Technical note The dissolution of uranium oxides: Thermodynamic and kinetic investigations Belkacem Bensmain a,b , Salah Chegrouche a, ⁎, Mahfoud Barkat a , Abdelhamid Mellah a, ⁎, Djamel Nibou b a Algerian Atomic Energy Commission, Nuclear Research Center of Draria, BP 43 16003 Draria, Algiers, Algeria b Laboratory of Materials Technology, USTHB, BP 32, El Alia, 16111 Bab Ezzouar, Algiers, Algeria abstract article info Article history: Received 2 June 2015 Received in revised form 3 November 2015 Accepted 7 December 2015 Available online 12 December 2015 This work investigates the dissolution of various uranium oxides in nitric acid medium and the most predominant occurring reaction was determined on the basis of the thermodynamic and kinetic studies. Six uranium oxides were dissolved and studied. The Gibbs free energies of all the reactions Δ r G°(T) were analyzed by Ulich model (Ulich, 1930) and the predominant dissolution reaction was found to be: 3U 3 O 8(s) + 20HNO 3(aq) → 9 UO 2 (NO 3 ) 2(aq) + 2NO (g) + 10H 2 O Three reaction order rate models namely the first, the second and the third order were applied on the predominant reaction reported above. According to the kinetics results, our reaction best fits the second order equation rate. © 2015 Elsevier B.V. All rights reserved. Keywords: Uranium oxide Uranyl nitrate Thermodynamic study Kinetic study 1. Introduction Uranium is the most representative actinide element that is of fundamental importance in the nuclear fuel cycle. Uranium dioxide powder is the starting material for the preparation of the fuel pellets used in nuclear power reactors. The nuclear fuel cycle involves several major steps consisting of the leaching of uranium ore in sulphuric acid (Guettaf et al., 2009), followed by the precipitation of uranium as yellow-cake (YC). The resulting impure material previously obtained is then purified by means of the TBP solvent extraction (Boualia and Mellah, 1989) and the precipitates obtained are identified as ammonium diuranate (ADU) or ammonium uranyl tricarbonate (AUC) (Venter and Boylett, 2009; Chegrouche and Kebir, 1992). Finally, the pure materials produced are dried, calcined and reduced to various uranium oxides. The physical properties of the oxides obtained are very important for their ultimate use as a fuel in nuclear reactors. It is now established that many of these properties are inherited from the precursor materials used (Ayaz and Bilge, 2000). Thus the method of the preparation of am- monium diuranate controls its particle size and determines the nature of the oxide obtained. This flow sheet has been studied by a number of authors (Benedict et al., 1981). The dissolution of the different uranium concentrates by nitric acid constitutes the principal operation of the uranium refining process flow sheet (Venter and Boylett, 2009; Morss et al., 2010). The major advantage of the use of the nitric acid is its powerful oxidizing property. In fact, the nitric acid oxidizes the uranium in its different compounds from the lower state (IV) to the highest oxidation state (VI). However, the biggest disadvantages of this reagent are respectively its high cost and its high consumption per mass unit of dissolved uranium. In addition, the dissolution of uranium oxides by nitric acid is accompanied by considerable volumes of gas emissions consisting mainly of the by- products of the nitric acid reduction such as nitrogen dioxide and monox- ide (Fukasawa and Ozawa, 1986; Sakura et al., 1988; Yasuike et al., 1995; Smirnov et al., 2012). Dyck et al. (1977) have studied the dissolution of mixed thorium– uranium oxide fuel in nitric acid/hydrofluoric acid to assist the design of equipment and procedures for reprocessing. The dissolution rate was found to depend upon the acid concentration and temperature with the optimum dissolution rate occurring at a concentration of 13 mol/L HNO 3 /0.05 mol/L HF and at a boiling temperature. The dissolu- tion of the uranium oxide with nitric acid was also addressed by Y. Ikeda et al., 1995; Homma et al., 1993). Our study focused on the dissolution of the most important uranium oxides in nitric acid, the following reactions have been considered: U 3 O 8ðs þ 8HNO 3 aq ð Þ →3UO 2 NO 3 ð Þ 2ðaq þ 2NO 2ðg þ 4H 2 O ð1Þ 3U 3 O 8ðs þ 20HNO 3 aq ð Þ →9 UO 2 NO 3 ð Þ 2 aq ð Þ þ 2NO g ðÞ þ 10H 2 O ð2Þ UO 2s ðÞ þ 4HNO 3 aq ð Þ →UO 2 NO 3 ð Þ 2 aq ð Þ þ 2NO 2g ðÞ þ 2H 2 O ð3Þ 3UO 2s ðÞ þ 8HNO 3 aq ð Þ →3UO 2 NO 3 ð Þ 2 aq ð Þ þ 2NO g ðÞ þ 4H 2 O ð4Þ 2UO 2s ðÞ þ 6HNO 3 aq ð Þ →2UO 2 NO 3 ð Þ 2 aq ð Þ þ NO 2g ðÞ þ NO g ðÞ þ 3H 2 O ð5Þ Hydrometallurgy 160 (2016) 73–78 ⁎ Corresponding authors. E-mail addresses: salahcheg@yahoo.fr (S. Chegrouche), a.mellah@crna.dz (A. Mellah). http://dx.doi.org/10.1016/j.hydromet.2015.12.003 0304-386X/© 2015 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Hydrometallurgy journal homepage: www.elsevier.com/locate/hydromet