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Purification Rate of Uranyl Nitrate Hexahydrate Crystal for Transuranium Elements on Isothermal Sweating Phenomenon Masaumi Nakahara,*,† Kazunori Nomura,‡ and Tsutomu Koizumi‡ Nuclear Fuel Cycle Engineering Laboratories and AdVanced Nuclear System Research and DeVelopment Directorate, Japan Atomic Energy Agency, 4-33 Muramatsu, Tokai-mura, Naka-gun, Ibaraki 319-1194, Japan
For the purpose of further increasing decontamination performance, the purification behavior of uranyl nitrate hexahydrate (UNH) crystals was studied in the static system. The approach to purification of UNH crystals recovered from the irradiated fast reactor core fuel using a dissolver solution was examined by an isothermal sweating process, which involves heating to the melting point of the crystal. Since transuranium (TRU) elements were present as liquid impurities in the UNH crystal, their decontamination factors (DFs) were improved using this process. The DFs of Cs and TRU elements for the UNH crystals had a tendency to increase with time and were kept virtually constant by 240 min. Not having been precipitated as Cs2Pu(NO3)6 during the crystallization, Pu remained in the mother liquor and was removed by the sweating process. The purification rate coefficients of UNH crystals for Cs and TRU elements were calculated using these experimental data. These values for Pu, 137Cs, 241Am, and 242Cm were evaluated to be 2.13 × 10-4, 2.73 × 10-4, 2.11 × 10-4, and 3.13 × 10-4 s-1 at 57 °C, respectively. 1. Introduction Uranium, Pu, and minor actinides (MAs) in an irradiated fast reactor fuel are planned for recovery to be reused in a nuclear reactor. An advanced aqueous reprocessing for a fast reactor fuel cycle named “New Extraction System for Transuranium (TRU) Recovery (NEXT)” has been proposed as one fast reactor fuel reprocessing method and is being developed in Japan.1 The NEXT process consists of highly efficient dissolution of the fuel with HNO3 solution, U crystallization for partial U recovery, simplified solvent extraction for U, Pu, and Np co-recovery using tri-n-butylphosphate (TBP) as an extractant, and extraction chromatography for recovery of Am and Cm from a raffinate. The crystallization process has been studied as part of NEXT, and U is recovered as uranyl nitrate hexahydrate (UNH) crystals from a HNO3 medium in its oxidation state, U(VI). In the NEXT process, U, which is the main element in the irradiated fuel, is separated partially as UNH crystal for the ingredient of blanket fuels by the crystallization. Therefore, it is desirable to remove TRU elements and fission products (FPs) from UNH crystals. The adoption of the crystallization process is expected to reduce the radioactive waste, equipment, and hot cell volume because the amount of throughput will be reduced in the simplified solvent extraction process. Among TRU elements, Pu, Am, and Cm are the important elements for the fast reactor fuel cycle, especially in nuclear waste management because of their long half-life and high radiotoxicity. In NEXT, Pu, Am, and Cm will be recovered from the nuclear fuel to decrease the environmental impact. For the efficient utilization of resources, TRU elements are recovered in reprocessing for loading into a fast reactor as core fuel. A dissolver solution of irradiated fast reactor mixed oxide (MOX) fuel contains a number of TRU elements and FPs, which contaminate the UNH crystal recovered from the dissolver * To whom correspondence should be addressed. Tel.: +81 29 282 1111. Fax: +81 29 282 9290. E-mail:
[email protected]. † Nuclear Fuel Cycle Engineering Laboratories, Japan Atomic Energy Agency. ‡ Advanced Nuclear System Research and Development Directorate, Japan Atomic Energy Agency.
