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High-temperature distillation and consolidation of U–Zr cathode product from molten salt electrorefining of simulated metallic fuel
Journal of Nuclear Materials 448 (2014) 259–269
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Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
High-temperature distillation and consolidation of U–Zr cathode product from molten salt electrorefining of simulated metallic fuel Masatoshi Iizuka ⇑, Masaaki Akagi, Tadafumi Koyama Central Research Institute of Electric Power Industry, Iwato-kita 2-11-1, Komae, Tokyo 201-8511, Japan
a r t i c l e
i n f o
Article history: Received 24 May 2013 Accepted 10 February 2014 Available online 18 February 2014
a b s t r a c t High-temperature distillation experiments were performed using U–Zr cathode products of various compositions to obtain knowledge on suitable operation conditions and equipment design such as the container material. The LiCl–KCl–UCl3 electrolyte adhering to the U–Zr cathode products was almost completely vaporized at 1273–1573 K, under pressure of 10–300 Pa. Massive ingots were obtained from the remaining cathode products by heating them at 1573–1673 K. Three different phases were identified in a distillation product of a higher Zr content. A U-rich bulk (3.9 wt% Zr) and a deposit of a relatively low Zr content (17.2 wt% Zr) were considered to be formed during the cooling process of the distillation product. Another Zr-rich deposit (64.7 wt% Zr), which might cause the inhomogeneity of product ingots, was expected to result from Zr-rich spots that originally existed in the cathode product. The Cl content in the cathode product was decreased by distillation to less than 1/200 of that after electrorefining, while it was markedly larger at a higher Zr concentration. To limit the amount of Zr-rich deposit and the Cl content, the amount of Zr in the distillation product should be controlled to a sufficiently low level by optimization of the operating procedures and conditions in the electrorefining and distillation steps. The zirconia coating material developed in this study showed superior performance in inhibiting reaction between the melted U–Zr alloy melt and the graphite crucible and also in the easy release of the U–Zr ingot from the crucible. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Development of innovative nuclear fuel cycle technology that has the advantages of economic and safe power generation as well as proliferation resistance is strongly expected to be an effective measure for achieving environmental sustainability and satisfying the increasing energy demand. Metallic fuel cycle technology, consisting of a metal fuel fast reactor, pyrometallurgical reprocessing and fuel fabrication by injection casting, has been attracting increasing attention as one of the most promising nuclear fuel cycle technologies for the next generation satisfying the above requirements [1–4]. In pyrometallurgical reprocessing, the recovery of actinide elements and the decontamination from fission product elements (FPs) are conducted in an electrorefining step [5] using molten lithium chloride-potassium chloride (LiCl–KCl) as an electrolyte (Fig. 1). The actinide elements are electrochemically dissolved from chopped spent metal fuel pins loaded in anode baskets made of ⇑ Corresponding author. Address: Central Research Institute of Electric Power Industry, Nuclear Technology Research Laboratory, Iwato-kita 2-11-1, Komae, Tokyo 201-8511, Japan. Tel.: +81 3 3480 2111; fax: +81 3 3480 7956. E-mail address: [email protected] (M. Iizuka). http://dx.doi.org/10.1016/j.jnucmat.2014.02.015 0022-3115/Ó 2014 Elsevier B.V. All rights reserved.
stainless steel. Uranium, which has the highest redox potential among the actinides, is preferentially reduced and collected on a solid steel cathode [6]. Plutonium and other transuranium elements are recovered into a liquid Cd cathode together with the remaining U since their activities are greatly reduced in liquid Cd [7–9]. Before electrorefining cathode products are sent to a fuel fabrication step, the electrolyte adhering to them has to be removed, and the products must be consolidated in the form of an ingot for storage and adjustment of the fuel composition. This treatment, called ‘cathode processing’, is carried out by hightemperature distillation at a reduced pressure. Recently, development of an electrochemical reduction method for oxide materials, either spent oxide fuels or products from light water reactor fuel reprocessing, is also in progress with the aim of supplying the metallic fuel for the start-up of fast reactors [10–12]. In the electrochemical reduction step, the oxide materials are immersed in LiCl–LiO2 electrolyte and cathodically reduced to metals. Before the reduction product is processed in the electrorefining step to remove lanthanide and noble metal FPs, the adherent electrolyte must be removed to inhibit the migration of O2 ions to the electrorefiner, which depletes the U in LiCl–KCl by formation of a UO2 deposit and to an increase in the melting point of the
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Fig. 1. Schematic diagram of electrorefining process for metallic fast reactor fuel.
electrorefiner electrolyte due to an increase in Li/(Li + K) ratio in the electrolyte. High-temperature distillation is also planned for the removal of the LiCl–LiO2 electrolyte adhering to the reduction product. Most of the fundamental thermodynamic properties related to the high temperature distillation process have been obtained, such as the temperature dependence of the vapor pressure of metals and chloride species contained in the cathode product from the electrorefining step [13,14]. It has also been found from small-scale distillation experiments that the vaporization of LiCl–KCl at reduced pressure begins at approximately 1073 K and that it is almost completed at approximately 1273 K even when UCl3 is contained in the processed chlorides [15,16]. At Idaho National Laboratory (INL), a large-scale ‘cathode processor’ with a maximum cathode product capacity of 80 kg has been developed for the treatment of spent metallic fuels from Experimental Breeder Reactor-II (EBR-II). The results of high temperature distillation using a large amount of solid cathode product were reported [17]. The salt adhering to the cathode product was vaporized and the products, consisting mainly of U metal, were melted normally at 1473 K and at less than 100 Pa, using a zirconia-based castable crucible (78 wt% zirconia, 20 wt% alumina and 2 wt% carbon) as a lining material for the graphite secondary container. The zirconia castable crucibles were durable against repeated use for at least 4–6 times, although the loss of U of a few percent inevitably occurred owing to the chemical reaction with the crucible material. Spent metallic fast reactor fuel contains about 10 wt% Zr. Since the actinides are anodically dissolved into the molten chloride electrolyte prior to Zr during the electrorefining step, the solid cathode product obtained near the end of each batch of electrorefining operation tends to be more Zr-rich than that recovered at the beginning [18] depending on the operating conditions. Therefore, the cathode processing step must be able to process cathode products of much wider composition range (U/Zr ratio) than that in spent fuel. However, operation conditions required for the treatment of such a variety of cathode products have not yet been clarified. In addition, an ideal cathode processor crucible material compatible with both the chemically active cathode product alloy melt and molten/vaporized chlorides has not yet been found. Means of minimizing the actinide loss due to the reaction with the crucible material and the waste generated in the cathode-processing step are also needed. In the present study, small-scale high-temperature distillation experiments were performed using U–Zr cathode products obtained by previous electrorefining tests [19] to obtain knowledge on suitable operation conditions for consolidating cathode
products of a variety of U/Zr ratios, equipment design and crucible materials compatible with both U–Zr alloy melt and chlorides. 2. Experimental 2.1. Experimental apparatus A schematic view of the high-temperature distillation apparatus is shown in Fig. 2. It was installed in a high-purity Ar glove box to handle chlorides and active metals, which are highly sensitive to oxygen and moisture. The apparatus consists of three major parts: vaporization part at the top that heats the cathode product, a connecting part in the middle where the vaporized chlorides diffuse to the lower-temperature region, and a condensation part at the bottom to collect the chlorides in the solid state. The three parts were made of graphite. The vaporization part can be heated to up to 1873 K by induction heater (20 kW, 28 kHz). An electric resistance heater is installed around the connecting part to avoid plugging by the solidification of vaporized chlorides. The condensation part is placed on a water-cooled base to attain a high recovery ratio of the vaporized chlorides by solidifying them. The temperature of each component was monitored with thermocouples during each experiment. The three major parts of the apparatus were placed in a watercooled gas-tight container made of stainless steel that could be evacuated to approximately 10 Pa. This container is divided into three sections in the vertical direction. The upper two sections can be moved digitally and remotely upward/downward using stepper motors for easier assembly/disassembly and maintenance of the apparatus. First, the vaporization area was designed to accommodate three small crucibles (29 mm in outer diameter, 53 mm in height) to examine the different types of cathode products and/or crucible materials simultaneously at a small batch size. This design was changed later to accommodate one larger crucible (up to 94 mm in outer diameter and 91 mm in height) so that a much larger amount of cathode product could be processed in a single experiment. After increasing the batch size, the distillation apparatus was also used as a ‘cathode processor’ for engineering-scale fuel cycle tests using simulated oxide/metal fuels [20]. 2.2. Crucible materials As candidate crucible materials compatible with the melted cathode product alloy and molten/vaporized chlorides, yttria
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Fig. 2. Schematic view of high-temperature distillation apparatus.
(Y2O3), zirconia (ZrO2) and zirconium nitride (ZrN) were selected. The yttria crucible used in the present study had a 29 mm O.D., a 25 mm I.D., a 53 mm height and a 99.5% in purity. Zirconia was used both as a material for the crucible itself and as a coating material applied on a graphite or W substrate. The zirconia crucible (stabilized with 3 mol% Y2O3) was of the same size as the yttria crucible. Although a commercially available zirconia coating material (ZYP coating corporation, ZO-MOD) was used in earlier experiments, an original coating material was developed in parallel to minimize the content of impurities, such as Al2O3 and P2O5 sintering additives, in the finished coating layer. This original coating material, which was used in later small-scale experiments and all of the large-scale experiments, was a mixture of zirconia beads (25–106 lm in size, about 5 mol% Y2O3) and powder (40 nm, 3 mol% Y2O3). This mixture can be coated by brushing to form a layer of more than 300 lm thickness without the occurrence of cracks during thermal treatment before use, unlike in the case of using blended ZrO2 powders of different particle sizes. A cross-sectional view of the original coating layer on a graphite substrate is shown in Fig. 3. The ZrN powder used in this study (ZR108PB, Kojundo Chemical Laboratory Co., Ltd., 98%) was very fine, and it was impossible to form a dense and thick layer of ZrN by brushing. Thus, this material was used to make a thinner (about 100 lm) secondary layer on a primary ZO-MOD layer that had been applied on the surface of a W metal substrate.
Fig. 3. Cross-sectional view of originally developed ZrO2 coating applied on graphite substrate.
After brushing the graphite or W substrate with the commercial or original zirconia coating material, the crucible was heated at 1373 K for 1 h under vacuum to form a mechanically durable layer. For the preparation of ZrN/ZrO2 double coating layers on the W substrate, ZrN powder was applied on a W crucible, that had already been coated with ZO-MOD by the above-mentioned procedure, then the crucible was heated again at 1373 K for 1 h. Although a much higher temperature is generally required to sinter ZrN powder, the pretreatment was carried out at a lower temperature in the present study to avoid the embrittlement of W due to its recrystallization and to inhibit interdiffusion between the ZrO2 and ZrN layers.
