Isothermal and isobaric kinetic modeling of the reaction of uranium dioxide
Résumé
The hydrofluorination of uranium dioxide is a key reaction in the conversion processes for uranium enrichment and nuclear fuel fabrication. The purpose of the study is to identify the model of transformation of this reaction in order to simulate the kinetics under isothermal and isobaric conditions.
UO2(s) + 4 HF(g) ⟺ UF4(s) + 2H2O(g)
The solid-gas reactions always occur according to nucleation and growth process. If the reaction admits a rate-determining step, it can be shown that the rate equation may be expressed as the product of two functions:
dα/dt=Φ(T,P)*Sm(α,r0) with dα/dt (s-1) ; Φ(T,P)*S (mol.m-2.s-1) and Sm(α,r0)(m².mol-1)
Thermogravimetric analysis of the hydrofluorination enabled to study the influence of the temperature and hydrogen fluoride partial pressure on the reaction rate, and thus determine ϕ(T, P). It follows that the order of reaction is equal to 1 towards hydrogen fluoride between 42 and 720 mbar, and the reaction follows an Arrhenius law between 375 and 450°C. Powder characterization showed that the space function Sm(α) follows a Carter-Valensi law, which corresponds to a shrinking core model with a rate limiting step of diffusion. The simulations show a phenomenon of kinetic slowdown from alpha = 0.6. The molar volumes between UO2 and UF4 being quite different (1,88 ratio), the dimensions of the pores inside the powder aggregates decreases when conversion increases. This is most likely the reason for the slowing down of the kinetic rate since the diffusion of reacting and produced gases through the smallest pores become more and more difficult. Thus isobaric conditions are no longer valid and the model deviates from the experience. The model can thus be improved by taking into account the transport phenomena in the powder agglomerates.