Chlorination Route E. Mendes1, L. Cassayre2, R. Malmbeck1, P
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O1_11 Recovery of Actinides from Actinide-Aluminium Alloys: Chlorination Route E. Mendes1, L. Cassayre2, R. Malmbeck1, P. Souček1, R. Jardin1, J.-P. Glatz1 1European Commission, JRC, Institute for Transuranium Elements, Postfach 2340, 76125 Karlsruhe, Germany 2Laboratoire de Génie Chimique (LGC), Université Paul Sabatier, UMR CNRS 5503, 118 route de Narbonne, 31062 Toulouse Cedex 04, France A method for recovery of actinides (An) from An-Al alloys formed by electrochemical separation of metallic spent nuclear fuel on solid aluminium electrodes in molten chloride salts is described. The proposed route consists of three main steps: Vacuum distillation of salt adhered on the electrodes, chlorination of An-Al alloy by pure chlorine gas and sublimation of formed AlCl3. A thermochemical study of the route was performed to determine important chemical reactions and to find optimum experimental conditions for all process steps. Vacuum distillation of the electrode is efficient for complete removal of remaining salt and most fission products (FP), full chlorination of the An-Al alloys is possible at any working temperature and evaporation of AlCl3 is achieved by heating under argon.. Experiments have been carried out using U-Al alloy in order to define parameters providing full alloy chlorination without formation of volatile UCl5 and UCl6. It was shown that full chlorination of An-Al alloys without An losses should be possible at a temperature approx. 150°C. INTRODUCTION to obtain the desired output product, e.g. An alloy. A method involving the chlorination of Al- A promising pyrochemical reprocessing option U spent fuel has been proposed by Bohe et al. [5, regarding spent nuclear fuels is the grouped- 6], but up to now is only based on selective separation of all actinides (An's) by thermochemical calculations without validation electrochemical methods in a LiCl-KCl molten by experimental studies involving Uranium. salt. An's are electrochemically grouped- selectively reduced from the mixture of fission Chlorination AlCl3 products (FP) dissolved in the molten salt. In Al-U-Pu-Np-Am Distillation with Cl2(g) + alloy Al-U-Pu-Np-Am An ITU a process based on electrorefining of + LiCl-KCl salt alloy chlorides metallic alloy fuel and the use of solid reactive + FP chlorides aluminium cathodes is under development. In Pure An An alloy recent work, the efficiency of the process and chlorides selectivity over lanthanides has been Chemical Heating under Ar : reduction AlCl3 sublimation demonstrated [1, 2]. It has been shown that the use of solid aluminium cathodes leads to the Fig. 1: Principle of the chlorination route formation of stable An-Al alloys [3], highly (An=actinides, FP=fission products) loaded in An (up to about 70 wt. %) [4]. In this work, a process using the chlorination of Efficient separation of An´s from Al-An is a key An-Al alloys by pure Cl2(g) is investigated. point for the feasibility of the process. The Initially, a detailed thermochemical study of all method to recover An´s proposed and studied in process steps was carried out. Based on this this work is a chlorination route based on a three study, experiments were focussed to optimise step procedure as presented in Fig.1. Initially, the process conditions for a full recovery of uranium salt, which is composed mainly of LiCl-KCl and from U-Al alloys. The reason is that due to the some FP chlorides, adhered on the electrode after high volatility of UCl5 and UCl6, U is the most the electrorefining process is removed by difficult element to convert to chloride form vacuum distillation. The second step is a without any losses. For that purpose, chlorination chlorination of the An-Al alloy in order to fully experiments on a UAl3 alloy were carried out in convert the alloy to chlorides. The last step the temperature range 150-170°C. consists of the sublimation of AlCl3. The final An chlorides can be further chemically processed ATALANTE 2008 Montpellier (France) May 19-22, 2008 1 O1_11 THERMOCHEMICAL STUDY melting point than pure metals, their vapour pressures are most likely lower than the one of The main idea of the chlorination route is to fully each individual metal. Therefore, the vapour chlorinate the alloy and removal of AlCl3 by pressure of alloyed Am will be lower than pure sublimation. Thermochemical calculations using Am. the Factsage software were performed, aiming to determine the most suitable conditions for the Pure LiCl-KCl salt is possible to remove by process. At first, a bibliographic study was vacuum distillation at temperatures around carried out to collect available thermochemical 800°C, which was confirmed experimentally in data (∆fH°, Sm° and Cp(T)) and phase