CONVERSION OF OXIDE INTO METAL OR CHLORIDE FOR THE PYROMETALLURGICAL PARTITIONING PROCESS Yoshiharu Sakamura, Masaki Kurata, Tsuyoshi Usami and Tadashi Inoue Central Research Institute of Electric Power Industry (CRIEPI), Iwadokita 2-11-1, Komae-shi, Tokyo 201-8511, Japan Abstract Oxide fuel is not directly treated in the pyrometallurgical partitioning process that consists of electrorefining and reductive extraction in LiCl-KCl eutectic system. Therefore, some reduction and chlorination techniques for removing oxygen have been developed by CRIEPI. Typical experimental results and characteristics of the pretreatment techniques are summarized in this paper. Introduction In the pyrometallurgical partitioning process, uranium, plutonium and minor actinide are dissolved into a LiCl-KCl eutectic salt and then are selectively collected by the electrorefining and/or the reductive extraction using a liquid metal [1-5]. It is easy for metal and nitride fuels to be anodically dissolved into the molten salt [6-8]. However, oxide fuel as well as the oxide obtained by denitration of HLLW is hardly dissolved. Actinide cation in the salt (e.g. U 3+, Pu 3+, etc.) immediately reacts with oxide ion to give a precipitate of oxide or oxychloride. The products of the electrorefining and reductive extraction steps are distilled at a high temperature to remove the adhering salt and cadmium from the actinide metal, when a carbon crucible coated with ZrO 2 or Y 2 O 3 is used for containing the corrosive materials [9]. In the distillation step, a part of actinide metal may react with crucible coating materials or oxygen impurities to give some dross consisting of actinide oxide and oxychloride. The actinide in the dross has to be recycled to attain a high actinide recovery ratio. Since the pyrometallurgical partitioning process cannot directly accept oxides or oxychlorides, a pretreatment step is necessary for removing oxygen. Therefore, CRIEPI has developed both reduction techniques to convert oxide into metal and chlorination techniques to convert oxide into chloride. They are listed as follows: - Lithium reduction - Electrochemical reduction - Chlorination using chlorine gas. - Chlorination using ZrCl 4. In this paper, typical experimental results and characteristics of the pretreatment techniques are
summarized. Reduction technique Lithium reduction In the lithium reduction process, actinide oxides are converted into metals by adding lithium metal reductant in a LiCl salt bath at 650 C. The reaction is expressed as MO 2 + 4 Li (LiCl) M + 2 Li 2 O (LiCl), (1) where MO 2 is an actinide oxide and (LiCl) denotes the molten LiCl phase. It was experimentally verified that UO 2, NpO 2, and PuO 2 could be reduced into metal [10]. As for AmO 2, the Li 2 O concentration in the salt had to be less than 5.1 wt% for the complete reduction into metal [11]. Figure 1 shows the results of a lithuim reduction test for a simulated spent MOX pellet containing U, Pu, Np, Am, Cm, Ce, Nd, Sm, Ba, Pd, Mo and Zr [12]. The cross section of the pellet after the reduction was metallic, while oxide solid solutions of rare earths were observed microscopically. Rare earth oxides could not be reduced to metals in this system. Moreover, a part of them were detected in the molten LiCl phase and on the bottom of the crucible. The rare earths dissolved into the salt easier as the Li 2 O concentration increased. Ln 2 O 3 + O 2- (LiCl) 2 LnO 2 - (LiCl) (2) U-Pu alloy Rare-earth (a) MOX before reduction (b) MOX after reduction BSE image Fig. 1 (c) Cross section of MOX after reduction X-ray mapping for Ce (d) Oxide solid solution of rare earth in the reduced MOX The simulated spent MOX pellet reduced by adding lithium metal in a LiCl salt bath at 650 C. The MOX pellet was sintered from oxides of U, Pu, Np, Am, Cm, Ce, Nd, Sm, Ba, Pd, Mo and Zr.
