Low temperature two-step sintering of nanocrystalline UO₂

Low temperature two-step sintering was used for the first time to obtain sintered pellets of UO₂ comprising uniform microstructure. Nanocrystalline UO₂ powders derived by citrate gel-combustion from mixtures containing different quantities of the fuel (citric acid) and the oxidant (nitrate ions) were used. Powders were cold compacted (60–335 MPa) to get green pellets with a density in the range of 45–58% T. D without the addition of binder or lubricant. These pellets were sintered in two steps under reducing atmosphere (Ar + 8% H₂) at 1073 K (2 h) as well as 1673 K (4 h). The axial and radial shrinkage, sintered density and porosity of these sintered pellets were studied. For the first time the "two-step" sintering has been used for obtaining high density UO₂. A density as high as 97.8% T. D could be obtained at 1673 K. The optimum compaction pressure was found to be 244 MPa. Scanning electron microscopy of these pellets revealed uniform grain growth and an insignificantly low closed porosity in the product. Shrinkage anisotropy factor (α) was found to vary linearly with the compaction pressure. For the first time it has been demonstrated that two-step sintering helps avert uncontrolled grain growth due to "run-away" sintering, in nanocrystalline UO₂.