Experimental approach of UO 2 compaction and analytical modelling of pellet diametric deformations

Abstract

UO₂ powder uniaxial compaction into cylindrical die was studied with a mechanical stress tester Instron, which is able to measure continuously the increase in density, the applied stress and the transmitted stress into both axial and radial direction. For this study, the authors used a spray dried UO₂ powder obtained by a wet process, and pressing was performed by single acting compaction until 600 MPa. The relationship between compact density and applied pressure was analysed successively using Niesz's equation between 10 and 100 MPa and Heckel's equation between 100 and 600 MPa. Observations and porosity measurements on green compacts at various degrees of densification permitted to identify which powder particles act on the successive densification stages, previously identified as regimes of Niesz and Heckel for densification. Then, using Thompson's and Heckel's equations, the authors calculated the density distribution into the compact under pressure. Adding analytical equations to calculate springback and shrinkage in function of density, simulated the compact ejection and sintering stages were simulated. This succession of four analytical equations gives the authors a good model of UO₂ pellets deformations obtained after a uniaxial powder compaction and sintering. Validation of calculated pellets deformation was performed, measuring diameters at different heights on pellets after sintering for 4 h at 1600°C under flowing Ar/H₂ humidified.