2.3. Cathode products used in high-temperature distillation experiments The cathode products used in the small-scale and part of the large-scale experiments in this study were obtained from engineering-scale electrorefining tests previously performed using U– Zr alloy [19]. As mentioned above, the Zr concentration in the solid cathode deposit tends to increase gradually during each batch of electrorefining operation. Since the cathode deposit was scraped off several times during the electrorefining tests and each deposit was separately stored, a variety of cathode product compositions (Zr content, morphology, and content of accompanying chloride electrolyte) were obtained in the engineering-scale electrorefining tests [19]. In this study, the behavior of these cathode products of different compositions upon high-temperature distillation was investigated. In Run Z-3 and Z-5, which were performed as part of the engineering-scale fuel cycle tests using simulated oxide/metal fuels [20], 700–1000 g of the cathode product was obtained from the electrorefining step carried out immediately prior to the distillation. The cathode product was crushed with a laboratory mill just before these runs to increase its packing density in the crucible. As in the case of the single electrorefining test, the cathode products obtained in the fuel cycle tests had different compositions depending on the time of their collection during each batch of electrorefining operation. Similarly, the samples taken from the surface of
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these cathode products after the electrorefining operation were not sufficiently representative of the cathode product. 2.4. Experimental procedure An example of temperature/pressure control in the high-temperature distillation apparatus during an experiment is shown in Fig. 4. After placing the crucibles loaded with the cathode products in the vaporization area, the whole apparatus was evacuated to about 1 Pa at room temperature. Then, it was disconnected from the vacuum line and the vaporization area was heated to 873– 973 K to melt the accompanying chlorides to prevent explosive boiling. Then, the apparatus was evacuated again to 5–10 Pa and the cathode product was heated to distil the chlorides and consolidate the remaining U–Zr. In earlier experiments, the distillation and consolidation were performed in separate stages as shown in Fig. 4. In these experiments, the chlorides were vaporized from the cathode product at 1273–1473 K. Although the apparatus was basically closed during the distillation, it was connected to the vacuum line whenever the internal pressure reached approximately 300 Pa. After the distillation, argon gas was introduced to a pressure of 50 kPa to avoid the revolatilization of the chlorides from the lower part of the apparatus, and the U–Zr alloy remaining in the vaporization part was further heated to 1673 K in an attempt to consolidate it. Finally, the apparatus was cooled slowly to prevent the breakage of the crucible due to a rapid temperature change. The connecting part was heated and maintained at 1073 K during the experiment to prevent it from being plugged by the solidification of vaporized chlorides. 2.5. Analytical methods The amount of chloride electrolyte accompanying the cathode product used for the high-temperature distillation was evaluated from the change in weight after washing with water to dissolve the chloride salts. The samples obtained from the U–Zr cathode products before and after distillation and the crucible materials including the coating layer were dissolved in a mixture of nitric/ hydrofluoric acid and analyzed by inductively coupled plasmaatomic emission spectroscopy (ICP-AES, Seiko Electronics SPS7700). Samples obtained from the cathode products before distillation were taken from the surface of dendritic cathode deposits. Their representativity of the whole product was not always satisfactory as mentioned above. Samples obtained from the cathode products after distillation were taken by cutting consolidated U– Zr ingots.
Fig. 4. Example of temperature/pressure control during distillation experiment.
The microstructure and distributions of U, Zr and Cl in both the cathode products after distillation and the crucible materials were observed by scanning electron microscopy/wavelength-dispersive X-ray spectrometry (SEM/WDS, JEOL JXA-8900RL). To quantify the remaining chlorine in the cathode products after distillation, some samples were subjected to nephelometric analysis based on silver chloride (AgCl) formation. In this method, silver nitrate (AgNO3) was added to an acid solution of the samples and the absorbance of AgCl at 460 and 610 nm was measured to determine the Cl concentration in the samples. The absorbance of the sample solutions diluted 10–100-fold was compared with those of standard solutions. Since the absorbance of standard solutions containing 0.2–10 ppm of Cl ions did not exhibit a linear relationship with the concentration, the concentration of the remaining Cl in the cathode products was expressed as a range that should include the exact value. 3. Results and discussion The major experimental conditions and results of the high temperature distillation tests performed in this study for the evaluation of crucible materials and the characterization of distillation products are summarized in Tables 1 and 2, respectively. 3.1. Behavior of vaporized chlorides In all the experiments performed in this study, no residual chlorides were visually observed in the crucibles containing the cathode products, indicating that the accompanying chlorides were almost completely vaporized. In the earlier experiments in this study, the vaporized chlorides were deposited on the inner surface of the connection part and began to plug their diffusion path to the condensation part. This undesirable solidification was considered to be due to the connection part being cooled from its bottom through thermal contact with the water-cooled connection part, despite being heated by the surrounding electric heater. In Run Z-2, the apparatus was modified by inserting an alumina ring as thermal insulation between the connection and condensation parts to avoid the excessive cooling of the connection part. As a result, the amount of chlorides deposited in the connection part was decreased and the recovery ratio of the chloride, which was defined as below, reached 95.6%. Recov ery ratio ¼ Amount of chloride collected in condensation part ðgÞ Amount of v aporized chloride ¼ Decrease in weight of v aporization part ðgÞ
Visual observation after the distillation experiments showed that most of the vaporized chloride did not directly reach the condensation part but precipitated in the connecting part, fell as drops and finally solidified in the condensation part. It may have been advantageous to recover the vaporized chloride in the liquid state to increase the packing density of the collected chloride and to inhibit the accumulation of chloride at the bottom of the condensation part. However this would require careful design to prevent both the revaporization of condensed chloride due to overheating and solidification in an undesirable shape through cooling in an unexpected manner. In Table 1, the decrease in the weight of the vaporization part and the total increase in the weight of the connecting and condensation parts are shown. In this study, the former was considered to correspond to the amount of vaporized chloride, which is almost equal to the amount of chloride electrolyte accompanying the cathode product. Similarly, the latter was considered equivalent to the amount of vaporized chloride, which was accounted for in the entire distillation apparatus. The amount of chloride accompanying
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M. Iizuka et al. / Journal of Nuclear Materials 448 (2014) 259–269 Table 1 Summary of experimental conditions and results of high temperature distillation tests for evaluation of crucible materials. Run no.
M-1
M-2
M-3
M-4
M-5
Crucible Dimension (O.D./height, in mm) Substrate material Coating material Thickness of coated layer (lm) 3
29/53 Y2O3 – –
29/53 ZrO2 – –
29/53 Graphite ZO-MOD1 500
29/53 Tungsten ZrN/ZO-MOD1 100/500
29/53 Graphite ZrO22 300
Used cathode product
LiCl–KCl–UCl3
Engineering-scale electrorefining test E-4 [19]
Engineering-scale electrorefining test E-4 [19]
Engineeringscale electrorefining test E-3 [19]
Engineeringscale electrorefining test E-4 [19]
Evaluation of cathode product before distillation4 Loaded amount (g) 10.03 Chloride content (wt%) 1005 Zr in metal (wt%) –
27.61 27.7 0.39
27.91 27.7 0.39
21.85 13.1 0.14
27.55 27.7 0.39
Distillation parameters Distillation temperature (K) Hold time at distillation temperature (h) Melting temperature (K) Hold time at melting temperature (h) Pressure during distillation (Pa)
1273 1 1673 1 10–300
1273 1 1673 1 10–300
1273 1 1673 0.5 5–300
1273 1 1673 1 10–300
Evaluation cathode product after distillation Chloride content (wt%) – Residual chlorine (ppm) – Weight of product(g) – Zr in metal (wt%) –
24.1 NA 20.96 1.03
18.5 NA 22.75 1.33
35.6 NA 14.07 1.01
18.4 125–312 22.49 1.50
Behavior of chlorides Vaporized chloride: a (g)6 Deposited at connection part: b (g) Deposited at condensation part: c (g) Deposited at other parts: d (g) Recovered ratio: c/a (%) Material mass balance = (b + c + d)/a (%)
29.6 1.14 19.79 7.51 66.9 96.1
16.87 5.34 3.88 7.28 23.0 97.8
Same as M-2
20.92 3.25 9.85 7.53 47.1 98.6
Same as M-2
Inner surface of crucibleturned gray or brown and stripped off
Poor release of ingot. Inner surface of crucible turned brown and stripped off
Ingot stuck to crucible and could not be taken out. Coated layer turned reddish brown and remained on crucible surface
Smooth release of ingot
Smooth release of ingot
Notes
1 2 3 4 5 6
1273 1 1673 0.5 10–100
Commercially available coating material with ZrO2 base. Estimated from weight of applied coating materials. Mixture of ZrO2 beads and powder originally developed for this study. Analytical results of cathode deposit obtained before distillation experiments. Chlorides were used alone to investigate interaction with crucible material. Estimated from decrease in weight of vaporization part of distillation apparatus.
the cathode product was precisely re-evaluated using the result of each distillation experiment, and is included in the same table for comparison with the value determined after the electrorefining operation using a small sample taken from the surface of the dendritic cathode product. The material mass balance of the chloride obtained through each distillation experiment was evaluated using these data and is also shown in Table 1. Fig. 5 shows the material mass balance of the chloride evaluated for all the high temperature distillation experiments performed at various batch sizes using the same apparatus. The material mass balance was nearly 100% and the leakage from the apparatus was slight, if any, in the small-scale experiments, in which less than 30 g of the cathode product was used. In the scaled-up experiments, however, the amount of leakage significantly increased, although the relationship between the material mass balance and the amount of the loaded cathode product was not completely consistent. This result indicates that the increase in the amount of vaporized chloride per unit time caused a temporary increase in pressure inside the distillation apparatus and consequently increased the loss of chloride due to leakage. On the other hand, the variation in the amount of leakage under similar conditions probably originated from the unequal physical contact between the graphite parts comprising the apparatus in each experiment. In the design of engineering-scale or practical distillation equipment, it is necessary to pay close attention to the gas-tightness at the joints between its compo-
nents, as well as to establish operation conditions for the distillation equipment, such as the control of the induction power, to prevent an excessive rate of chloride vaporization. 3.2. Evaluation of crucible materials The compatibility of various crucible materials for high-temperature distillation with the chemically active cathode product alloy melt and molten/vaporized chlorides was investigated by performing small-scale experiments using the electrorefiner electrolyte and the cathode products obtained in previous studies. LiCl–KCl–UCl3 (2.25 wt% U) was placed in an Y2O3 crucible and heated to 1273–1673 K under vacuum (Run M-1). Although the chlorides were completely vaporized from the crucible in this operation, the inner surface of the crucible turned gray or brown and peeled off. This peeled off layer contained uranium (71.8 wt%) and yttrium (4.2 wt%) as major constituents, indicating that it is primarily UO2 generated by the reaction
Y2 O3 þ 3UCl3 ! 2YCl3 þ 3UOCl:
ð1Þ
The results from previous injection casting studies [21,22] have shown that Y2O3 has excellent compatibility with uranium and U– Zr alloy melt. However, this material was found to be unsuitable for use in high-temperature distillation owing to the significant reaction with UCl3 as observed in this study.
