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Study on the leaching behavior of salt waste forms prepared by using zeolite and lime glass
Journal of Industrial and Engineering Chemistry 15 (2009) 293–298
Contents lists available at ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Study on the leaching behavior of salt waste forms prepared by using zeolite and lime glass Hwan-Young Kim *, Jeong-Guk Kim, In-Tae Kim, Hwan-Seo Park Korea Atomic Energy Research Institute, Yuseong, Daejon 305-353, Republic of Korea
A R T I C L E I N F O
Article history: Received 12 February 2008 Accepted 17 November 2008 Keywords: Free salt Zeolite Waste form Electrolytic reduction process Lime glass Borosilicate glass
A B S T R A C T
When the reaction of salt and zeolite was used to minimize the free salt in waste forms (r = 0.1), Cs showed the lowest leaching rate, 1.015 101 g/m2 d. Because alkali chloride is chemically stable, the reaction that alkali elements become components of glass does not happen and thus the leach resistance of the waste form solidified with soda glass was not much different from that solidified with borosilicate glass. The crystalline phase containing Cl was sodalite, but the tendency that Cs exists prior to sodalite phase was not confirmed. From a result of a long-term leaching, the predicted leaching fraction of Cs in 900 days was as high as 5.13%, but that of Sr was as low as 0.24%. The leaching experiment with a varying pH showed the major nuclides such as Cs, Sr, and Li in salt waste had different leaching characteristics each other. ß 2009 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
1. Introduction Radioactive salt waste generated from the electrolytic reduction process, which reduces spent nuclear fuel electrolytically, has some alkali, alkaline earth, and rare earth nuclides that have been reduced formerly to metals by lithium metal in the electrolytic reduction process and converted to chlorides in the molten salt of LiCl or that are easier to exist as chlorides out of a metal conversion in a molten LiCl salt. Of course, besides these nuclides, there are also some daughter nuclides generated from the beta decay of the nuclides contained in a salt. Although these daughter nuclides are highly likely to exist as non-radioactive chemical species due to their short half lives, they also affect the leach resistance of waste form. So we need to consider their effect on waste forms. In addition, along with nuclides, a large amount of LiCl used as a molten salt medium is generated as a waste. Because they are highly water-soluble and too difficult to form a highly integrated waste form, they cannot be disposed as they are. Thus, two-step method to fabricate a final waste form for salt waste, which is composed of conversion into a water-insoluble form and then transformation into a monolithic form, is generally suggested. US ANL (Argonne National Laboratory) actually showed an enhancement for the leach resistance of salt waste form by adsorbing the radionuclides with zeolite and immobilizing the salt in zeolite as a chemical species of product other than a free salt [1,1a,2]. Such result
* Corresponding author. E-mail address: [email protected] (H.-Y. Kim).
was obtained for a LiCl–KCl eutectic salt waste, while an application for a LiCl salt waste was recently accomplished by Kim et al. [3]. When LiCl salt containing Cs, Sr, or Ba reacts with zeolite 4A at 1188 K, a sodalite, which is stable even in water, is formed and a free salt is immobilized within the sodalite, as shown in follow reaction. Na12 ½AlO2 Þ12 ðSiO2 Þ12 nH2 O þ 4LiCl ! 2Na6 Li2 ½AlO2 Þ6 ðSiO2 Þ6 Cl2 In this reaction, 1 mol zeolite reacts with 4 mol LiCl and produces 2 mol sodalite, and the weight ratio of sodalite to LiCl is 100:9.95, which shows an 10 times increase of product compared to its original salt waste. In this work, we examined the leaching characteristic of waste forms solidified SLZ (salt-loaded zeolite) with lime glass (SINIL) or borosilicate glass (R7T7) used in solidification of high-level waste (HLW) in France, respectively. In addition, we also attempted to characterize the waste forms by XRD (X-ray diffractometer) and SEM (scanning electron microscope). Also the leaching characteristic of the waste forms solidified with a reduced amount of zeolite A was compared with the results above in order to establish the possibility of reducing the quantity of the waste forms. 2. Experimental 2.1. Chemicals The molten LiCl salt was prepared by heating commercial LiCl powder (Aldrich, 99+%) to 923 K. The simulated salt waste was prepared by a physical mixing of LiCl, CsCl (Aldrich, 99.9%), SrCl2 (Aldrich, 99.9%), and BaCl2 (Aldrich, 99.9%) powders in an Ar glove
1226-086X/$ – see front matter ß 2009 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jiec.2008.11.009
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box and then a heating under Ar gas flow conditions. A granulartype zeolite A [Aldrich, Molecular Sieves, 4A, Na12(AlSiO4)12] with an 8–12 mesh size was used as an immobilization medium. Zeolite A was dehydrated before use by heating it with a purge of nearly pure nitrogen (99.999+%); it was stored in an Ar glove box. 2.2. Apparatus 2.2.1. Dehydrator The as-received zeolite A contained up to 20 wt% of water. If the water in the zeolite is not extracted during the salt-loading process, the radioactive nuclides and chlorides might not be fully loaded onto the zeolite or they might become hydroxides. Dry nitrogen gas was passed through a packed zeolite column, which was heated to 820 K by a cylindrical furnace. After a dehydration, the water content of the zeolite was below 0.5 wt%. 2.2.2. Blender Salt-loaded zeolite (SLZ) samples were prepared by a V-type blender. A rotating V-type blender located within an electric furnace was used to mix the zeolite A and LiCl salt at r = 0.4, 0.25, and 0.1. The blender was made of stainless steel (SUS-316) and it was connected to a vacuum pump via a needle valve and could be maintained at 923 5 K. 2.2.3. Furnace A muffle furnace was used for fabrication of a final waste form sample at 1193 K.