solution after solid-liquid separation. Therefore, the UNH crystal is washed with a HNO3 solution in order to remove the mother liquor on the surface of the UNH crystal. However, the inclusions within the crystal are not removed by washing the crystal. The grown crystalline particles are generally purified to increase its purity at an industrial plant in the food, medicine, and chemical fields.2 One crystal purification method, known as “sweating”, is applied to organic materials and metals, which are purified by heating up to as high as the melting point of the crystal and then liquid impurities are removed from them.3 Further decontamination of the UNH crystal is of great advantage in terms of the reduction of radiation shielding needed in fuel fabrication and effective utilization of recovered U. The inclusions within the UNH crystal contain a number of elements, which are assumed to have a different behavior in the crystal purification process. It is therefore necessary to prove the benefit of the applications for the sweating process with UNH crystals recovered from the dissolver solution of irradiated fast reactor fuel. Although the removal of Zr, Ru, Ce, and Eu was confirmed by the sweating operation in the previous study, the decontamination factors (DFs) of Cs and Pu did not improve.4 It is known that the reaction of Cs and Pu(IV) ions in highly concentrated HNO3 solution results in the crystalline precipitate of Cs2Pu(NO3)6,5 and the precipitation behavior of Cs2Pu(NO3)6 affects the decontamination for the UNH crystal in the U crystallization process. Since TRU elements are key materials in the NEXT process, it is important to understand the behavior of Pu, Am, and Cm in the crystallization and crystal purification operation. Additionally, the behavior of Cs needs to be examined in terms of the precipitation of Cs2Pu(NO3)6 because it will be useful to estimate the behavior of Pu. In this study, we investigated the behavior of Cs, Pu, Am, and Cm in the crystallization process. Moreover, the decontamination effect of Cs, Pu, Am, and Cm by the sweating phenomenon was also examined using the UNH crystal recovered from the dissolver solution derived from the irradiated fast reactor core fuel. The crystallization and crystal purification experiments were carried out at the Chemical Processing Facility (CPF), Japan Atomic Energy Agency (JAEA).
10.1021/ie1012097 2010 American Chemical Society Published on Web 09/17/2010
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2. Theory
Table 1. Composition of the Feed Solution
The volume of liquid in the crystal increases by heating to as high as the crystal’s melting point, and then the liquid is exuded to the surface of the crystal by increased interior pressure. This phenomenon is called sweating, which can remove the incorporated liquid impurities in the crystal. Because the obtained crystals possess a number of defects and grain boundaries, the inclusion is drained away to the outside of the crystal along some defects and grain boundaries by Ostwald ripening or pressure force arising from the increase in liquid volume during the sweating operation.6 The crystals obtained from the solution contain excess impurities, which exist primarily either on the crystal surfaces as an adhered mother liquor or inside the crystal as inclusions. Such crystals can be modeled as a mixture of the pure crystals in equilibrium at the crystallization temperature and the adhered or entrapped mother liquor.7 In an organic crystal system, the following purification rate coefficient, k, was defined because a simple expression was desirable for analysis, and the purification rate equation was solved with mass balance equations,7
HNO3 (mol/dm3)
U (g/dm3)
Pu (g/dm3)
137 Cs (Bq/cm3)
241 Am (Bq/cm3)
242 Cm (Bq/cm3)
3.2
4.16 × 102
4.20 × 101
5.24 × 108
6.40 × 108
8.60 × 107
dws ) k(w*s - ws) dt
(1)
where ws* is the local equilibrium concentration that the composition of the crude crystals approach. When crude crystals are placed in a higher temperature atmosphere than when they were crystallized, changes in the crystal composition with time are observed and can be analyzed using eq 1. Since the reprocessing purity of the product is evaluated by the impurities in the DFs, the purification rate of the UNH crystal is expressed as the following equation considering the operation in a steady state: dβp,j ) kd(βp,j∞ - βp,j) dt
(2)
Here, the DFs of Cs and TRU elements for the UNH crystal in the sweating process were calculated by the following equation:
βp,j
Cb,j Cb,U ) Ca,j Ca,U
(3)
where βp,j is the DF of Cs or TRU elements j for the UNH crystal in the sweating process; Cb,j is concentration of Cs or TRU elements j in the crystal before the sweating process (mg/ g); Cb,U is the concentration of U in the crystal before the sweating process (mg/g); Ca,j is the concentration of Cs or TRU elements j in the crystal after the sweating process (mg/g); and Ca,U is the concentration of U in the crystal after the sweating process (mg/g). The purification rate coefficient kd for the UNH crystal was defined in the sweating process. By integration of eq 2 with the initial condition that βp,j ) βp,j0 at t ) 0, the DFs of Cs and TRU elements are given as βp,j ) βp,j∞ - (βp,j∞ - βp,j0) exp(-kdt)
(4)
When the UNH crystal has the initial condition, βp,j0, and the sweating temperature is kept constant, the DFs of liquid impurities for the UNH crystal finally stay constant, βp,j∞. The changes in their DFs with time can be analyzed by the use of eq 4.