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Table 2 Summary of experimental conditions and results of high temperature distillation tests for characterization of distillation product and its dependence on zirconium content. Run no.
Z-1
Z-2
Z-3
Z-4
Z-5
Crucible Dimension (O.D./height, in mm) Substrate material Coating material Thickness of coated layer (lm)2
29/53 Graphite ZrO21 300
78/95 Graphite ZrO21 300
94/91 Graphite ZrO21 300
29/53 Graphite ZrO21 300
94/91 Graphite ZrO21 300
Engineeringscale electrorefining test E-5 [19]
Engineering-scale electrorefining test E-5 [19]
Electrorefining step immediately before in fuel cycle test [20]
Engineering-scale electrorefining test E-5 [19]
Electrorefining step immediately before in fuel cycle test [20]
Evaluation of cathode product before distillation3 Loaded amount (g) 37.04 Chloride content (wt%) 46.5 Zr in metal (wt%) 6.32
380.1 46.5 6.32
1008 N/A4 N/A4
62.2 80.4 16.29
839 N/A4 N/A4
Distillation parameters Distillation temperature (K) Hold time at distillation temperature (h) Melting temperature (K) Hold time at melting temperature (h) Pressure during distillation (Pa)
1473 1 1673 1 10–300
1273 2 1673 1 10–300
1573 2 15735 –5 10–300
1273 2 1673 1 10–300
1573 2 15735 –5 10–300
Evaluation cathode product after distillation Chloride content (wt%) Residual chlorine (ppm) Weight of product(g) Zr in metal (wt%)
20.6 140–280 29.42 0.89
21.4 118–236 288.7 0.89
32.9 N/A 676 3.66
76.1 943–1885 4.81 11.9
48.2 N/A 435 12.8
Behavior of chlorides Vaporized chloride: a (g)6 Deposited at connection part: b (g) Deposited at condensation part: c (g) Deposited at other parts: d (g) Recovered ratio: c/a (%) Material mass balance (b + c + d)/a (%)
24.15 5.91 16.25 2.23 67.3 101.0
81.21 2.24 77.66 0.68 95.6 99.2
332 8 284 9 85.5 90.7
47.84 42.82 0.01 1.79 0.0 93.3
404 20 363 1 89.9 95.0
Dense and uniform product
Massive product thin gray/ brown surface layer on the top of ingot dense section view with metallic shine
Massive product
Unified and dense product Zr-rich surface layer Zr-rich phase of around 100 lm in size deposited
Many vacancy remained in product
Used cathode product
Notes
1 2 3 4 5 6
Mixture of ZrO2 beads and powder originally developed for this study. Estimated from weight of applied coating materials. Analytical results of cathode deposit obtained before distillation experiments. Exact value are not available since two or three stock of cathode products were used together. Distillation and melting operations were conducted in one step. Estimated from decrease in weight of vaporization part of distillation apparatus.
In Run M-2, where a cathode product of a low Zr content (1.03 wt%) was placed in a ZrO2 crucible without any coating, the U–Zr ingot after the distillation stuck to the crucible and could not be removed without applying s strong impact to the crucible. The inner surface of the ZrO2 crucible turned brown and peeled off. Since this material contained 89.3 wt% U, this indicates that the surface of this crucible was attacked by a reaction between ZrO2 and the U–Zr alloy melt or between UCl3 and Y2O3 which was contained in the ZrO2 crucible at a concentration of 5 mol% as a stabilizing agent. By considering that ZrO2 castable crucible could be used repeatedly in cathode processing tests carried out in INL [17], the temperature at which Run M-2 was performed (up to 1673 K) may have been too high to expect compatibility with the U–Zr alloy melt. Similarly, the U–Zr ingot after distillation strongly stuck to the graphite crucible coated with the commercial ZrO2 coating material (ZO-MOD) and could not be removed without breaking the crucible (Run M-3). Although most of the coated layer remained on the crucible surface, it turned reddish brown, probably owing to the reaction between ZrO2 and U–Zr alloy or between UCl3 and Y2O3 as in the case of the pure ZrO2 crucible. On the other hand, the visual observation after breaking the crucible indicated that the coated layer did not strongly stick to the U–Zr ingot and
could be easily separated. When the same coating was applied to a W crucible, the U–Zr ingot could be removed without causing any damage to the crucible. These results indicate that the most important factor determining the difficulty in using this coating material is its interaction with the graphite substrate rather than that with the U–Zr alloy melt. In Run M-4, where a W crucible double-coated with ZO-MOD and ZrN was used, the U–Zr ingot was released from the crucible without causing any damage to it. However, no further advantages over the simple originally developed ZrO2 coating mentioned below could be found, since part of the coated layer on the W substrate was removed with the ingot and a laborious coating procedure could have to be performed again before the reuse of the crucible. When considering the use of W as a crucible material in the high-temperature distillation process, it also has to be considered that the embrittlement of W due to its recrystallization above 1300 K probably limits its maximum number of uses. When the graphite crucible was used with the originally developed ZrO2 coating material, the U–Zr ingot could be removed by simply inverting the crucible (Run M-5). The coated layer was sometimes removed with the ingot and sometimes remained inside the crucible. In either case, the coated layer was easily removed and did not strongly adhere to the crucible, as shown in
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Fig. 5. Relation between material mass balance of chloride and initial amount of it contained in cathode product used in distillation experiments.
Fig. 6. A cross-sectional view of the removed coated layer observed by SEM and WDS is shown in Fig. 7. The exchange of Zr in the coated layer for U in the U–Zr alloy melt occurred within approximately 50 lm from the interface, indicating that ZrO2 reacted with U. In the inner region of the coated layer, the original structure formed by ZrO2 beads and powder remained as it was before use and hardly any U was found. Fig. 8 shows a cross-sectional view of the graphite crucible near the interface with the coated layer. It can be seen that a thin Zr-rich layer of about 10 lm thickness formed at the surface and that a small amount of U entered the graphite substrate to a depth of approximately 200 lm. The mechanism of such U penetration is not clear at present, since it is shown in Fig. 7 that only a very small amount of U reached further than the interfacial region of the ZrO2 coating layer. In any case, however, the amount of U in the graphite substrate was too small to cause problems concerning the mechanical strength or demolding of the crucible. By adopting the ZrO2 coating developed in the present study, the release of U–Zr ingots and the removal of the coated layer from both the U–Zr ingot and the crucible by remote handling are expected to become easier, although it is inevitable that some U will be lost in the form of UO2 by the reaction between the U–Zr alloy melt and the coating material. Table 3 shows the estimated distribution of U before and after one of the scaled-up distillation experiments, where the proposed coating material was applied to a graphite crucible. The U contents in the cathode product, U–Zr ingot and coated layer after distillation were determined by the
Fig. 7. Cross-sectional view of originally developed ZrO2 coating layer after distillation experiment.
Fig. 8. Cross-sectional view of graphite crucible after removal of originally developed ZrO2 coating layer.
Table 3 Estimated distribution of uranium before and after a scaled-up high temperature distillation test.
Fig. 6. U–Zr distillation product ingot and graphite crucible after removal of originally developed ZrO2 coating layer.
Evaluation of cathode product before distillation Amount used in distillation (g) Chloride content in cathode product (wt%) U in metallic part of cathode product (wt%) U in metallic part of cathode product (g) = a
1034 53.4 98.3 474
Evaluation of cathode product after distillation Amount of distillation product (g) U in distillation product (wt%) U in distillation product (g) = b
482 98.9 477
Evaluation of coating later after distillation Amount of coating layer (g) U in coating layer (wt%) U in coating layer (g) = c
30.0 43.2 12.9
Material mass balance = (b + c)/a (%)
103
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its weight ratio should decrease from the above mentioned value with future scaling-up of the apparatus. The UO2 remaining in the coated layer after distillation can be converted to its chloride by reaction with Zr and ZrCl4 [23], and has no effect on the final material mass balance in the pyrometallurgical process. 3.3. Effect of Zr in distillation product on its characteristics
Fig. 9. Outer and vertical cross-sectional views of U–Zr ingot obtained in scaled-up distillation experiment using cathode product of lower Zr content (Run Z-2).
chemical analysis of samples taken from them. The content of the electrolyte accompanying the cathode product was evaluated from the change in the weight of the vaporization part of the distillation apparatus during the experiment. This table shows that 12.9 g of U reacted with the coating material, which corresponds to 2.72% of the initial loading (474 g) in the distillation experiment. Since the amount that reacts should increase in proportion to the interface area between the melted cathode product and the coated layer,
Outer and vertical cross-sectional views of the U–Zr distillation product obtained in a scaled-up distillation experiment using a cathode product of a lower Zr content (0.89 wt%., Run Z-2) are shown in Fig. 9. The top of the ingot was uneven and gray or brown, but showed a metallic shine after a thin surface layer was removed. The bottom was flat and a small amount of the coated layer was adhered to it. Although the upper part in the cross-sectional view appeared to be black, the other part showed a metallic shine and formed a dense block. In fact, no difference was found between the chemical compositions of the black and shiny parts of this ingot. Fig. 10 shows the cross-sectional view of the central part of this distillation product observed by SEM and WDS. The product ingot was dense and also homogeneous at the microscopic scale. No segregation of Zr was found. Mo and Pd, which were used as simulated FPs in the electrorefining test [19], were also distributed uniformly in the ingot. A cathode product of intermediate Zr content (3.66 wt%, Run Z3) was also formed into a massive ingot and the unevenness of its surface was similar to that observed for the product of a lower Zr content. When the Zr content in the loaded cathode product was much larger (11.9 wt%, Run Z-4), the roughness of the distillation product
Fig. 10. Vertical cross-sectional view of central part of U–Zr ingot obtained in Run Z-2.
Fig. 11. Outer view of U–Zr ingot obtained in distillation experiment using cathode product of high Zr content (Run Z-4).
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Fig. 12. Cross-sectional view of U–Zr ingot obtained in distillation experiment using cathode product of high Zr content (Run Z-4).
Fig. 13. U–Zr binary phase diagram [24] and composition of phases observed in distillation product obtained in Run Z-4.