Table 1 The compositions of the used glasses. Component
Mixed ratio (wt%) Lime glass (SIN)
Borosilicate glass (R7T7)
SiO2 Na2O K2O CaO ZnO MgO BaO SrO Al2O3 Fe2O3 Li2O TiO2 B2O3
55.0 0.5 – 21.8 – 0.5 – – 14.5 0.2 – 0.5 7.5
53.3 11.5 – 4.7 2.9 – – – 8.8 – 2.3 – 16.4
(Micrometrics ASAP 2400, USA). An international standards organization (ISO) method is an evaluation method for the leach resistance of the waste form under long-term controlled conditions. The test was accomplished at 343 K for more than 365 days. The materials characterization center test no. 1 (MCC-1), which is accomplished at 343 K for more than 7 days, can be used to characterize a corrosion mechanism. The both ratios of the volumes of demineralized water for ISO and buffer solution for MCC-1 to surface areas of waste form samples are both 10. 3. Results and discussion
2.3. Procedure
3.1. Leaching characteristic according to mixing ratio of salt to zeolite
2.3.1. SLZ preparation The SLZ was formed during blending granular zeolite A and a mixture of salt powders were loaded into V-type blender. The mixture of salt powders was prepared by adding of 5.32 wt% CsCl, 2.89 wt% SrCl2, and 4.96 wt% BaCl2 to a lithium compound (97 wt% LiCl and 3 wt% Li2O). The blender was sealed and placed in the furnace, which could heat the whole vessel to 923 K. Before a heating, the blending vessel was evacuated using a mechanical vacuum pump. The vessel was rotated at 20 rpm under a heating rate of 5 K/min. Once the furnace temperature had reached 923 K, vacuum conditions were re-established prior to 20-h isothermal blending with a rotation. After being cooled, the blended zeolite was stored in an Ar-atmosphere glove box.
A mixing ratio of salt to zeolite, r (=salt/zeolite) had a significant influence on a leaching characteristic. The leaching rate of Cs at r = 0.1 was 1.015 101 g/m2 d, but the leaching rates at r = 0.25 and 0.4 were 1.148 and 1.430 g/m2 d, respectively, increasing by around 10 times. Such effect was the same on the leaching rate of Sr. The leaching rate of Sr at r = 0.1 was 2.089 102 g/m2 d, showing a high leach resistance, but the leaching rates at r = 0.25 and 0.4 were 9.221 101 and 2.026 101 g/m2 d, respectively, increasing significantly. Even if we consider that the amount of Sr added to the waste form was smaller than that of Cs, the result shows that Sr is leached away less than Cs. These results of Cs and Sr show that an optimum mixing ratio exists to form a good waste form with a high leach resistance. When LiCl was incorporated with zeolite of 10 times more weight, the leaching rate approached a considerably low value, as discussed in a theoretical review, because most of the LiCl reacted with zeolite and transformed to a component of sodalite having very low solubility in water. Therefore, if the mixing ratio of salt to zeolite increases, a free salt, not-reacted excessive LiCl, would also increase significantly, resulting in an increase of the leaching rate of radionuclides, such as Cs and Sr.