3. Experimental Section 3.1. Reagents and Feed Solution. The composition of the feed solution used in the crystallization experiment is shown in Table 1. In the experiments, HNO3 was purchased from Junsei Chemical Co., Ltd., and used without further purification. Stock solutions of HNO3 were standardized by acid-base titration and diluted to appropriate concentrations. The dissolver solution was prepared by dissolution of sheared pieces of irradiated core fuel from fast reactor “JOYO” Mk-III with an average burn-up of 53 GWd/t and a cooling time of 3 years. Sheared pieces comprising 130 g of heavy metal were dissolved with 325 cm3 of 8 mol/dm3 HNO3 solution at 95 °C for 600 min. Concentrations of HNO3 and heavy metal were adjusted by evaporation and addition of HNO3 solution. The initial volume of the feed solution was 175 cm3, and the H+ concentration was 3.2 mol/ dm3 in the feed solution. For stabilizing Pu(IV) in the feed solution, NOx gas was produced by reacting 5 mol/dm3 HNO3 solution and 5 mol/dm3 NaNO2 (Wako Pure Chemical Industries, Ltd.) solution for 120 min. 3.2. Procedure. Figure 1 shows a schematic diagram of the crystal-forming apparatus. The crystal-forming apparatus made from Pyrex glass was used for cooling the solution. The volume capacity was 200 cm3, and it had a cooling jacket for cooling and heating media whose temperature was controlled by a thermostat. The dissolver solution was cooled from 50 to 4 °C while being stirred. The spontaneously nucleated and grown crystalline particles were quickly separated from the mother liquor using a centrifuge (H-112, KOKUSAN Co., Ltd.) at 3000 rpm for 20 min. The recovered UNH crystals were washed using 8 mol/dm3 HNO3 solution at 4 °C and then centrifuged at 3000 rpm for 20 min. In the sweating experiments, ∼14 g of UNH crystals were placed in a glass vial, which was closed with a lid. The glass vials were kept isothermally in a thermostat bath maintained at the sweating temperature of 56 or 57 °C for 240 min. The sampled crystals were provided for analysis of the metal concentrations both before and after the sweating process to determine the effectiveness of the sweating process in separating the U from the dissolver solution of fast reactor fuel. 3.3. Analysis. The acidity of the solution was determined by acid-base titration (COM-2500, Hiranuma Sangyo Co., Ltd.), and the Pu valence in the feed solution was confirmed as Pu(IV) by optical spectrometry (V-570DS, JASCO Corporation) of the ultraviolet (UV)-visible region. The concentrations of U and Pu were measured by colorimetry. Americium and Cm concentrations were measured by R-ray spectrometry (CU017450-100, detector and NS920-8MCA, pulse height analyzer, ORTEC). The concentration of Cs was analyzed by γ-ray spectrometry (GEN10, detector and 92XMCA, pulse height analyzer, ORTEC). 4. Results and Discussion 4.1. Crystallization Process. The DFs of Pu, 137Cs, 241Am, and 242Cm in the UNH crystal before washing and after washing in the crystallization process are summarized in Table 2.