Fig. 14. Cross-sectional view of U–Zr ingot obtained in distillation experiment using cathode product of highest Zr content (Run Z-5).
was slightly greater than in the above cases. However, the entire cathode product completely melted and formed a unified ingot, as shown in Fig. 11. A cross-sectional view of this ingot (Fig. 12) also shows that it was dense and favorably consolidated. Three different phases were identified by SEM/WDS analysis: a U-rich phase A (3.9 wt% Zr), which occupied the bulk of the ingot, a deposit B (17.2 wt% Zr) of relatively low Zr content and a highly Zr-rich deposit C (64.7 wt% Zr). The formation path of these phases is explained in Fig. 13 using a U–Zr binary phase diagram [24]. The result of analyzing the sample taken from the distillation product after cooling (11.9 wt% Zr, shown in Table 2) is considered to most appropriately represent the average composition of the metallic part of the initial material used in Run Z-4. This composition at the melting temperature (1673 K) is plotted as point P in Fig. 13. However, the Zr-rich phase C is thought to have continued to exist as a dependent solid phase throughout the distillation experiment as mentioned later. Thus, the actual composition of the melt formed at the melting point might be somewhat more U-rich than
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that indicated by point P in Fig. 13. In the distillation experiment, it is supposed that the metallic part of the loaded material formed a uniform melt at 1673 K and then phase B was deposited during the cooling period. Finally, the remaining part is considered to have solidified as phase A. According to the phase diagram shown in Fig. 13, it is unlikely that the Zr-rich phase C of such a large deposit with an inhomogeneous distribution was formed during the cooling process of the homogeneous melt of the composition indicated by P. In the WDS observation of the cathode deposit [19] used in Run Z4, it was found that the distributions of U and Zr in this material were not uniform and that small spots of a very high Zr concentration existed. These findings indicate that it is highly possible that phase C in the distillation product from Run Z4 originated from the Zr-rich spot found in the cathode product used in this experiment and that this Zr-rich material continued to exist in the solid state throughout the distillation step and finally formed phase C after simultaneous agglomeration and equilibration with the surrounding U–Zr melt. In addition to the Zr-rich spots in the cathode product, ZrO2 beads existed in the coating layer on the graphite crucible as a potential Zr source which located close to U–Zr melt. However, this material may not be related to the formation of phase C even if the ZrO2 beads were reduced by U metal very slowly, since it is unlikely that the resulting Zr metal formed such a large Zr-rich deposit at 1673 K rather than simply dissolved into the U–Zr melt. From the viewpoints of process control and nuclear material accountancy, samples taken from the distillation product are required to have appropriate representativity in its composition, and the existence of the Zr-rich deposit phase C, which might result in inhomogeneity of the product ingot, is undesirable. Various means can be proposed to decrease the amount of this Zr-rich phase remaining in the distillation product. One possibility is to inhibit the electrotransport of Zr from the spent metallic fuel in the anode to the cathode by controlling the operating conditions of the electrorefining step. Although this method may increase the ratio of actinides remaining at the anode, the recovery ratio in the entire pyrometallurgical reprocessing can be maintained sufficiently high by the adoption of suitable designed treatment process for the anode residue from the electrorefining step [25]. Another possibility would be to extend the melting time or electromagnetically stir the U–Zr melt during the distillation step. The plots in Fig. 13 indicate that the entire cathode product will melt to form a uniform U–Zr mixture after phase equilibrium is attained at 1673 K provided that its average composition is within the range investigated in the present study. Even if these means are employed, the deposition of phase B containing a higher Zr concentration than the surrounding bulk may be unavoidable. However, the effect of this phase on the representativity of samples taken from the distillation product is not expected to be significant because of its small dimensions and uniform distribution, as shown in Fig. 12. On the other hand, increasing the melting temperature to shorten the time required to attain phase equilibrium in the cathode product should be discouraged since a higher temperature clearly facilitates the reaction between the U–Zr melt and the crucible or coating material. Fig. 14 shows a cross-sectional view of the distillation product obtained in Run Z-5, which contained a high Zr concentration (12.8 wt%). Although this value was not much larger than that in Run Z-4, this product melted only locally and many vacancies remained in the ingot. The reason for this was simply the insufficient melting temperature (1573 K) to completely melt this material, whose composition could not be exactly evaluated before the distillation since it was a mixture with an unclear ratio that consisted of a few cathode products recovered different times during the electrorefining test.
Mechanical milling of the cathode product introduced in Run Z3 and Z-5 potentially has an influence on the characteristics of the distillation product through homogenization of Zr concentration in the cathode product and greater oxidation by increased surface area. Up to the present, however, effect by these factors has been overcome by the influence of averaged Zr content and not appeared definitely. 3.4. Cl content in distillation products The Cl contents in some of the U–Zr ingots obtained in the small-scale distillation experiments using cathode products of various Zr contents are shown in Tables 1 and 2. The amount of Cl remaining in the ingots of a lower Zr content was limited to a few hundred ppm (Runs M-5, Z-1 and Z-3). On the other hand, it was much higher, probably over 1000 ppm, in the ingot containing more Zr (Run Z-4). These results indicate that the Cl content in the cathode products was decreased to less than 1/200 of that after electrorefining by performing the high-temperature distillation. In the fabrication of U–Pu–Zr metallic fuel elements for irradiation tests at Joyo [26], the maximum allowed Cl impurity concentration in fuel alloy slugs was determined as 1000 ppm on the basis of the results of previous studies on metallic fuel fabrication in Japan and the specifications of EBR-II driver fuels. Although the cathode products after distillation obtained from Runs M-5, Z-1 and Z-3 satisfy this criterion, that obtained from Run Z-4 does not meet the criterion. The Cl tended to be distributed at the voids between metal grains, or at the gaps between the consolidated metal and the Zrrich layer formed on the surface of the ingot. Zirconium preferentially captures oxygen around it and forms a layer of its oxide at the surface of the metal grains or the entire ingot. This oxide layer is considered to hamper the complete melting of the cathode product to form a massive ingot, as seen above, as well as the distillation of the anode residue from electrorefining [25]. Such behavior of Zr probably affected the difference in the amount of residual Cl among the distillation products of various Zr contents. Considering these results and findings together, the amount of Zr in the distillation product should be limited below a certain level to decrease the Cl content in the product ingot fed to the subsequent fuel fabrication steps. Such control would be possible by adjusting the operating conditions of the electrorefining step or by blending cathode products of different Zr contents, for example. In addition, attention should be directed to the oxidation of Zr in the cathode product both before and during the high-temperature distillation step, which leads to the formation of voids and gaps in the product ingot and could isolate chlorides contained in the cathode product from vaporization. 4. Conclusions High-temperature distillation experiments were performed using U–Zr cathode products of various compositions obtained in previous electrorefining tests to obtain knowledge on suitable operation conditions and equipment design such as the container material compatible with both high-temperature chlorides and the chemically active U–Zr alloy melt. The LiCl–KCl–UCl3 electrolyte adhering to the U–Zr cathode products was almost completely vaporized at 1273–1573 K under pressure of 10–300 Pa. Although the material balance of chlorides was excellent for a smaller amount of the loaded cathode product, it deteriorated with increasing quantity of the processed material, probably owing to leakage from the distillation apparatus caused by the temporary increase in internal pressure.
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After the vaporization of the chlorides, the remaining cathode product was further heated at 1573–1673 K for consolidation. Massive ingots were obtained from cathode products of a wide range of Zr contents (0–11.9 wt%). For cathode products of a higher Zr content (12.8 wt%), the melting temperature of 1573 K was found to be insufficient for complete consolidation. Three different phases were identified in the distillation product containing an average Zr content of 11.9 wt%. A U-rich bulk (3.9 wt% Zr) and a deposit of a relatively low Zr content (17.2 wt% Zr) were considered to be formed during the cooling process of the distillation product. It is highly possible that another Zr-rich deposit in the product (64.7 wt% Zr) resulted from Zr-rich spots that originally existed in the cathode product used in this experiment. The amount of this phase should be decreased by some means, such as by adjusting the operating conditions of the elecrorefining step, extending the melting time, or electromagnetic stirring of the U–Zr melt, to ensure sufficient homogeneity of the distillation product. The Cl content in the cathode products was decreased to less than 1/200 of that after electrorefining by performing distillation. The amount of residual Cl was limited to a few hundred ppm in the ingots of a lower Zr concentration, while it was markedly larger at a higher Zr content. To maintain the Cl content below an appropriate level in materials used for recycled fuel fabrication, the Zr content in the distillation product should be controlled to a sufficiently low level by optimization of the operating procedures and conditions in the electrorefining and distillation steps. The zirconia coating material originally developed in this study showed superior performance in inhibiting the reaction between the melted U–Zr alloy melt and the graphite crucible and also in easy the release of the U–Zr ingot from the crucible. Although a small amount of the U in the cathode product reacted with this coating material, no effect on the material mass balance of recovery ratio in the entire pyrometallurgical process is expected since the UO2 remaining in the coated layer can be converted to its chloride by reaction with Zr metal and ZrCl4 and finally returned to the process. Acknowledgements The authors are grateful to Messrs. H. Nakamura and K. Murakami of Toshiba Corporation for their experimental and analytical support as well as Dr. K. Nakamura of CRIEPI for his useful suggestion and discussion on the SEM/EPMA results. This paper presents the results of the ‘‘Development of technologies for processing of
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anode residue and cathode product from electrorefining of metallic nuclear fuel’’ and ‘‘Development of metal fuel cycle technology for closing FBR fuel cycle’’ projects entrusted to CRIEPI by the Ministry of Education, Culture, Sports, Science and Technology (MEXT). References [1] T. Koyama, T. Ogata, M. Myochin, Y. Arai, in: Proc. GLOBAL 2011, Makuhari, Japan, Dec. 11–16, 2011. [2] T.A. Todd, L.K. Felker, J.D. Vienna, J. Bresee, S. Lesica, in: Proc. GLOBAL 2009, Paris, France, Sep. 8–11, 2009. [3] H. Lee, J.M. Hur, D.H. Ahn, I.T. Kim, J.H. Lee, in: Proc. GLOBAL 2009, Paris, France, Sep. 811, 2009. [4] K.V.G. Kutty, P.R.V. Rao, B. Raj, in: Proc. GLOBAL 2009, Paris, France, Sep. 8–11, 2009. [5] T. Koyama, R. Fujita, M. Iizuka, Y. Sumida, Nucl. Technol. 110 (1995) 357–368. [6] T. Koyama, M. Iizuka, Y. Shoji, R. Fujita, H. Tanaka, T. Kobayashi, M. Tokiwai, J. Nucl. Sci. Technol. 34 (1997) 384–393. [7] M. Iizuka, K. Uozumi, T. Inoue, T. Iwai, O. Shirai, Y. Arai, J. Nucl. Mater. 299 (2001) 32–42. [8] K. Uozumi, M. Iizuka, T. Kato, T. Inoue, O. Shirai, T. Iwai, Y. Arai, J. Nucl. Mater. 325 (2004) 34–43. [9] T. Kato, T. Inoue, T. Iwai, Y. Arai, J. Nucl. Mater. 357 (2006) 105–114. [10] Y. Sakamura, T. Omori, Nucl. Technol. 171 (2010) 266–275. [11] M. Iizuka, Y. Sakamura, T. Inoue, J. Nucl. Mater. 359 (2006) 102–113. [12] T. Kato, T. Murakami, K. Uozumi, T. Koyama, M. Ougier, A. Rodrigues, S.V. Winckel, R. Malmbeck, J.-P. Glatz, in: Proc. GLOBAL 2011, Makuhari, Japan, Dec. 11–16, 2011. [13] O.K. Kubachewski, C.B. Alock, Metallurgical Thermochemistry, fifth ed., Pergamon Press, 1979. [14] E.A. Brandes (Ed.), Smithells Metals Reference Book, sixth ed., Butterworths, London, 1983. [15] T. Hijikata, T. Koyama, T. Usami, S. Kitawaki, T. Hinozaki, T. Kobayashi, in: Proc. GLOBAL 2003, New Orleans, LA, USA, Nov. 16–20, 2003. [16] T. Hijikata, T. Koyama, T. Usami, S. Kitawaki, T. Shinozaki, M. Fukushima, in: I. May, R. Alvares, N. Bryan (Eds.), Recent Advances in Actinide Science, Royal Society of Chemistry Publishing, 2006, pp. 763–772. [17] B.R. Westphal, K.C. Marsden, J.C. Price, D.V. Laug, Nucl. Eng. Technol. 40 (2008) 163–174. [18] M. Iizuka, T. Omori, T. Tsukada, J. Nucl. Sci. Technol. 47 (2010) 244–254. [19] M. Iizuka, K. Uozumi, T. Ogata, T. Omori, T. Tsukada, J. Nucl. Sci. Technol. 46 (2009) 699–716. [20] M. Iizuka, K. Kinoshita, Y. Sakamura, T. Ogata, T. Koyama, Nucl. Technol. 184 (2013) 107–120. [21] T. Ogata, T. Tsukada, in: Proc. GLOBAL 2007, Boise, Idaho, USA, Sep. 9–13, 2007. [22] K. Nakamura, T. Ogata, T. Iwai, K. Nakajima, T. Kato, Y. Arai, K. Uozumi, T. Hijikata, T. Koyama, T. Omori, H. Miura, W. Itagaki, T. Soga, T. Aoyama, Trans. Atom. Energy Soc. Jpn. 10 (2011) 245–256. [23] Y. Sakamura, T. Inoue, T. Iwai, H. Moriyama, J. Nucl. Mater. 340 (2005) 39–51. [24] H. Okamoto, J. Phase Equilib. Diff. 28 (2007) 499–500. [25] M. Iizuka, M. Akagi, T. Omori, Nucl. Technol. 181 (2013) 507–525. [26] K. Nakamura, T. Ogata, H. Kikuchi, T. Iwai, K. Nakajima, T. Kato, Y. Arai, T. Koyama, W. Itagaki, T. Soga, T. Aoyama, in: Proc. GLOBAL 2011, Makuhari, Japan, Dec. 11–16, 2011.