where Ln 2 O 3 denotes rare earth oxide. Rare earths were supposed to dissolve into the salt from the pellet where the Li 2 O concentration might be high, and then to precipitate in the bulk salt. Small amounts of Pu, Am and Cm were also detected on the bottom of the crucible. These actinides might be soluble in the salt but the solubility limit seemed much smaller than that of rare earths. Almost all of U and Np remained in the reduced pellet. Barium was dissolved in the salt almost completely. As oxide fuels are processed, Li 2 O accumulates in the salt bath. So, electrowinning for decomposing the Li 2 O must be performed to keep the Li 2 O concentration lower and to recycle the lithium metal. The reduction step and the electrowinning step are repeated one after the other. Anode: 2 O 2- (LiCl) O 2 + 4 e -, (3) Cathode: Li + (LiCl) + e - Li. (4) It was experimentally demonstrated that the Li 2 O concentration was decreased from 3.0 wt% to 0.2 wt% with a current efficiency higher than 90% [13]. Since the solubility of lithium metal in LiCl is no more than 0.6 at% at 650 C, almost all of the lithium metal was collected at the iron cathode surrounded by a MgO shroud. Electrochemical reduction Recently, an electrochemical reduction technique has been developed to make the reduction process more efficient. By electrolysis with a cathode where actinide oxide is loaded, the oxide ion is released into the salt and the actinide metal remains at the cathode. The oxide ion is transported to the anode and oxygen gas is evolved at the anode. When a carbon anode is employed, CO 2 or CO is evolved. Cathode: MO 2 + 4 e - M + 2 O 2- (5) Anode: 2 O 2- O 2 + 4 e -, (6) 2 O 2- + C CO 2 + 4 e -, O 2- + C CO + 2 e -. (7) Advantages of the electrochemical reduction technique are as follows: - The reduction and electrowinning steps are performed simultaneously. - The concentration of oxide ion in the salt is almost constant and can be maintained at a low value. - The amount of the salt bath may be small because oxide ion do not accumulate. An electrochemical reduction technique has been developed for an economical process to produce titanium metal [14]. A CaCl 2 salt bath having low oxygen potential is employed for the TiO 2 reduction because the affinity of oxygen to titanium is very large. If the CaCl 2 salt bath is used for the oxide nuclear fuel, not only actinide oxides but also rare earth oxides will be reduced to metals. However, the operating temperature for the CaCl 2 system (melting point of CaCl 2 : 772 C) is higher than that for the LiCl system, which may make the design of a facility more challenging. CRIEPI is now investigating the electrochemical reduction of UO 2 and mixed oxides in both of LiCl and CaCl 2 salt baths. Figure 2 shows typical uranium metal products obtained in UO 2 reduction tests using about 0.2 g of sliced UO 2 pellets. The sample in the LiCl salt bath shrank during the reduction and the uranium metal having uniform porosity was obtained as shown in Fig. 2(a). In the LiCl salt bath at 650 C, the reduction progressed quite satisfactorily.
In case of the CaCl 2 salt bath at 820 C, the uranium metal product had a big cave in the center as shown in Fig. 2(b). The reduced uranium metal condensed at the surface of the UO 2 piece, which might be due to the high operating temperature. The melting point of uranium metal is 1132 C. The dense uranium metal membrane formed at the surface often prevented the reduction from proceeding. However, the dense uranium metal membrane was not observed in the reduction tests for MOX and UO 2 -Gd 2 O 3. A large scale test was carried out in a LiCl salt bath. At the cathode, 100 g of UO 2 (0.3-1.0 mm grain) was loaded in a stainless steel basket. Figure 3 shows the uranium metal product. It was demonstrated that UO 2 was successfully reduced to uranium metal. MOX fuel reduction tests were carried out and in result U-Pu alloys were obtained [15]. During the reduction, uranium was reduced prior to plutonium and Pu-spots were observed in the product. It was suggested that the reduction rate of MOX was higher than that of pure UO 2. 1mm (a) A product in a LiCl salt bath at 650 1mm (b) A product in a CaCl 2 salt bath at 820 Fig. 2 The cross section of sliced UO 2 pellet (83% of theoretical density) after reduction in LiCl at 650 C (a) and in CaCl 2 at 820 C (b). 10mm Fig. 3 The product of 100g-UO 2 reduction test in LiCl at 650 C. Chlorination technique Chlorination using chlorine gas A chlorination process using chlorine gas and carbon has been developed. When uranium, plutonium and rare earth oxides are converted to chloride, the following chemical reactions occur:
