FIB-SEM advanced characterization of uranium based powders: Contribution to understanding gas-solid reactions
Résumé
The hydrofluorination of uranium dioxide is a key reaction in the conversion processes for uranium enrichment and nuclear fuel fabrication. Improvement in process performance can be achieved by a detailed characterization and modeling of the powder microstructure. High-quality and detailed microstructure data is necessary, and can be recently obtained by advanced techniques as focused ion beam (FIB) and scanning electron microscopy (SEM) combined with image processing. The FIB procedure uses Ga ions to mill thin sections from the exposed powder surface, which is alternatively imaged by SEM. Sequential milling and imaging yield a serial set of consecutive images of the powder agglomerates. The 3D reconstruction obtained concerns about 300µm3.This 3D reconstruction gives a valuable basis for the determination of essential microstructure and morphological parameters necessary for kinetic modeling: surface area, porosity, pore size distribution and tortuosity.
UO2(s) + 4 HF(g) ⟺ UF4(s) + 2H2O(g)
The aim of this study is to focus on uranium based powders, in order to determine microstructural evolution during hydrofluorination. It is already known that the molar volume of UF4 being 1,88 times greater than UO2, the dimensions of the pores inside the powder aggregates decrease when conversion increases. This is most likely the reason for the observed slowing down of the kinetic rate since the diffusion of reacting and produced gases through the smallest pores becomes more and more difficult.
3 samples have been studied: UO2, UF4 and a partially transformed powder (50% conversion). FIB-SEM results show that, as expected, porosity decreases, and pore size distribution is shifted towards small pores, whereas tortuosity and the amount of closed pores are increasing throughout the reaction. All these results have been implemented in a numerical model which simulates the diffusion of gases into porous media. Coupling with a kinetic model have allowed to correctly simulate the hydrofluorination reaction at the scale of a small amount of powder.