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Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
High-temperature distillation and consolidation of U–Zr cathode product from molten salt electrorefining of simulated metallic fuel Masatoshi Iizuka ⇑, Masaaki Akagi, Tadafumi Koyama Central Research Institute of Electric Power Industry, Iwato-kita 2-11-1, Komae, Tokyo 201-8511, Japan
a r t i c l e
i n f o
Article history: Received 24 May 2013 Accepted 10 February 2014 Available online 18 February 2014
a b s t r a c t High-temperature distillation experiments were performed using U–Zr cathode products of various compositions to obtain knowledge on suitable operation conditions and equipment design such as the container material. The LiCl–KCl–UCl3 electrolyte adhering to the U–Zr cathode products was almost completely vaporized at 1273–1573 K, under pressure of 10–300 Pa. Massive ingots were obtained from the remaining cathode products by heating them at 1573–1673 K. Three different phases were identified in a distillation product of a higher Zr content. A U-rich bulk (3.9 wt% Zr) and a deposit of a relatively low Zr content (17.2 wt% Zr) were considered to be formed during the cooling process of the distillation product. Another Zr-rich deposit (64.7 wt% Zr), which might cause the inhomogeneity of product ingots, was expected to result from Zr-rich spots that originally existed in the cathode product. The Cl content in the cathode product was decreased by distillation to less than 1/200 of that after electrorefining, while it was markedly larger at a higher Zr concentration. To limit the amount of Zr-rich deposit and the Cl content, the amount of Zr in the distillation product should be controlled to a sufficiently low level by optimization of the operating procedures and conditions in the electrorefining and distillation steps. The zirconia coating material developed in this study showed superior performance in inhibiting reaction between the melted U–Zr alloy melt and the graphite crucible and also in the easy release of the U–Zr ingot from the crucible. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Development of innovative nuclear fuel cycle technology that has the advantages of economic and safe power generation as well as proliferation resistance is strongly expected to be an effective measure for achieving environmental sustainability and satisfying the increasing energy demand. Metallic fuel cycle technology, consisting of a metal fuel fast reactor, pyrometallurgical reprocessing and fuel fabrication by injection casting, has been attracting increasing attention as one of the most promising nuclear fuel cycle technologies for the next generation satisfying the above requirements [1–4]. In pyrometallurgical reprocessing, the recovery of actinide elements and the decontamination from fission product elements (FPs) are conducted in an electrorefining step [5] using molten lithium chloride-potassium chloride (LiCl–KCl) as an electrolyte (Fig. 1). The actinide elements are electrochemically dissolved from chopped spent metal fuel pins loaded in anode baskets made of ⇑ Corresponding author. Address: Central Research Institute of Electric Power Industry, Nuclear Technology Research Laboratory, Iwato-kita 2-11-1, Komae, Tokyo 201-8511, Japan. Tel.: +81 3 3480 2111; fax: +81 3 3480 7956. E-mail address: [email protected] (M. Iizuka). http://dx.doi.org/10.1016/j.jnucmat.2014.02.015 0022-3115/Ó 2014 Elsevier B.V. All rights reserved.
stainless steel. Uranium, which has the highest redox potential among the actinides, is preferentially reduced and collected on a solid steel cathode [6]. Plutonium and other transuranium elements are recovered into a liquid Cd cathode together with the remaining U since their activities are greatly reduced in liquid Cd [7–9]. Before electrorefining cathode products are sent to a fuel fabrication step, the electrolyte adhering to them has to be removed, and the products must be consolidated in the form of an ingot for storage and adjustment of the fuel composition. This treatment, called ‘cathode processing’, is carried out by hightemperature distillation at a reduced pressure. Recently, development of an electrochemical reduction method for oxide materials, either spent oxide fuels or products from light water reactor fuel reprocessing, is also in progress with the aim of supplying the metallic fuel for the start-up of fast reactors [10–12]. In the electrochemical reduction step, the oxide materials are immersed in LiCl–LiO2 electrolyte and cathodically reduced to metals. Before the reduction product is processed in the electrorefining step to remove lanthanide and noble metal FPs, the adherent electrolyte must be removed to inhibit the migration of O2 ions to the electrorefiner, which depletes the U in LiCl–KCl by formation of a UO2 deposit and to an increase in the melting point of the
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Fig. 1. Schematic diagram of electrorefining process for metallic fast reactor fuel.
electrorefiner electrolyte due to an increase in Li/(Li + K) ratio in the electrolyte. High-temperature distillation is also planned for the removal of the LiCl–LiO2 electrolyte adhering to the reduction product. Most of the fundamental thermodynamic properties related to the high temperature distillation process have been obtained, such as the temperature dependence of the vapor pressure of metals and chloride species contained in the cathode product from the electrorefining step [13,14]. It has also been found from small-scale distillation experiments that the vaporization of LiCl–KCl at reduced pressure begins at approximately 1073 K and that it is almost completed at approximately 1273 K even when UCl3 is contained in the processed chlorides [15,16]. At Idaho National Laboratory (INL), a large-scale ‘cathode processor’ with a maximum cathode product capacity of 80 kg has been developed for the treatment of spent metallic fuels from Experimental Breeder Reactor-II (EBR-II). The results of high temperature distillation using a large amount of solid cathode product were reported [17]. The salt adhering to the cathode product was vaporized and the products, consisting mainly of U metal, were melted normally at 1473 K and at less than 100 Pa, using a zirconia-based castable crucible (78 wt% zirconia, 20 wt% alumina and 2 wt% carbon) as a lining material for the graphite secondary container. The zirconia castable crucibles were durable against repeated use for at least 4–6 times, although the loss of U of a few percent inevitably occurred owing to the chemical reaction with the crucible material. Spent metallic fast reactor fuel contains about 10 wt% Zr. Since the actinides are anodically dissolved into the molten chloride electrolyte prior to Zr during the electrorefining step, the solid cathode product obtained near the end of each batch of electrorefining operation tends to be more Zr-rich than that recovered at the beginning [18] depending on the operating conditions. Therefore, the cathode processing step must be able to process cathode products of much wider composition range (U/Zr ratio) than that in spent fuel. However, operation conditions required for the treatment of such a variety of cathode products have not yet been clarified. In addition, an ideal cathode processor crucible material compatible with both the chemically active cathode product alloy melt and molten/vaporized chlorides has not yet been found. Means of minimizing the actinide loss due to the reaction with the crucible material and the waste generated in the cathode-processing step are also needed. In the present study, small-scale high-temperature distillation experiments were performed using U–Zr cathode products obtained by previous electrorefining tests [19] to obtain knowledge on suitable operation conditions for consolidating cathode
products of a variety of U/Zr ratios, equipment design and crucible materials compatible with both U–Zr alloy melt and chlorides. 2. Experimental 2.1. Experimental apparatus A schematic view of the high-temperature distillation apparatus is shown in Fig. 2. It was installed in a high-purity Ar glove box to handle chlorides and active metals, which are highly sensitive to oxygen and moisture. The apparatus consists of three major parts: vaporization part at the top that heats the cathode product, a connecting part in the middle where the vaporized chlorides diffuse to the lower-temperature region, and a condensation part at the bottom to collect the chlorides in the solid state. The three parts were made of graphite. The vaporization part can be heated to up to 1873 K by induction heater (20 kW, 28 kHz). An electric resistance heater is installed around the connecting part to avoid plugging by the solidification of vaporized chlorides. The condensation part is placed on a water-cooled base to attain a high recovery ratio of the vaporized chlorides by solidifying them. The temperature of each component was monitored with thermocouples during each experiment. The three major parts of the apparatus were placed in a watercooled gas-tight container made of stainless steel that could be evacuated to approximately 10 Pa. This container is divided into three sections in the vertical direction. The upper two sections can be moved digitally and remotely upward/downward using stepper motors for easier assembly/disassembly and maintenance of the apparatus. First, the vaporization area was designed to accommodate three small crucibles (29 mm in outer diameter, 53 mm in height) to examine the different types of cathode products and/or crucible materials simultaneously at a small batch size. This design was changed later to accommodate one larger crucible (up to 94 mm in outer diameter and 91 mm in height) so that a much larger amount of cathode product could be processed in a single experiment. After increasing the batch size, the distillation apparatus was also used as a ‘cathode processor’ for engineering-scale fuel cycle tests using simulated oxide/metal fuels [20]. 2.2. Crucible materials As candidate crucible materials compatible with the melted cathode product alloy and molten/vaporized chlorides, yttria
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Fig. 2. Schematic view of high-temperature distillation apparatus.