2.3.2. Solidification The granular SLZ crushed to be fine powder below 100 mesh, and then mixed with glass powder of 100–200 mesh. The waste form samples were obtained by heating the mixture in an electric furnace at 1193 K for 72 h. The glass frits used in this experiment were soda glass (SINIL, Korea) and borosilicate glass (simulated R7T7), which was used to treat HLW in France, shown in Table 1. The glass powder were added up to 25(glass was 1/3 of SLZ) and 40 wt% of final waste form. The final waste form had 3.5–6% less weight than before solidification because of LiCl volatilization. Solidification was carried out with an injection of argon gas to minimize an oxidation of carbon vessel filled with the mixture of SLZ and glass powder. 2.3.3. Leaching test The leaching characteristics were tested by three different methods. A product consistency test (PCT) is an evaluation method for the chemical durability of a waste form. 30 mL demineralized water was used for 3 g waste form sample, which was crushed in 100–200 mesh. The PCT was performed at 90 8C for more than 7 days. The surface area was measured through a BET analysis
3.2. The crystallinity of the waste forms obtained according to the type and amount of glass frit Table 2 shows the XRD analysis (by Bruker D8 Advance with TOPAS V3 software) results crystallized substances for the final waste forms, which were fabricated by an incorporation of SLZ with r = 0.1 and two different glass frits. In the soda glass frit (SIN), the contents of SiO2 and Al2O3 are high and CaO exists as a substitute for the alkali and boron. Compared to borosilicate glass (R7T7), its thermal characteristic (glass point and mp) is relatively high and thus its characteristic of a reaction with SLZ at a given manufacturing temperature 1188 K is different. Due to the difference solubility of the sodalite in glasses at the same temperature, waste forms incorporated with borosilicatie glass are more amorphous than
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Sample
Result of XRD
respectively. These phenomena is considered due to that the matrix of waste form is changed more dense structure, because the effects of dissolution in glass and binding the crystals increase when the content of glass increase.
SIN-25
Sodalite 46.69% Albite (NaAlS3O8) 53.31%
3.4. Morphology of the Cs
SIN-40
Sodalite 28.49% Anorthite (CaAl2S2O8) 71.59%
R7T7-25
Sodalite 91.65% Halite 8.35%
R7T7-40
–
Table 2 The results of the XRD analysis of the crystallized substance in the waste forms.
those with soda glass, more transparent and compact waste forms from borosilicate glass were obtained. In the waste forms incorporated with soda glass (40 wt%), there is sodalite as well as crystallized calcium aluminum silicate that appears through a annealing of metallic aluminum silicate over Tg (723–823 K). This phenomenon shows that the glass plays the role of a binder that enables a physical binding without a perfect solubility to the sodalite. The soda glass seems to be more favorable for the salt waste containing lots of heat generation nuclides such as Cs and Sr because it helps to form a high crystalline product. 3.3. Compressive strength of the waste forms according to amount of glass
Fig. 1 shows the SEM photographs (by EPMA, JEOL JAX-8600) of the waste form (SIN-25) incorporated with soda glass. It shows the morphology between the glass medium and the sodalite phase. Because the elements of the added glass matrix are almost identical to the major elements of sodalite, we examined the phase through a mapping for the elements only existing in the glass and those in the sodalite. As seen in the figure, the part where Ca exists is clearly distinguished from the part where Cl exists, and they are mixed with each other at a level of 100 mm. They seem to be distributed at a similar level to the particle size of each phase during the incorporation. The sodalite phase contained Cl, but did not likely to contain Cs. Because the amount of Cs was small, its existence could not be confirmed through the SEM photograph. The glass and sodalite phases, two major components of waste form, were not only separated physically by the glass matrix but also combined with each other chemically by reaction at interface, thus clear phase boundary by a phase separation was not observed. Therefore, the used glass is considered as a usable binder. 3.5. Short-term leaching characteristic
The compressive strength and density of waste form with 25 wt% soda glass were 146.8 MPa and 2040 kg/m3, respectively, while those of 40 wt% soda glass were 230.9 MPa and 2260 kg/m3,
As a short-term evaluation for a hydrochemical stability of the waste form, the leaching rates, measured by PCT-A at 363 K for 7
Fig. 1. Photographs of the SEM and element mapping of the SIN-25 waste forms.