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Figure 1. Schematic diagram of the crystal-forming apparatus. Table 2. DFs of Cs and TRU Elements for UNH Crystal in Crystallization Process process
Pu (-)
before washing after washing
25.9 134
137
Cs (-)
27.9 173
241
Am (-) 28.0 174
242
Cm (-)
30.4 179
The DFs of Cs and TRU elements for the UNH crystal in the crystallization process were calculated by the following equation: CF,j CF,U βc,j ) CP,j CP,U
(5)
where βc,j is the DF of Cs or TRU elements j for the UNH crystal in the crystallization process; CF,j is the concentration of Cs or TRU elements j in the feed solution (g/dm3); CF,U is the concentration of U in the feed solution (g/dm3); CP,j is the concentration of Cs or TRU elements j in the crystal (mg/g); and CP,U is the concentration of U in the crystal (mg/g). The DFs of Pu, 137Cs, 241Am, and 242Cm for the UNH crystal were 25.9, 27.9, 28.0, and 30.4 before washing, respectively. Then, their DFs increased 5-6 times when the UNH crystal was washed with HNO3 solution. These elements were kept in the mother liquor during UNH crystallization by cooling the solution. Therefore, the mother liquor adhered on the surface of UNH crystal after solid-liquid separation was washed out with the HNO3 solution. It is known that the Pu(IV) reacts with Cs and forms Cs2Pu(NO3)6 in highly concentrated HNO3 solution.5 It is difficult to remove the solid impurities in the UNH crystal by crystal washing with the HNO3 solution.8 If Cs2Pu(NO3)6 precipitates in the course of the U crystallization, it makes the DFs of Pu and Cs low even after crystal washing. However, the DFs of Pu and Cs increased to 134 and 173 after the UNH crystal was washed in the experiments, respectively. Because higher DFs of Pu and Cs were achieved by washing the crystal, it was assumed that Cs2Pu(NO3)6 did not form, and Pu and Cs remained in the mother liquor. This compound tends to be generated in high concentrations of HNO3 and Cs in a HNO3 solution.8 The concentrations of HNO3 and Cs in the feed solution were only 3.2 mol/dm3 and 1.0 g/dm3, respectively; therefore, their concentrations were low enough to avoid the formation of Cs2Pu(NO3)6. 4.2. Sweating Process. The temperature change at the center of the crystal bed is shown in Figure 2.
Figure 2. Temperature changes in crystal bed in sweating process.
The temperature at the center of crystal bed attained the target value within 20 min of the glass vials being immersed in the thermostat bath and then remained constant until the end of the sweating experiments. Figures 3, 4, 5, and 6 show the time and temperature dependences of DFs of Pu, 137Cs, 241Am, and 242Cm in the sweating process, respectively. The DFs of Pu, 137Cs, 241Am, and 242Cm for the UNH crystal were calculated by eq 3. In the experiments, the decontamination behavior of Cs and TRU elements for the UNH crystal showed the same tendency at 56 and 57 °C in the static system. The DFs of Cs and TRU elements increased with time and stayed constant by 240 min. High sweating temperature made the DFs of Cs and TRU elements for the UNH crystal higher, as seen by the fact that Cs and TRU elements sweating at 57 °C were more decontaminated than those sweating at 56 °C. The relationship between sweating time and crystal yield at 56 and 57 °C is shown in Figure 7. More UNH crystal was melted by increasing the sweating time at 56 and 57 °C. The experimental results showed that there was a close connection between the sweating time/the sweating temperature and the UNH crystal yield. Since liquid impurities on the surface of the crystal were washed out with the molten UNH crystal, the amount of molten crystal increased the DFs of Cs and TRU elements for the UNH crystal. The dependence of the sampling position on the DFs of Pu, 137Cs, 241Am, and 242Cm was examined at 56 and 57 °C. The dependence of the sampling position on the DFs of Pu, 137Cs, 241Am, and 242Cm in the crystal bed are shown in Figures 8, 9, 10, and 11, respectively. The Cs
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Figure 3. Influence of sweating time and sweating temperature on DF of Pu for UNH crystal.