(Y2O3), zirconia (ZrO2) and zirconium nitride (ZrN) were selected. The yttria crucible used in the present study had a 29 mm O.D., a 25 mm I.D., a 53 mm height and a 99.5% in purity. Zirconia was used both as a material for the crucible itself and as a coating material applied on a graphite or W substrate. The zirconia crucible (stabilized with 3 mol% Y2O3) was of the same size as the yttria crucible. Although a commercially available zirconia coating material (ZYP coating corporation, ZO-MOD) was used in earlier experiments, an original coating material was developed in parallel to minimize the content of impurities, such as Al2O3 and P2O5 sintering additives, in the finished coating layer. This original coating material, which was used in later small-scale experiments and all of the large-scale experiments, was a mixture of zirconia beads (25–106 lm in size, about 5 mol% Y2O3) and powder (40 nm, 3 mol% Y2O3). This mixture can be coated by brushing to form a layer of more than 300 lm thickness without the occurrence of cracks during thermal treatment before use, unlike in the case of using blended ZrO2 powders of different particle sizes. A cross-sectional view of the original coating layer on a graphite substrate is shown in Fig. 3. The ZrN powder used in this study (ZR108PB, Kojundo Chemical Laboratory Co., Ltd., 98%) was very fine, and it was impossible to form a dense and thick layer of ZrN by brushing. Thus, this material was used to make a thinner (about 100 lm) secondary layer on a primary ZO-MOD layer that had been applied on the surface of a W metal substrate.
Fig. 3. Cross-sectional view of originally developed ZrO2 coating applied on graphite substrate.
After brushing the graphite or W substrate with the commercial or original zirconia coating material, the crucible was heated at 1373 K for 1 h under vacuum to form a mechanically durable layer. For the preparation of ZrN/ZrO2 double coating layers on the W substrate, ZrN powder was applied on a W crucible, that had already been coated with ZO-MOD by the above-mentioned procedure, then the crucible was heated again at 1373 K for 1 h. Although a much higher temperature is generally required to sinter ZrN powder, the pretreatment was carried out at a lower temperature in the present study to avoid the embrittlement of W due to its recrystallization and to inhibit interdiffusion between the ZrO2 and ZrN layers.
2.3. Cathode products used in high-temperature distillation experiments The cathode products used in the small-scale and part of the large-scale experiments in this study were obtained from engineering-scale electrorefining tests previously performed using U– Zr alloy [19]. As mentioned above, the Zr concentration in the solid cathode deposit tends to increase gradually during each batch of electrorefining operation. Since the cathode deposit was scraped off several times during the electrorefining tests and each deposit was separately stored, a variety of cathode product compositions (Zr content, morphology, and content of accompanying chloride electrolyte) were obtained in the engineering-scale electrorefining tests [19]. In this study, the behavior of these cathode products of different compositions upon high-temperature distillation was investigated. In Run Z-3 and Z-5, which were performed as part of the engineering-scale fuel cycle tests using simulated oxide/metal fuels [20], 700–1000 g of the cathode product was obtained from the electrorefining step carried out immediately prior to the distillation. The cathode product was crushed with a laboratory mill just before these runs to increase its packing density in the crucible. As in the case of the single electrorefining test, the cathode products obtained in the fuel cycle tests had different compositions depending on the time of their collection during each batch of electrorefining operation. Similarly, the samples taken from the surface of
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these cathode products after the electrorefining operation were not sufficiently representative of the cathode product. 2.4. Experimental procedure An example of temperature/pressure control in the high-temperature distillation apparatus during an experiment is shown in Fig. 4. After placing the crucibles loaded with the cathode products in the vaporization area, the whole apparatus was evacuated to about 1 Pa at room temperature. Then, it was disconnected from the vacuum line and the vaporization area was heated to 873– 973 K to melt the accompanying chlorides to prevent explosive boiling. Then, the apparatus was evacuated again to 5–10 Pa and the cathode product was heated to distil the chlorides and consolidate the remaining U–Zr. In earlier experiments, the distillation and consolidation were performed in separate stages as shown in Fig. 4. In these experiments, the chlorides were vaporized from the cathode product at 1273–1473 K. Although the apparatus was basically closed during the distillation, it was connected to the vacuum line whenever the internal pressure reached approximately 300 Pa. After the distillation, argon gas was introduced to a pressure of 50 kPa to avoid the revolatilization of the chlorides from the lower part of the apparatus, and the U–Zr alloy remaining in the vaporization part was further heated to 1673 K in an attempt to consolidate it. Finally, the apparatus was cooled slowly to prevent the breakage of the crucible due to a rapid temperature change. The connecting part was heated and maintained at 1073 K during the experiment to prevent it from being plugged by the solidification of vaporized chlorides. 2.5. Analytical methods The amount of chloride electrolyte accompanying the cathode product used for the high-temperature distillation was evaluated from the change in weight after washing with water to dissolve the chloride salts. The samples obtained from the U–Zr cathode products before and after distillation and the crucible materials including the coating layer were dissolved in a mixture of nitric/ hydrofluoric acid and analyzed by inductively coupled plasmaatomic emission spectroscopy (ICP-AES, Seiko Electronics SPS7700). Samples obtained from the cathode products before distillation were taken from the surface of dendritic cathode deposits. Their representativity of the whole product was not always satisfactory as mentioned above. Samples obtained from the cathode products after distillation were taken by cutting consolidated U– Zr ingots.
Fig. 4. Example of temperature/pressure control during distillation experiment.
The microstructure and distributions of U, Zr and Cl in both the cathode products after distillation and the crucible materials were observed by scanning electron microscopy/wavelength-dispersive X-ray spectrometry (SEM/WDS, JEOL JXA-8900RL). To quantify the remaining chlorine in the cathode products after distillation, some samples were subjected to nephelometric analysis based on silver chloride (AgCl) formation. In this method, silver nitrate (AgNO3) was added to an acid solution of the samples and the absorbance of AgCl at 460 and 610 nm was measured to determine the Cl concentration in the samples. The absorbance of the sample solutions diluted 10–100-fold was compared with those of standard solutions. Since the absorbance of standard solutions containing 0.2–10 ppm of Cl ions did not exhibit a linear relationship with the concentration, the concentration of the remaining Cl in the cathode products was expressed as a range that should include the exact value. 3. Results and discussion The major experimental conditions and results of the high temperature distillation tests performed in this study for the evaluation of crucible materials and the characterization of distillation products are summarized in Tables 1 and 2, respectively. 3.1. Behavior of vaporized chlorides In all the experiments performed in this study, no residual chlorides were visually observed in the crucibles containing the cathode products, indicating that the accompanying chlorides were almost completely vaporized. In the earlier experiments in this study, the vaporized chlorides were deposited on the inner surface of the connection part and began to plug their diffusion path to the condensation part. This undesirable solidification was considered to be due to the connection part being cooled from its bottom through thermal contact with the water-cooled connection part, despite being heated by the surrounding electric heater. In Run Z-2, the apparatus was modified by inserting an alumina ring as thermal insulation between the connection and condensation parts to avoid the excessive cooling of the connection part. As a result, the amount of chlorides deposited in the connection part was decreased and the recovery ratio of the chloride, which was defined as below, reached 95.6%. Recov ery ratio ¼ Amount of chloride collected in condensation part ðgÞ Amount of v aporized chloride ¼ Decrease in weight of v aporization part ðgÞ
Visual observation after the distillation experiments showed that most of the vaporized chloride did not directly reach the condensation part but precipitated in the connecting part, fell as drops and finally solidified in the condensation part. It may have been advantageous to recover the vaporized chloride in the liquid state to increase the packing density of the collected chloride and to inhibit the accumulation of chloride at the bottom of the condensation part. However this would require careful design to prevent both the revaporization of condensed chloride due to overheating and solidification in an undesirable shape through cooling in an unexpected manner. In Table 1, the decrease in the weight of the vaporization part and the total increase in the weight of the connecting and condensation parts are shown. In this study, the former was considered to correspond to the amount of vaporized chloride, which is almost equal to the amount of chloride electrolyte accompanying the cathode product. Similarly, the latter was considered equivalent to the amount of vaporized chloride, which was accounted for in the entire distillation apparatus. The amount of chloride accompanying
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M. Iizuka et al. / Journal of Nuclear Materials 448 (2014) 259–269 Table 1 Summary of experimental conditions and results of high temperature distillation tests for evaluation of crucible materials. Run no.
M-1
M-2
M-3
M-4
M-5
Crucible Dimension (O.D./height, in mm) Substrate material Coating material Thickness of coated layer (lm) 3
29/53 Y2O3 – –
29/53 ZrO2 – –
29/53 Graphite ZO-MOD1 500
29/53 Tungsten ZrN/ZO-MOD1 100/500
29/53 Graphite ZrO22 300
Used cathode product
LiCl–KCl–UCl3
Engineering-scale electrorefining test E-4 [19]
Engineering-scale electrorefining test E-4 [19]
Engineeringscale electrorefining test E-3 [19]
Engineeringscale electrorefining test E-4 [19]
Evaluation of cathode product before distillation4 Loaded amount (g) 10.03 Chloride content (wt%) 1005 Zr in metal (wt%) –
27.61 27.7 0.39
27.91 27.7 0.39
21.85 13.1 0.14
27.55 27.7 0.39
Distillation parameters Distillation temperature (K) Hold time at distillation temperature (h) Melting temperature (K) Hold time at melting temperature (h) Pressure during distillation (Pa)
1273 1 1673 1 10–300
1273 1 1673 1 10–300
1273 1 1673 0.5 5–300
1273 1 1673 1 10–300
Evaluation cathode product after distillation Chloride content (wt%) – Residual chlorine (ppm) – Weight of product(g) – Zr in metal (wt%) –
24.1 NA 20.96 1.03
18.5 NA 22.75 1.33
35.6 NA 14.07 1.01
18.4 125–312 22.49 1.50
Behavior of chlorides Vaporized chloride: a (g)6 Deposited at connection part: b (g) Deposited at condensation part: c (g) Deposited at other parts: d (g) Recovered ratio: c/a (%) Material mass balance = (b + c + d)/a (%)
29.6 1.14 19.79 7.51 66.9 96.1
16.87 5.34 3.88 7.28 23.0 97.8
Same as M-2
20.92 3.25 9.85 7.53 47.1 98.6
Same as M-2
Inner surface of crucibleturned gray or brown and stripped off
Poor release of ingot. Inner surface of crucible turned brown and stripped off
Ingot stuck to crucible and could not be taken out. Coated layer turned reddish brown and remained on crucible surface
Smooth release of ingot
Smooth release of ingot
Notes
1 2 3 4 5 6
1273 1 1673 0.5 10–100
Commercially available coating material with ZrO2 base. Estimated from weight of applied coating materials. Mixture of ZrO2 beads and powder originally developed for this study. Analytical results of cathode deposit obtained before distillation experiments. Chlorides were used alone to investigate interaction with crucible material. Estimated from decrease in weight of vaporization part of distillation apparatus.
