Influence of different external lubricants and their deposition mode on green nuclear fuel pellets during cold compaction
Résumé
In the framework of manufacturing future nuclear fuels, some solutions set out to optimize the nuclear powder compaction process. Lubrication is used in the fabrication process to reduce friction between the actinide oxide grains and press tools. A higher plutonium content and an isotopic composition with more $^{238}$Pu for the purpose of plutonium multi-recycling would increase the level of radioactivity and the temperature in future UO$_2$ + PuO$_2$ powder mixtures. This temperature could be detrimental to maintaining the lubricant-s properties within the mixture.A solution to this problem could be to replace the internal lubricant by a lubricant deposit on the die wall (external lubrication) during compaction process. This lubrication technique, when combined with internal lubrication, is known to enhance the mechanical strength and density of the pellets produced by powder compaction.This paper investigates the influence of this kind of lubricant deposition on the die wall for UO$_2$ 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 zinc stearate solid by lubricant powder pelletization with spraying. Results show that the tensile strength of green pellets is enhanced and surface defects are reduced when the lubricant is sprayed. Furthermore, the application of viscous oil on the die wall does not make it possible to manufacture UO$_2$ pellets, which is probably due to the tribological inefficiency of this form of lubricant under our conditions, while 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 acts on the wall friction, the friction index, the ejection force, and the characteristics of the final green pellets. The ejection force seems to be more sensitive with respect to assessing the performance of a lubricant when only external lubrication is used in nuclear powder compaction.