Figure 4. Influence of sweating time and sweating temperature on DF of 137 Cs for UNH crystal.
and TRU elements in the mother liquor and the molten UNH crystal exuded from the interior of the UNH crystal at the top of the glass vials. Discharged mother liquor and U melt from the crystal were collected at the bottom of the glass vial by the sweating operation, and it brought about lower DFs for the UNH crystal near the bottom of the glass vial. The incorporated liquid impurities such as Cs and TRU elements in the UNH crystal were removed by the sweating phenomenon. Sweating crystals for a long time at high temperatures brings about high DFs of liquid impurities for the UNH crystal. The change of DFs of liquid impurities with time can be evaluated by eq 4. The curves calculated by eq 4 are also drawn in Figures 3, 4, 5, and 6, respectively. The purification rate coefficients of the UNH crystals for Cs and TRU elements in the sweating process are summarized in Table 3. 4.3. Decontamination Performance of Liquid Impurities from Crystallization to Sweating Processes. The DFs of Cs and TRU elements from the crystallization to sweating processes in the static system were evaluated in Table 4. The values were calculated using the DFs after washing in the crystallization process and those at 57 °C for 240 min into
Figure 5. Influence of sweating time and sweating temperature on DF of 241 Am for UNH crystal.
Figure 6. Influence of sweating time and sweating temperature on DF of 242 Cm for UNH crystal.
Figure 7. Relationship between sweating time and crystal yield at 56 and 57 °C.
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Figure 8. Relationship between DF of Pu and sampling position of UNH crystal at 56 and 57 °C.
Figure 9. Relationship between DF of 137Cs and sampling position of UNH crystal at 56 and 57 °C.
the sweating process. It was confirmed that their DFs were enhanced by crystal washing and sweating operations. The crystal purification equipment, Kureha Crystal Purifier (KCP),9 has been applied in industrial plants using organic matter. The schematic diagram of KCP is shown in Figure 12.4 The equipment has been developed in the following fashion: feed stock is charged as solids at the bottom of the column, the heating unit is set at the top of the column, and it is possible to contact the melt with crude crystal countercurrently. The KCP features high purity, high yield, energy savings, little maintenance, and a long, stable operation.9 The crude crystal is fed at the bottom of the column and then is carried to the upper side of a column by a double screw conveyor, and then part of the crystal is molten by a heating unit at the top of the column and the melt trickles downward among the crude crystal. The equipment performs countercurrent contact between the crystal and reflux melts in the course of being conveyed upward, and the crude crystal is washed by a portion of the melt. Therefore, higher DFs of liquid impurities are expected by the crystal purifier, KCP, because the liquid impurities were washed with
Figure 10. Relationship between DF of UNH crystal at 56 and 57 °C.
241
Figure 11. Relationship between DF of UNH crystal at 56 and 57 °C.