the cathode product was precisely re-evaluated using the result of each distillation experiment, and is included in the same table for comparison with the value determined after the electrorefining operation using a small sample taken from the surface of the dendritic cathode product. The material mass balance of the chloride obtained through each distillation experiment was evaluated using these data and is also shown in Table 1. Fig. 5 shows the material mass balance of the chloride evaluated for all the high temperature distillation experiments performed at various batch sizes using the same apparatus. The material mass balance was nearly 100% and the leakage from the apparatus was slight, if any, in the small-scale experiments, in which less than 30 g of the cathode product was used. In the scaled-up experiments, however, the amount of leakage significantly increased, although the relationship between the material mass balance and the amount of the loaded cathode product was not completely consistent. This result indicates that the increase in the amount of vaporized chloride per unit time caused a temporary increase in pressure inside the distillation apparatus and consequently increased the loss of chloride due to leakage. On the other hand, the variation in the amount of leakage under similar conditions probably originated from the unequal physical contact between the graphite parts comprising the apparatus in each experiment. In the design of engineering-scale or practical distillation equipment, it is necessary to pay close attention to the gas-tightness at the joints between its compo-
nents, as well as to establish operation conditions for the distillation equipment, such as the control of the induction power, to prevent an excessive rate of chloride vaporization. 3.2. Evaluation of crucible materials The compatibility of various crucible materials for high-temperature distillation with the chemically active cathode product alloy melt and molten/vaporized chlorides was investigated by performing small-scale experiments using the electrorefiner electrolyte and the cathode products obtained in previous studies. LiCl–KCl–UCl3 (2.25 wt% U) was placed in an Y2O3 crucible and heated to 1273–1673 K under vacuum (Run M-1). Although the chlorides were completely vaporized from the crucible in this operation, the inner surface of the crucible turned gray or brown and peeled off. This peeled off layer contained uranium (71.8 wt%) and yttrium (4.2 wt%) as major constituents, indicating that it is primarily UO2 generated by the reaction
Y2 O3 þ 3UCl3 ! 2YCl3 þ 3UOCl:
ð1Þ
The results from previous injection casting studies [21,22] have shown that Y2O3 has excellent compatibility with uranium and U– Zr alloy melt. However, this material was found to be unsuitable for use in high-temperature distillation owing to the significant reaction with UCl3 as observed in this study.
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Table 2 Summary of experimental conditions and results of high temperature distillation tests for characterization of distillation product and its dependence on zirconium content. Run no.
Z-1
Z-2
Z-3
Z-4
Z-5
Crucible Dimension (O.D./height, in mm) Substrate material Coating material Thickness of coated layer (lm)2
29/53 Graphite ZrO21 300
78/95 Graphite ZrO21 300
94/91 Graphite ZrO21 300
29/53 Graphite ZrO21 300
94/91 Graphite ZrO21 300
Engineeringscale electrorefining test E-5 [19]
Engineering-scale electrorefining test E-5 [19]
Electrorefining step immediately before in fuel cycle test [20]
Engineering-scale electrorefining test E-5 [19]
Electrorefining step immediately before in fuel cycle test [20]
Evaluation of cathode product before distillation3 Loaded amount (g) 37.04 Chloride content (wt%) 46.5 Zr in metal (wt%) 6.32
380.1 46.5 6.32
1008 N/A4 N/A4
62.2 80.4 16.29
839 N/A4 N/A4
Distillation parameters Distillation temperature (K) Hold time at distillation temperature (h) Melting temperature (K) Hold time at melting temperature (h) Pressure during distillation (Pa)
1473 1 1673 1 10–300
1273 2 1673 1 10–300
1573 2 15735 –5 10–300
1273 2 1673 1 10–300
1573 2 15735 –5 10–300
Evaluation cathode product after distillation Chloride content (wt%) Residual chlorine (ppm) Weight of product(g) Zr in metal (wt%)
20.6 140–280 29.42 0.89
21.4 118–236 288.7 0.89
32.9 N/A 676 3.66
76.1 943–1885 4.81 11.9
48.2 N/A 435 12.8
Behavior of chlorides Vaporized chloride: a (g)6 Deposited at connection part: b (g) Deposited at condensation part: c (g) Deposited at other parts: d (g) Recovered ratio: c/a (%) Material mass balance (b + c + d)/a (%)
24.15 5.91 16.25 2.23 67.3 101.0
81.21 2.24 77.66 0.68 95.6 99.2
332 8 284 9 85.5 90.7
47.84 42.82 0.01 1.79 0.0 93.3
404 20 363 1 89.9 95.0
Dense and uniform product
Massive product thin gray/ brown surface layer on the top of ingot dense section view with metallic shine
Massive product
Unified and dense product Zr-rich surface layer Zr-rich phase of around 100 lm in size deposited
Many vacancy remained in product
Used cathode product
Notes
1 2 3 4 5 6
Mixture of ZrO2 beads and powder originally developed for this study. Estimated from weight of applied coating materials. Analytical results of cathode deposit obtained before distillation experiments. Exact value are not available since two or three stock of cathode products were used together. Distillation and melting operations were conducted in one step. Estimated from decrease in weight of vaporization part of distillation apparatus.
In Run M-2, where a cathode product of a low Zr content (1.03 wt%) was placed in a ZrO2 crucible without any coating, the U–Zr ingot after the distillation stuck to the crucible and could not be removed without applying s strong impact to the crucible. The inner surface of the ZrO2 crucible turned brown and peeled off. Since this material contained 89.3 wt% U, this indicates that the surface of this crucible was attacked by a reaction between ZrO2 and the U–Zr alloy melt or between UCl3 and Y2O3 which was contained in the ZrO2 crucible at a concentration of 5 mol% as a stabilizing agent. By considering that ZrO2 castable crucible could be used repeatedly in cathode processing tests carried out in INL [17], the temperature at which Run M-2 was performed (up to 1673 K) may have been too high to expect compatibility with the U–Zr alloy melt. Similarly, the U–Zr ingot after distillation strongly stuck to the graphite crucible coated with the commercial ZrO2 coating material (ZO-MOD) and could not be removed without breaking the crucible (Run M-3). Although most of the coated layer remained on the crucible surface, it turned reddish brown, probably owing to the reaction between ZrO2 and U–Zr alloy or between UCl3 and Y2O3 as in the case of the pure ZrO2 crucible. On the other hand, the visual observation after breaking the crucible indicated that the coated layer did not strongly stick to the U–Zr ingot and
could be easily separated. When the same coating was applied to a W crucible, the U–Zr ingot could be removed without causing any damage to the crucible. These results indicate that the most important factor determining the difficulty in using this coating material is its interaction with the graphite substrate rather than that with the U–Zr alloy melt. In Run M-4, where a W crucible double-coated with ZO-MOD and ZrN was used, the U–Zr ingot was released from the crucible without causing any damage to it. However, no further advantages over the simple originally developed ZrO2 coating mentioned below could be found, since part of the coated layer on the W substrate was removed with the ingot and a laborious coating procedure could have to be performed again before the reuse of the crucible. When considering the use of W as a crucible material in the high-temperature distillation process, it also has to be considered that the embrittlement of W due to its recrystallization above 1300 K probably limits its maximum number of uses. When the graphite crucible was used with the originally developed ZrO2 coating material, the U–Zr ingot could be removed by simply inverting the crucible (Run M-5). The coated layer was sometimes removed with the ingot and sometimes remained inside the crucible. In either case, the coated layer was easily removed and did not strongly adhere to the crucible, as shown in
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Fig. 5. Relation between material mass balance of chloride and initial amount of it contained in cathode product used in distillation experiments.
Fig. 6. A cross-sectional view of the removed coated layer observed by SEM and WDS is shown in Fig. 7. The exchange of Zr in the coated layer for U in the U–Zr alloy melt occurred within approximately 50 lm from the interface, indicating that ZrO2 reacted with U. In the inner region of the coated layer, the original structure formed by ZrO2 beads and powder remained as it was before use and hardly any U was found. Fig. 8 shows a cross-sectional view of the graphite crucible near the interface with the coated layer. It can be seen that a thin Zr-rich layer of about 10 lm thickness formed at the surface and that a small amount of U entered the graphite substrate to a depth of approximately 200 lm. The mechanism of such U penetration is not clear at present, since it is shown in Fig. 7 that only a very small amount of U reached further than the interfacial region of the ZrO2 coating layer. In any case, however, the amount of U in the graphite substrate was too small to cause problems concerning the mechanical strength or demolding of the crucible. By adopting the ZrO2 coating developed in the present study, the release of U–Zr ingots and the removal of the coated layer from both the U–Zr ingot and the crucible by remote handling are expected to become easier, although it is inevitable that some U will be lost in the form of UO2 by the reaction between the U–Zr alloy melt and the coating material. Table 3 shows the estimated distribution of U before and after one of the scaled-up distillation experiments, where the proposed coating material was applied to a graphite crucible. The U contents in the cathode product, U–Zr ingot and coated layer after distillation were determined by the
Fig. 7. Cross-sectional view of originally developed ZrO2 coating layer after distillation experiment.
Fig. 8. Cross-sectional view of graphite crucible after removal of originally developed ZrO2 coating layer.
Table 3 Estimated distribution of uranium before and after a scaled-up high temperature distillation test.
Fig. 6. U–Zr distillation product ingot and graphite crucible after removal of originally developed ZrO2 coating layer.
Evaluation of cathode product before distillation Amount used in distillation (g) Chloride content in cathode product (wt%) U in metallic part of cathode product (wt%) U in metallic part of cathode product (g) = a
1034 53.4 98.3 474
Evaluation of cathode product after distillation Amount of distillation product (g) U in distillation product (wt%) U in distillation product (g) = b
482 98.9 477
Evaluation of coating later after distillation Amount of coating layer (g) U in coating layer (wt%) U in coating layer (g) = c
30.0 43.2 12.9
Material mass balance = (b + c)/a (%)
103
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its weight ratio should decrease from the above mentioned value with future scaling-up of the apparatus. The UO2 remaining in the coated layer after distillation can be converted to its chloride by reaction with Zr and ZrCl4 [23], and has no effect on the final material mass balance in the pyrometallurgical process. 3.3. Effect of Zr in distillation product on its characteristics
Fig. 9. Outer and vertical cross-sectional views of U–Zr ingot obtained in scaled-up distillation experiment using cathode product of lower Zr content (Run Z-2).
chemical analysis of samples taken from them. The content of the electrolyte accompanying the cathode product was evaluated from the change in the weight of the vaporization part of the distillation apparatus during the experiment. This table shows that 12.9 g of U reacted with the coating material, which corresponds to 2.72% of the initial loading (474 g) in the distillation experiment. Since the amount that reacts should increase in proportion to the interface area between the melted cathode product and the coated layer,
Outer and vertical cross-sectional views of the U–Zr distillation product obtained in a scaled-up distillation experiment using a cathode product of a lower Zr content (0.89 wt%., Run Z-2) are shown in Fig. 9. The top of the ingot was uneven and gray or brown, but showed a metallic shine after a thin surface layer was removed. The bottom was flat and a small amount of the coated layer was adhered to it. Although the upper part in the cross-sectional view appeared to be black, the other part showed a metallic shine and formed a dense block. In fact, no difference was found between the chemical compositions of the black and shiny parts of this ingot. Fig. 10 shows the cross-sectional view of the central part of this distillation product observed by SEM and WDS. The product ingot was dense and also homogeneous at the microscopic scale. No segregation of Zr was found. Mo and Pd, which were used as simulated FPs in the electrorefining test [19], were also distributed uniformly in the ingot. A cathode product of intermediate Zr content (3.66 wt%, Run Z3) was also formed into a massive ingot and the unevenness of its surface was similar to that observed for the product of a lower Zr content. When the Zr content in the loaded cathode product was much larger (11.9 wt%, Run Z-4), the roughness of the distillation product
Fig. 10. Vertical cross-sectional view of central part of U–Zr ingot obtained in Run Z-2.
