Formalization of the kinetics for autocatalytic dissolutions. Focus on the dissolution of uranium dioxide in nitric medium

Abstract

Abstract. Uranium dioxide dissolution in nitric acid is a complex reaction. On the one hand, the dissolution produces nitrous oxides (NO X ), which makes it a triphasic reaction. On the other hand, one of the products accelerates the kinetic rate; the reaction is hence called autocatalytic. The kinetics for these kinds of reactions need to be formalized in order to optimize and design innovative dissolution reactors. In this work, the kinetics rates have been measured by optical microscopy using a single particle approach. The advantages of this analytical technique are an easier management of species transport in solution and a precise following of the dissolution rate. The global rate is well described by a mechanism considering two steps: a non-catalyzed reaction, where the catalyst concentration has no influence on the dissolution rate, and a catalyzed reaction. The mass transfer rate of the catalyst was quantified in order to discriminate when the reaction was influenced by catalyst accumulated in the boundary layer or uncatalyzed. This first approximation described well the sigmoid dissolution curve profile. Moreover, experiments showed that solutions filled with catalyst proved to lose reactivity over time. Results pointed out that the higher the liquid-gas exchanges, the faster the kinetic rate decreases with time. Thus, it was demonstrated, for the first time, that there is a link between catalyst and nitrous oxides. The outcome of this study leads to new ways for improving the design of dissolvers. Gas-liquid exchanges are indeed a lever to impact dissolution rates. Temperature and catalyst concentration can be optimized to reduce residence times in dissolvers.

Effect of surface orientation on dissolution rate and surface dynamics of UO2 single crystals in nitric acid

“…Then, this solution was used to dissolve UO2 (111)- single crystal. Cu has been already used to simulate UO2 dissolution in nitric acid [14,19,42]. The reaction is similar, as it is also autocatalytic and produces NOx and nitrous acid [43].…”

“…Then, in aerated nitric acid solutions, Cu(I) is likely to undergo oxidation to Cu(II). However, Charlier et al [14] compared dissolution rates of UO2 particles in solutions loaded with species produced by dissolving the J o u r n a l P r e -p r o o f same molar quantity of copper or uranium dioxide. For the same experimental conditions, they obtained the same dissolution rate of UO2.…”

“…) × S geo (14) The corrected surface area Scorr (t) (m²) was estimated and the corrected normalized weight loss N L corr (t) (g.m -2 ) was calculated using the following equations: (16) where tind (d) is the induction time defined as the dissolution time corresponding to the end of the first kinetic regime (i.e. 15 ± 1 days under standard conditions).…”

“…As HNO2 is a product of reaction (1), the dissolution mechanism of UO2 in nitric acid solutions was described as an autocatalytic reaction [13,14]. Several studies [10,15,16] underlined on the one hand, the increase of the dissolution rate when adding nitrite salts to the nitric acid solutions and on the other hand, the decrease of the dissolution rate when nitrous acid scavengers, like hydrazinium ions or urea, are added to nitric acid.…”

“…It is also important to note that the products of reaction (3) can recombine in nitric acid solutions to produce nitrous acid, according to reactions (4) and ( 5) [14,16]: NO2(aq) + H2O  HNO2(aq) + HNO3(aq) (…”