Review of the metallurgical studies carried out at the CEA on the in-service behaviour of the fuel elements of the gas graphite type

After a brief review of the three types of fuel elements used in the EdF reactors and a description of working conditions, the principal stages of five metallurgical investigations concerning these elements are presented. In each of these studies, success was possible only by combining research into basic mechanisms with an effort to find quickly the most suitable solution for the operator. The following are described in turn: (a) the reduction of growth, creep, and swelling of uranium to acceptable levels by controlling the mechanisms of these three elementary modes of distortion and by checking any interaction; (b) how, in the case of annular element of the Bugey 1 reactor, a metallurgical bond between fuel and cans has considerably limited the possible consequences of a burst can; (c) the main lines of investigations into the release of fission gases usually occluded in the uranium, but which under certain conditions can produce an interruption in thermal continuity between can and fuel; (d) the problems raised by the diffusion of plutonium into Mg-Zr alloy cans and the solutions which have been found; (e) the mechanism of rupture of the lugs positioning the fuel elements in their graphite sleeves and the slight modification in the design of the fuel element which prevents fracture. The conclusion emphasizes the advantage obtained by the operator from the multiple links established between himself, the fuel design team, the manufacturers, and the test services. (author)