UO 2 + C + 2 Cl 2 UCl 4 + CO 2, (8) PuO 2 + C + 2 Cl 2 PuCl 4 + CO 2, (9) 2 Ln 2 O 3 + 3 C + 3 Cl 2 2 LnCl 3 + 3 CO 2. (10) Two chlorination experiments were performed for the oxide obtained by the denitration of a simulated high level liquid waste [16]. The oxide was loaded in a LiCl-KCl eutectic salt bath contained in a pyro-coating graphite crucible. Then chlorine gas was introduced into the salt through a graphite tube at 700 C. The results were shown in Fig. 4. All of elements containing uranium, rare earth, noble metal, alkali, alkaline earth, etc. could be converted to chloride, when some transition elements such as molybdenum, iron and zirconium volatilized with ease. It was suggested that the volatilized chloride should be collected because 5 % of the detected uranium was volatilized during the chlorination. The salt product was supplied for the reductive extraction test and consequently the uranium was collected in the liquid cadmium phase. recovered ratio in molten salt bath volatilized ratio 100 100 80 80 Mass balance (%) 60 40 Mass balance (%) 60 40 20 20 0 Na Ba Y Ce Nd Sm Ru Rh Pd Ni Cr Cs Zr Fe Mo Re Se Te Fig. 4 Mass balance in chlorination tests using the oxide obtained by the denitration of a simulated high level liquid waste. 0 U Na Cs Sr Ce Nd Ru Pd Cr Zr Fe Mo Re (a) Run 1 (b) Run 2 Chlorination using ZrCl 4 ZrCl 4 has a high reactivity with oxygen, but is not corrosive to refractory metals such as steel. The actinide and rare earth oxides were allowed to react with ZrCl 4 in a LiCl-KCl eutectic salt at 500 C to give metal chlorides dissolving in the salt and a precipitate of ZrO 2 [17]. When rare earth oxides are converted to chloride, the following chemical reactions occur: 2 Ln 2 O 3 + 3 ZrCl 4 (LiCl-KCl) 4 LnCl 3 (LiCl-KCl) + 3 ZrO 2 (11) where (LiCl-KCl) denotes the molten LiCl-KCl phase. For chlorinating actinide dioxides, an addition of zirconium metal was efficient as a reductant, because trivalent actinide ion is stable in the salt. UO 2 + 3/4 ZrCl 4 (LiCl-KCl) + 1/4 Zr UCl 3 (LiCl-KCl) + ZrO 2, (12) PuO 2 + 3/4 ZrCl 4 (LiCl-KCl) + 1/4 Zr PuCl 3 (LiCl-KCl) + ZrO 2. (13)
Divalent zirconium in the salt phase, which is formed by the disproportionation of ZrCl 4 and zirconium metal, might accelerate the chlorination [18]. Figures 5 and 6 indicate that UO 2 and PuO 2 were completely chlorinated and dissolved into the salt by adding an excess of ZrCl 4. Thermodynamic considerations indicate that the minor actinide oxides will be also chlorinated. When the oxides were in powder form, the reaction was observed to progress rapidly. By keeping the system quite still, the solution settled so that the ZrO 2 precipitate could be separated. Advantages of the ZrCl 4 chlorination technique are as follows: - Very simple. - The reaction rate is sufficient. - ZrCl 4 is not corrosive to refractory metals such as steel. - Flexibility for scale, molten salt composition and temperature. - Actinide metals as well as oxides are chlorinated. - The by-product of ZrO 2 is quite stable. 0.15 0.10 U and Zr in the salt ( g ) 0.10 0.05 U Zr Pu and Zr in the salt ( g ) 0.08 0.06 0.04 0.02 Pu Zr 0.00 0.0 0.1 0.2 0.3 0.4 ZrCl 4 added ( g ) Fig. 5 Results of a UO 2 chlorination test. The amount of U and Zr dissolved in the salt as a function of the amount of ZrCl 4 added. The solid lines are the calculated values based on the mass balance. 0.00 0 5 10 15 20 25 Time / hr Fig. 6 Results of a PuO 2 chlorination test. The amount of Pu and Zr dissolved in the salt after the salt melted at 500. The dotted lines are the calculated values based on the mass balance. Conclusions Oxide fuel and dross are not directly treated in the pyrometallurgical partitioning and reprocessing process which consist of electrorefining and reductive extraction in LiCl-KCl eutectic system. Therefore, some reduction and chlorination techniques have been developed by CRIEPI. Their characteristics are summarized in Table 1. Since each technique has its own advantages, the suitable pretreatment techniques will be selected as the needs of the case demand. Acknowledgments A part of this work was supported by MEXT (Ministry of Education, Culture, Sports, Science, and Technology of Japan), Development of Innovative Nuclear Technologies Program.
