Impact of fine particles on rheological properties of uranium dioxide powders
Résumé
In industry, the use of powders is very common. Understanding granular flow in silos, hoppers or
pneumatic conduits is necessary. This study investigates the characterization of the rheological
properties of uranium oxide powders in the context of a powder metallurgy process for nuclear fuel
pellets manufacturing. The flow and arrangement properties of these powders must be compatible
with a reproducible feed of press moulds. This may require a modification of the physical
characteristics of the powders. In particular, particle size is known to have a great impact on the
rheological properties and a significant effect of fine particles (less than 100 µm) is suspected. In this
study, we have quantified this influence of particle size distribution on rheological properties of UO2
powders, especially the influence of the amount of fine particles. Two complementary approaches
have been used. The first approach is to characterize the powder in a static state: the density,
compressibility and shear measurements make it possible to understand the behaviour of the
powder when it is moved from static state to dynamic state. The second approach is to characterize
the behaviour of the powder in the dynamic state. It allows understanding the flow capacity of a
powder in the process. To answer the problem of reproducible filling of the press moulds, it is
essential to characterize the two regimes, that is to say the flow of the powder from a hopper or a
shoe, then the behaviour in the dynamic state. The amount of fine particles has been more especially
varied because the cohesive forces are considered predominant with respect to gravitational forces
for particles smaller than 100 μm. A threshold of the rate of fines has been demonstrated in both
types of measurements. Beyond this threshold, the behaviour of the powder is approximatively the
same.
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