Fig. 11. Outer view of U–Zr ingot obtained in distillation experiment using cathode product of high Zr content (Run Z-4).
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Fig. 12. Cross-sectional view of U–Zr ingot obtained in distillation experiment using cathode product of high Zr content (Run Z-4).
Fig. 13. U–Zr binary phase diagram [24] and composition of phases observed in distillation product obtained in Run Z-4.
Fig. 14. Cross-sectional view of U–Zr ingot obtained in distillation experiment using cathode product of highest Zr content (Run Z-5).
was slightly greater than in the above cases. However, the entire cathode product completely melted and formed a unified ingot, as shown in Fig. 11. A cross-sectional view of this ingot (Fig. 12) also shows that it was dense and favorably consolidated. Three different phases were identified by SEM/WDS analysis: a U-rich phase A (3.9 wt% Zr), which occupied the bulk of the ingot, a deposit B (17.2 wt% Zr) of relatively low Zr content and a highly Zr-rich deposit C (64.7 wt% Zr). The formation path of these phases is explained in Fig. 13 using a U–Zr binary phase diagram [24]. The result of analyzing the sample taken from the distillation product after cooling (11.9 wt% Zr, shown in Table 2) is considered to most appropriately represent the average composition of the metallic part of the initial material used in Run Z-4. This composition at the melting temperature (1673 K) is plotted as point P in Fig. 13. However, the Zr-rich phase C is thought to have continued to exist as a dependent solid phase throughout the distillation experiment as mentioned later. Thus, the actual composition of the melt formed at the melting point might be somewhat more U-rich than
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that indicated by point P in Fig. 13. In the distillation experiment, it is supposed that the metallic part of the loaded material formed a uniform melt at 1673 K and then phase B was deposited during the cooling period. Finally, the remaining part is considered to have solidified as phase A. According to the phase diagram shown in Fig. 13, it is unlikely that the Zr-rich phase C of such a large deposit with an inhomogeneous distribution was formed during the cooling process of the homogeneous melt of the composition indicated by P. In the WDS observation of the cathode deposit [19] used in Run Z4, it was found that the distributions of U and Zr in this material were not uniform and that small spots of a very high Zr concentration existed. These findings indicate that it is highly possible that phase C in the distillation product from Run Z4 originated from the Zr-rich spot found in the cathode product used in this experiment and that this Zr-rich material continued to exist in the solid state throughout the distillation step and finally formed phase C after simultaneous agglomeration and equilibration with the surrounding U–Zr melt. In addition to the Zr-rich spots in the cathode product, ZrO2 beads existed in the coating layer on the graphite crucible as a potential Zr source which located close to U–Zr melt. However, this material may not be related to the formation of phase C even if the ZrO2 beads were reduced by U metal very slowly, since it is unlikely that the resulting Zr metal formed such a large Zr-rich deposit at 1673 K rather than simply dissolved into the U–Zr melt. From the viewpoints of process control and nuclear material accountancy, samples taken from the distillation product are required to have appropriate representativity in its composition, and the existence of the Zr-rich deposit phase C, which might result in inhomogeneity of the product ingot, is undesirable. Various means can be proposed to decrease the amount of this Zr-rich phase remaining in the distillation product. One possibility is to inhibit the electrotransport of Zr from the spent metallic fuel in the anode to the cathode by controlling the operating conditions of the electrorefining step. Although this method may increase the ratio of actinides remaining at the anode, the recovery ratio in the entire pyrometallurgical reprocessing can be maintained sufficiently high by the adoption of suitable designed treatment process for the anode residue from the electrorefining step [25]. Another possibility would be to extend the melting time or electromagnetically stir the U–Zr melt during the distillation step. The plots in Fig. 13 indicate that the entire cathode product will melt to form a uniform U–Zr mixture after phase equilibrium is attained at 1673 K provided that its average composition is within the range investigated in the present study. Even if these means are employed, the deposition of phase B containing a higher Zr concentration than the surrounding bulk may be unavoidable. However, the effect of this phase on the representativity of samples taken from the distillation product is not expected to be significant because of its small dimensions and uniform distribution, as shown in Fig. 12. On the other hand, increasing the melting temperature to shorten the time required to attain phase equilibrium in the cathode product should be discouraged since a higher temperature clearly facilitates the reaction between the U–Zr melt and the crucible or coating material. Fig. 14 shows a cross-sectional view of the distillation product obtained in Run Z-5, which contained a high Zr concentration (12.8 wt%). Although this value was not much larger than that in Run Z-4, this product melted only locally and many vacancies remained in the ingot. The reason for this was simply the insufficient melting temperature (1573 K) to completely melt this material, whose composition could not be exactly evaluated before the distillation since it was a mixture with an unclear ratio that consisted of a few cathode products recovered different times during the electrorefining test.
Mechanical milling of the cathode product introduced in Run Z3 and Z-5 potentially has an influence on the characteristics of the distillation product through homogenization of Zr concentration in the cathode product and greater oxidation by increased surface area. Up to the present, however, effect by these factors has been overcome by the influence of averaged Zr content and not appeared definitely. 3.4. Cl content in distillation products The Cl contents in some of the U–Zr ingots obtained in the small-scale distillation experiments using cathode products of various Zr contents are shown in Tables 1 and 2. The amount of Cl remaining in the ingots of a lower Zr content was limited to a few hundred ppm (Runs M-5, Z-1 and Z-3). On the other hand, it was much higher, probably over 1000 ppm, in the ingot containing more Zr (Run Z-4). These results indicate that the Cl content in the cathode products was decreased to less than 1/200 of that after electrorefining by performing the high-temperature distillation. In the fabrication of U–Pu–Zr metallic fuel elements for irradiation tests at Joyo [26], the maximum allowed Cl impurity concentration in fuel alloy slugs was determined as 1000 ppm on the basis of the results of previous studies on metallic fuel fabrication in Japan and the specifications of EBR-II driver fuels. Although the cathode products after distillation obtained from Runs M-5, Z-1 and Z-3 satisfy this criterion, that obtained from Run Z-4 does not meet the criterion. The Cl tended to be distributed at the voids between metal grains, or at the gaps between the consolidated metal and the Zrrich layer formed on the surface of the ingot. Zirconium preferentially captures oxygen around it and forms a layer of its oxide at the surface of the metal grains or the entire ingot. This oxide layer is considered to hamper the complete melting of the cathode product to form a massive ingot, as seen above, as well as the distillation of the anode residue from electrorefining [25]. Such behavior of Zr probably affected the difference in the amount of residual Cl among the distillation products of various Zr contents. Considering these results and findings together, the amount of Zr in the distillation product should be limited below a certain level to decrease the Cl content in the product ingot fed to the subsequent fuel fabrication steps. Such control would be possible by adjusting the operating conditions of the electrorefining step or by blending cathode products of different Zr contents, for example. In addition, attention should be directed to the oxidation of Zr in the cathode product both before and during the high-temperature distillation step, which leads to the formation of voids and gaps in the product ingot and could isolate chlorides contained in the cathode product from vaporization. 4. Conclusions High-temperature distillation experiments were performed using U–Zr cathode products of various compositions obtained in previous electrorefining tests to obtain knowledge on suitable operation conditions and equipment design such as the container material compatible with both high-temperature chlorides and the chemically active U–Zr alloy melt. The LiCl–KCl–UCl3 electrolyte adhering to the U–Zr cathode products was almost completely vaporized at 1273–1573 K under pressure of 10–300 Pa. Although the material balance of chlorides was excellent for a smaller amount of the loaded cathode product, it deteriorated with increasing quantity of the processed material, probably owing to leakage from the distillation apparatus caused by the temporary increase in internal pressure.
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After the vaporization of the chlorides, the remaining cathode product was further heated at 1573–1673 K for consolidation. Massive ingots were obtained from cathode products of a wide range of Zr contents (0–11.9 wt%). For cathode products of a higher Zr content (12.8 wt%), the melting temperature of 1573 K was found to be insufficient for complete consolidation. Three different phases were identified in the distillation product containing an average Zr content of 11.9 wt%. A U-rich bulk (3.9 wt% Zr) and a deposit of a relatively low Zr content (17.2 wt% Zr) were considered to be formed during the cooling process of the distillation product. It is highly possible that another Zr-rich deposit in the product (64.7 wt% Zr) resulted from Zr-rich spots that originally existed in the cathode product used in this experiment. The amount of this phase should be decreased by some means, such as by adjusting the operating conditions of the elecrorefining step, extending the melting time, or electromagnetic stirring of the U–Zr melt, to ensure sufficient homogeneity of the distillation product. The Cl content in the cathode products was decreased to less than 1/200 of that after electrorefining by performing distillation. The amount of residual Cl was limited to a few hundred ppm in the ingots of a lower Zr concentration, while it was markedly larger at a higher Zr content. To maintain the Cl content below an appropriate level in materials used for recycled fuel fabrication, the Zr content in the distillation product should be controlled to a sufficiently low level by optimization of the operating procedures and conditions in the electrorefining and distillation steps. The zirconia coating material originally developed in this study showed superior performance in inhibiting the reaction between the melted U–Zr alloy melt and the graphite crucible and also in easy the release of the U–Zr ingot from the crucible. Although a small amount of the U in the cathode product reacted with this coating material, no effect on the material mass balance of recovery ratio in the entire pyrometallurgical process is expected since the UO2 remaining in the coated layer can be converted to its chloride by reaction with Zr metal and ZrCl4 and finally returned to the process. Acknowledgements The authors are grateful to Messrs. H. Nakamura and K. Murakami of Toshiba Corporation for their experimental and analytical support as well as Dr. K. Nakamura of CRIEPI for his useful suggestion and discussion on the SEM/EPMA results. This paper presents the results of the ‘‘Development of technologies for processing of
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