+) Work performed within the association in the field of fast reactors between the European Atomi~Energy Community and Gesellschaft für Kernforschung mbH., Karlsruhe -1 -SUMHARY The fabrication of vibratory-compacted U0 2 -Pu0 2 fuel bearing pins is described under the aspect of test irradiations under the development program of a Fast Breeder Reactor. Such a fabrication always will start with a design which is based on theoretical studies and calculation of parameters needed. Thus astart for a

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... fuel theory had to be done, the first results of it are in use already. The procedure is such as to put in the aS calculated parameters of specific power, diameter, fuel and cladding properties in formulae and thus find the maximum available burnup and the density of the fuel pine After that the final design is made. Then fuel and cladding selection is done and the materials are tested. Special items of the fuel control are homogeneity of the mechanically mixed U0 2 -Pu0 2 , the Pu-analysis and the density control. Great efforts have been done on the tubing control. Here a long list of points has to be checked in a quality test. Because of its special problems the vibratory compaction method is being described though both it and the pellet line are generally followed in the development program. These special problems are the fuel density, contamination and welding. Here the developm~nt is proceeding. THE THEORETICAL DESIGN BASIS The target of the design is to reach a maximum burnup of about 100 000 MWd/t or in other words an operation time of 20 000 hours maximum. So it is important to know the behaviour of the fuel pin over that time. For the analysis a simple model is used: Two limitations are given for the fuel and the cladding. First the fuel-cladding contacts should be reduced to aminimum. Then the fuel operation time will come to an end after the most critical fuel cross section has been completely filled by fission-induced fuel swelling. Second the maximum inside pressure-induced plastic diametral creep deformation must not exceed 1%. ccording to the temperature distribution the fuel cross-section is divided into a plastic zone V pl ' the columnar grain region, with -2 ,.