242
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Am and sampling position of
Cm and sampling position of
Table 3. Purification Rate Coefficients of UNH Crystal for Cs and TRU Elements in Sweating Process temperature (°C)
Pu (s-1)
56 57
2.47 × 10-4 2.13 × 10-4
Cs (s-1)
Am (s-1)
Cm (s-1)
137
241
242
2.09 × 10-4 2.73 × 10-4
2.04 × 10-4 2.11 × 10-4
2.10 × 10-4 3.13 × 10-4
Table 4. DFs of Cs and TRU Elements for UNH Crystal from Crystallization to Sweating Processes process crystallization (after washing) sweating (57 °C, 240 min) total
Pu (-) 134 1.77 237
137
Cs (-)
173 1.75 303
241
Am (-)
174 1.83 318
242
Cm (-)
179 1.80 322
U melt in addition to sweating. The pure product exits as crystal from the top of the column. In the previous study,4 the UNH crystal recovered from uranyl nitrate solution containing Sr was purified by the KCP. Although the DFs of Cs and TRU elements in the static system were ∼1.8 at 57 °C for 240 min, the DF of Sr was 50 by the KCP in the
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2.11 × 10-4, and 3.13 × 10-4 s-1 at 57 °C, respectively. If the liquid impurities are contained in the UNH crystal, these impurities will be removed by the sweating operation. However, further experimental investigation into the behavior of other elements is required. Literature Cited
Figure 12. Schematic diagram of crystal purification equipment, KCP.4
sweating process. The liquid impurity such as Sr was removed from the UNH crystal not only by the sweating phenomenon but also by washing with reflux melt, which was produced by the melter at the top of the column in the KCP. 5. Conclusions The crystal purification experiments were conducted with UNH crystals recovered from the dissolver solution of irradiated fast reactor core fuel by isothermal sweating process. The purification rate coefficients of UNH crystals and the DFs of Cs and TRU elements were measured. The DFs of Cs and TRU elements increased with increasing the sweating time and sweating temperature in the static system. The more molten UNH generated, the higher were the DFs of Cs and TRU elements for the UNH crystal. It was experimentally confirmed that liquid impurities of Cs and TRU elements incorporated in the UNH crystals could be removed by the isothermal sweating phenomenon. The purification rate coefficients of UNH crystal for Pu, 137Cs, 241Am, and 242Cm were 2.13 × 10-4, 2.73 × 10-4,
(1) Koyama, T.; Washiya, T.; Nakabayashi, H.; Funasaka, H. Current status on reprocessing technology of fast reactor fuel cycle technology development (FaCT) project in JapansOverview of reprocessing technology development. Proceedings of the International Conference on the Nuclear Fuel Cycle: Sustainable Options and Industrial Perspectives (GLOBAL 2009), Paris, France, September 6-11, 2009; Paper no. 9100. (2) Yamada, J.; Shimizu, C.; Saitoh, S. Industrial Crystallization 81; Jancic, S. J., de Jong, E. J., Eds.; North-Holland Pub. Co.: Amsterdam, The Netherlands, 1982. (3) Zief, M.; Wilcox, W. R. Fractional Solidification; Marcel Dekker Inc.: New York, 1967. (4) Yano, K.; Nakahara, M.; Nakamura, M.; Shibata, A.; Nomura, K.; Nakamura, K.; Tayama, T.; Washiya, T.; Chikazawa, T.; Kikuchi, T.; Hirasawa, I. Research and development of crystal purification for product of uranium crystallization process. Proceedings of the International Conference on the Nuclear Fuel Cycle: Sustainable Options and Industrial Perspectives (GLOBAL 2009), Paris, France, September 6-11, 2009; Paper no. 9093. (5) Anderson, H. H. Alkali plutonium(IV) nitrates. In The Transuranium Elements, National Nuclear Energy Series, IV, 14B; Seaborg, G. T., Katz, J. J., Manning, W. M., Eds.; McGraw-Hill Book Co.: New York, 1949; p 964. (6) Matsuoka, M.; Ohishi, M.; Kasama, S. Purification of p-dichlorobenzene and m-chloronitrobenzene crystalline particles by sweating. J. Chem. Eng. Jpn. 1986, 19, 181. (7) Matsuoka, M.; Fukuda, T.; Takagi, Y.; Takiyama, H. Purification of organic solid solutions by melt crystallization: Comparison between layer and suspension crystallization. J. Cryst. Growth 1996, 166, 1035. (8) Nakahara, M.; Nomura, K.; Washiya, T.; Chikazawa, T.; Hirasawa, I. Influence of nitric acid and plutonium concentrations in dissolver solution of mixed oxide fuel on decontamination factors for uranyl nitrate hexahydrate crystal. Radiochim. Acta 2010, 98, 315. (9) Otawara, K.; Matsuoka, T. Axial dispersion in a Kureha Crystal Purifier (KCP). J. Cryst. Growth 2002, 237s239, 2246.
ReceiVed for reView June 3, 2010 ReVised manuscript receiVed August 27, 2010 Accepted August 31, 2010 IE1012097
