical.