Die wall lubrication for UO2 pellets pressing: A case study

摘要

This paper investigates the tribological performance of different external lubricants on the die wall for UO2 powder pelletization without any admixed internal lubricant. Different lubricants and various ways of depositing lubricant on the die wall were investigated through different parameters during powder compaction and pellets characterization. We have thus compared depositing solid zinc stearate (StZn) lubricant by two ways (StZn powder pelletization vs. StZn spray). Results show that the tensile strength of green pellets (compacts before sintering process) is enhanced and surface defects are reduced when the lubricant is sprayed. Furthermore, the application of silicon oil on the die wall does not make it possible to manufacture UO2 pellets, which is probably due to its tribological inefficiency under pressing conditions considered in this work. By contrast, an industrial grease makes it possible to obtain green pellets with good properties and fewer surface defects. The lubrication mechanism changes from one lubricant to another and influences the pressing parameters (wall friction, friction index, ejection force, etc.) and the characteristics of the final UO2 green pellets. The ejection force seems to be more sensitive for the lubricants performance assessment in case where only external lubrication is used for nuclear powder compaction.