Table 1 Summary of the pretreatment techniques for applying pyrometallurgical partitioning to oxide fuel Reduction technique Chlorination technique Li reduction Electrochemical reduction Cl 2 method ZrCl 4 method Molten salt bath LiCl LiCl CaCl 2 LiCl-KCl etc. LiCl-KCl etc. Temperature 650 650 800 700 500 Active material Li metal None None Cl 2, C ZrCl 4, Zr Product U, Pu metal U, Pu metal U, Pu metal UCl 4, PuCl 3 UCl 3, PuCl 3 By-product Li 2 O O 2,CO 2 O 2,CO 2,CO CO 2 ZrO 2 FP chemical form Separated from actinides. Accompany actinides Noble metal Matal Matal Matal Chloride Metal Rare earth Oxide or oxychloride Oxide or oxychloride Matal Chloride Chloride Alkali, alkaline earth Chloride Chloride Chloride Chloride Chloride References [1] T. Inoue, M. Sakata, H. Miyashiro, M. Matsumura, A. Sasahara, N. Yoshiki, Nucl. Technol., 93, 206 (1991). [2] Y. Sakamura, T. Hijikata, K. Kinoshita, T. Inoue, T.S. Storvick, C.L. Krueger, L.F. Grantham, S.P. Fusselman, D.L. Grimmett and J.J. Roy, J. Nucl. Sci. Technol., 35[1], 49 (1998). [3] K. Kinoshita, T. Inoue, S.P. Fusselman, D.L. Grimmett, J.J. Roy, R.L. Gay, C.L. Krueger, C.R. Nabelek and T.S. Storvick, J. Nucl. Sci. Technol., 36[2], 189 (1999). [4] K. Kinoshita, M. Kurata and T. Inoue, J. Nucl. Sci. Technol., 37[1], 75 (2000). [5] K. Kinoshita, T. Inoue, S.P. Fusselman, D.L. Grimmett, C.L. Krueger, C.R. Nabelek and T.S. Storvick, J. Nucl. Sci. Technol., 40[7], 524 (2003). [6] R.W.Benedict, J.R.Krsul, R.D.Mariani, K. Park and G.M.Teske, in Proceedings of Future Nuclear Systems: Emerging Fuel Cycles and Waste Disposal Options, Global'93, Vol. 2, Seattle, Washington, Sept. 12-17,1993, p.1331. [7] K. Kinoshita, T. Koyama, T. Inoue, M. Ougier and J.-P. Glatz, J. Physics and Chemistry of Solid, to be submitted. [8] O. Shirai, T. Iwai, K. Shiozawa, Y. Suzuki, Y. Sakamura and T. Inoue, J. Nucl. Mater., 277, 226 (2000). [9] B.R. Westphal, D. Vaden, T.Q. Hua, J.L. Willet and D.V. Laug, in Proceedings of 5th Topical Meeting, DOE Owned Spent Nuclear Fuel and Fissile Materials Management, Charleston, SC, Sept. 17-20, 2002. [10] T. Usami, M. Kurata, T. Inoue, H.E. Sims, S.A. Beetham and J.A. Jenkins, J. Nucl. Mater. 300, 15 (2002). [11] T. Usami, T. Kato, M. Kurata, T. Inoue, H.E. Sims, S.A. Beetham and J.A. Jenkins, J. Nucl. Mater. 304, 50 (2002). [12] T. Kato, T. Usami, M. Kurata, T. Inoue, H.E. Sims and J.A. Jenkins, CRIEPI Report T01003 (2001) in Japanese. [13] A. Kawabe, T. Kato and M. Kurata, CRIEPI Report T02041 (2003) in Japanese. [14] G.Z. Chen, D.J. Frey and T.W. Farthing, Nature, 407, 361 (2000). [15] M. Kurata, T. Inoue, J. Serp, M. Ougier and J-P. Glatz, J. Nucl. Mater. 328, 97 (2004). [16] M. Kurata, K. Kinoshita, T. Hijikata and T. Inoue, J. Nucl. Sci. Technol., 37[8], 682 (2000). [17] Y. Sakamura, T. Inoue, T. Iwai and H. Moriyama, submitted for publication in J. Nucl. Mater.. [18] Y. Sakamura, J. Electrochem. Soc., 151 [3], C187 (2004).