  The dry conversion process for making UO2 nuclear fuel pellets consists of
two steps: hydrolysis of UF6 to UO2F2, followed by reducing pyrohydrolysis to
UO2 in a rotary kiln. The physical characteristics (morphology, particle size
distribution) of the powder obtained at the kiln end determine the final
properties (sinterability, flow-ability, green strength). We developed a
mathematical model describing the morphological evolution of the powder in the
rotary kiln, which enables us to predict morphological characteristics of UO2
powder as functions of the processing conditions. Firstly, the powder flow in
the kiln was modelled, including the exchanges between a dense phase (powder
bed) and an airborne phase (particles showering down). An original feature of
this model is to consider the effect of lifters for calculating the dynamic
variables. Secondly, the phenomena responsible for the changes in morphology
and grain size were identified and modelled. A population of fractal
agglomerates was considered, whose number and size change due to Brownian and
sedimentation agglomeration, sintering, breaking up, and chemical reactions.
The model is based on population balances and the particle size distribution is
divided into sections. Results of both dynamic and morphological calculations
are compared to available measurements. Lastly, the influence of the different
mechanisms of morphological evolution on the final size distribution is
analysed.

Materials Science