diagrams. the laboratory scale (2g of LiCl-KCl were Most thermochemical data concerning the An’s completely distilled at 900°C and 5.10-5 bar). chlorides are tabulated, apart from AmCln and However, other compounds, especially Ln CmCln gaseous compounds [7-11]. Among An- chlorides, need stronger conditions to be Al alloys, the Gibbs energy of formation of removed. Available thermodynamical data UAl3, UAl4 and PuAl4 are available, together indicate that a temperature of 1000°C is required with phase diagrams for U-Al and Pu-Al systems to fully remove all lanthanide (Ln) chloride [12]. In both systems, the melting point of the compounds, with a vacuum of 10-5 bar, see intermetallic compounds increases with the An horizontal dotted line in Fig 2. However, content, UAl2 and PuAl2 having the highest distillation under these conditions is possibly still melting temperature (1620°C and 1540°C, not sufficient for complete removal of some FP respectively). No phase diagram was found for chlorides (e.g. MoCl2, SrCl2 and BaCl2). Np-Al and Am-Al systems. However, three Np- 1000 °C Al intermetallic compounds are reported in the 0 ZrCl4 Inorganic Crystal Structure Database [13]. No AlCl3 PrCl InCl ternary or quaternary An-Al system was found. -2 3 CeCl3 -4 First step: Distillation of remaining salt SmCl2 GdCl Al 3 SnCl2 vacuum 10 -2 mbar To ensure a sufficient purity of both end -6 MoCl2 AgCl products (An and Al), it is necessary to clean the / bar (P) log LaCl 3NdCl 3 Np Am cathode from the residual frozen salt adhering at -8 TbCl3 Cm the surface after the electrorefining step. This RhCl2 EuCl3 CdCl2 -10 PdCl2 KCl LiCl salt contains a mixture of LiCl-KCl with YCl SrCl 3 Pu 2 U dissolved FP chlorides. The proposed technique BaCl RuCl3 CsCl is distillation of the salt from the An-Al alloys -12 0.0005 0.001 0.0015 0.002 and recycling of the salt back in the 1/T / K electrorefiner. This requires sufficient difference Fig. 2: Vapour pressure vs. reverse of in vapour pressures between the salt and the temperature for various metals and chlorides. alloy. No thermochemical data exist to fully evaluate the vapour pressure of the salt mixture Second step: Chlorination of An-Al alloy (LiCl-KCl + FP) and of the alloy (Al-U-Pu-Am- Np-Cm). However, as a first estimation, the The main target of the chlorination is to separate vapour pressure, PM, of each individual actinides from aluminium. All metal elements compound (FP chlorides and An metals) can be (U, Pu, Np, Am, Cm, Al) of the alloy have to be calculated from available data according to: fully converted to chlorides and significantly ° more volatile aluminium chloride is ∆rG(T) (1) consecutively evaporated in the following step. log PM = − 2.303RT Each of presented metals forms a chloride where ∆rG° is the Gibbs energy of the compound in equilibrium with pure chlorine gas, equilibrium M(s,liq)=M(g). even at relatively low temperatures (25°C). Therefore, the key point of the process is to Results for the most important An's and FP are control working conditions in order to provide plotted in Fig. 2. It can be seen that, except Am, formation of desired species only. Formation of all An metals exhibit a lower vapour pressure gaseous actinide chlorides (e.g. UCl5 and UCl6) than all chloride compounds. As AnAl3 - AnAl2 has to be suppressed to avoid actinide losses by intermetallic compounds have a much higher volatilization ATALANTE 2008 Montpellier (France) May 19-22, 2008 2 O1_11 The thermochemical study of the chlorination of Al(s) AlCl3(s) An-Al alloys is a complex problem, as many 6 compounds can form, and the reactions are strongly dependent on the applied conditions. As a first approach, two main parameters of the 4 process have been studied: - Temperature of the reaction, moles - Molar ratio Cl2/alloy. 2 3 UCl3(s) For calculations, a constant volume of 1 dm was UCl4(s) UAl4(s) PuCl3(s) Al2Cl6(g) used and the Cl2/alloy molar ratio was fixed by PuAl4(s) the considered amount of actinide alloy. The 0 following calculations have been performed: 0 5 10 15 20 moles Cl (g) 2 Fig. 3: Chlorination of UAl (1.0 mol) and PuAl - Relative stability of An chlorides, 4 4 (0.5 mol) at 150°C - Influence of the amount of - Chlorination of UAl and PuAl at 150°C, 4 4 Cl (g). - Chlorination of UAl in the 100°C-1000°C 2 3 temperature range for a fixed Cl /UAl ratio. 2 3 These calculations show that Pu and U are chlorinated in preference to Al, leaving non- Chlorination of pure An metals chlorinated Al metal. The behaviour of the alloys The relative stability of An chlorides was is thermodynamically very close to the behaviour calculated according to the Gibbs energy of the of a mixture of pure compounds. However, this reaction: calculation shows only the equilibrium state 2 2 An(s,l) + Cl2 (g) = AnClx (s,l) (2) towards which the system should tend.