Revisiting the sintering of uranium dioxide

The sintering of uranium dioxide was investigated for humidified Ar/5 vol% H₂ gas mixtures up to 1700 °C. Three relative humidity levels were used: 100, 1200 and 7500 vpm. The sintering map has been built. It is unique and independent of the thermo-atmospheric conditions of the sintering cycle. All samples have a $O/U$ ratio of around 2.00. Whatever the value of the relative humidity level of the sintering gas mixture, it is proposed that densification is controlled by volume diffusion of the uranium cations and that grain growth is controlled by the grain boundaries. An apparent activation energy for densification has been calculated. The parameters of a shortened sintering cycle are finally proposed. They allow a reduction of approximately 50 % in the cold-to-cold duration, compared to what is currently done in the context of the manufacture of uranium dioxide fuel pellets for PWR reactors.