Research of Medium Model Floating Pressing Technology in Fast Reactor MOX Fuel

Green pellets pressing is critical step in the MOX fuel production which decides density, strength and microstructure of green pellets and pellets. MOX fuel is pressed by medium mode floating pressing technology for the first time in China, in order to master the medium mode floating pressing technology systematically, Al2O3was used as the simulation material to study the effect of pressing pressure, pressing time and zinc stearate content on the green billet and pellets. The results show that the density, axial strength, radial strength of green pellets increase significantly with the increase of pressing pressure, and the axial strength is much larger than the radial strength. With the increase of pressing pressure, the arrangement and deformation of particles become more sufficient. As a result, the porosity between particles reduces, and the density of green pellets increases gradually. When the pressing pressure increases, the displacement and deformation of the powder particles become more significant which makes the bonding between the particles closer. The strength of the green pellets is obviously improved. The pressing curve is in good agreement with the powder pressing curve and Huang Peiyun's double logarithmic equation. The results show that Al2O3is an inelastic compression in the pressing process, and there are work hardening and friction in the medium mode. The pellet density increases with the increase of pressing pressure and sintering temperature. The pellet density is determined by the diffusion of the sintering process. The diffusion coefficients of volume diffusion and surface diffusion arc proportional to the sintering temperature. Therefore, with the increase of sintering temperature, the diffusion rate and the pellet density increase, but pellets strength is independent of green pellets strength. The microstructure of pellets has clear grain boundaries and smaller voids which indicates that pellets have high density in sintering temperature 1 650 °C. The high pressing pressure causes cracks on the surface of the green pellets and pellets. The pressing time and zinc stearate content have no obvious effect on the green pellet density or strength. With the extension of the pressing time, the more fully the pressure is transferred in the process of billet formation, the more sufficient time the gas between the powders is discharged from the billet, and the green pellet density tends to increase, so prolonging the pressing time is beneficial to the blank pressing process. The effect of pressing time on density is small, which may be due to the too short pressing time. In a word, the green pellet density and strength are related to medium mold floating pressing pressure, and the pellet density is related to the sintering temperature and pressing pressure. © 2023 Atomic Energy Press. All rights reserved.