Some factors affecting densification and grain growth in the sintering of uranium dioxide – a brief review

The sintering of uranium dioxide, like many other oxide ceramics, is influenced by the starting particle size distribution, additives, physical homogeneity of the green body (with respect to density and porosity), the temperature-time profile in sintering and the composition of the sintering atmosphere (oxygen potential in the reducing atmosphere) etc. However, things can go wrong in manufacturing leading to de-sintering and other defects. The presence of agglomerates within the UO2 powder may be one source of defects. The agglomerate may be that of UO2 or that of an additive to UO2, such as U3O8, Gd2O3 or Nb2O5. There is a critical size of the additive agglomerates above which would leave residual stable porosity in the sintered ceramic. Some authors have attributed the difficulties in sintering to the inherent nature of the additives instead of to their agglomerated state in the powder before pressing. The often-cited Kirkendall effect causing the slowdown in sintering can be easily remedied by restricting the size of the agglomerate of the powder or the additive. Researchers have explained the sintering mechanism through point defects and diffusion ignoring the conventional role of additives in grain boundary pinning. A unified explanation of the mechanism of additive and atmosphere has been presented. The processes of densification and grain growth may unintentionally interfere with and inhibit each other instead of proceeding with synergy. Hence, these need to be suitably moderated through additives and atmosphere control for achieving high-density large grain sized and defect free sintered UO2 bodies.