Finite element modelling of the pressing of nuclear oxide powders to predict the shape of LWR fuel pellets after die compaction and sintering

Die pressing and sintering processes are extensively used in the fabrication of LWR UO2 and MOX fuels. However, perfect cylindrical sintered pellets cannot be directly obtained by the current processes because slight heterogeneities of density are inevitably present in the green body as a result of frictional forces between powder and die. A cooperative program between COGEMA and CEA has been undertaken to improve the forming process in order to decrease diametrical scattering of sintered product. In this context, Finite Element Modeling (FEM) of the cold compaction stage has been used to simulate the die pressing stage of nuclear oxide powders and to predict the green density distributions. The shape of sintered pellet resulting from a non uniform shrinkage of the green compact can also be calculated by this numerical code developed by the CEA and called PreCAD. The simulation shows that the axial repartition of frictional forces may be changed during die pressing by a modification of the sequence of punches displacement. Thus, it has been established that the pressing cycle may have an influence on the shape of sintered pellet through the axial repartition of green density. The code PreCAD. has consequently been used to optimize the sequence in which the tools move during pressing, in order to improve the cylindricity of pellets. Indeed, the displacement of press tools, i.e. die and upper punches, can be separately fitted on current presses. The main trends, confirmed through experiments, are finally discussed. (author)