H.-Y. Kim et al. / Journal of Industrial and Engineering Chemistry 15 (2009) 293–298
296
Table 3 The leaching rate of the waste forms solidified with varying compositions and amounts of glass. Sample
Leaching rate of Cs
Leaching rate of Sr
Leaching rate of Ba
SIN-25 SIN-40 R7T7-25 R7T7-40
3.892 101 4.013 101 2.389 101 2.1722 103
7.826 104 1.445 103 2.555 103 2.231 102
<4.540 104 5.423 104 4.860 104 <5.899 104
days, are showed in Table 3. Both leaching rates of Sr and Ba were 104 to 103 g/m2 d, that of Cs, however, was 101 g/m2 d, except waste form with 40 wt% borosilicate glass (=103 g/m2 d). Such a high leach resistance of a borosilicate glass-rich waste form may come from a change to glass without any crystalline substance. On the other hand, the leaching rates for Sr and Ba were both very low, which suggests that these nuclides are easily immobilized with stable chemicals having low solubility in water during the incorporation. 3.6. Long-term leaching characteristic Fig. 2 shows the leaching fractions of the waste forms incorporated with soda glass (SIN-25) for a year by the ISO method, which is one of the long-term leaching methods. Leaching fraction was high by the order of boron, the alkali elements, the major elements of the waste forms and the alkaline earth elements. The leaching fractions for the major nuclides Cs, Sr and Ba were 2.83%, 0.11% and 0.06%, respectively. As similar to the PCT method, the leaching fractions of the alkaline earth elements such as Ba and Sr were very low too. Based on the data from the experiments above, a 900 days’ longterm leaching characteristic, by using a semi-empirical equation, can be predicted. The model equation is expressed as follows. CFL ¼ k1 ð1 expðk2 tÞÞ þ k3 t 0:5 þ k4 t Fig. 3 shows the predicted long-term behavior of the major elements of the waste forms and the coefficient of each term of the equation above. The predicted 900 days’ leaching fraction is 5.13% for Cs and 0.24% for Sr. The 900 days’ leaching fractions of Si, Al and Ca, which are major elements of the waste forms, are expected to
Fig. 2. Leaching fractions of the elements obtained by the ISO method.
be 2.85%, 1.94% and 1.00%, respectively. Considering k3 and k4, which shows a diffusion and a dissolution, respectively, it is believed that most of the elements existing in the waste forms except for B and K are leached out through a diffusion mechanism for a long period of time. 3.7. Leaching characteristic according to the pH in the MCC-1P method Fig. 4 shows the leaching behavior according to the pH in the MCC-1 leaching method at 343 K. Buffer solution used at each pH showed almost no change in the pH after the experiment, and the given pH value can be taken as an equilibrium pH. Each element showed a different leaching behavior in acidic, neutral and alkali conditions, but in general the leaching rate was lowest in the neutral region. When the leaching behavior was examined using data from a 28 days’ leaching for a sufficient equilibrium time, Li, Cs and Sr showed different behaviors according to the pH. Li showed a high leaching rate in the acidic or alkali regions, and although this tendency was also observed in the other elements, the leaching rate of Li was higher in the acidic region than in the alkali region. Different from Li, Cs showed a higher leaching rate in the alkali region, and Sr showed a high leaching rate at pH 4 and almost no change in the other conditions.
Fig. 3. Results of predicting the leaching behavior obtained using a semi-empirical equation from the results of an experiment by the ISO method.
H.-Y. Kim et al. / Journal of Industrial and Engineering Chemistry 15 (2009) 293–298
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Fig. 4. Leaching behaviors of the waste forms according to the pH using the MCC-1P method. Normalized leach rate (g/m2).
At a high pH, leached alkaline earth metal elements such as Sr are converted to a hydroxide with a very low solubility and thus can be reprecipitated, and consequently they may show a low leaching rate in an alkali region. But as in Fig. 4, a very high leaching rate was observed at pH 4 and this suggests that most of the Sr is immobilized in a very stable matrix and it is hardly possible to exist in an unstable form (free salt). For Cs, pH 6 and 8 are the transition points of the leaching rate. Cs shows a high leaching rate in the acidic region because of the dissolution of the matrix as the immobilizer of Cs and also in the alkali region because of the desorption of Cs by the chemical change of the matrix due to the
existence of competing ions (Na+ and K+ from buffer solution) and because of the dissolution of the matrix material composed of Si and Al that are a amphoteric elements. Li shows a higher leaching rate in the acidic region than in the alkali region, and this is also because of a dissolution of the immobilizer of Li by an acid such as Cs. Because, with the rise of the pH, there is the possibility of a reprecipitation such as the conversion of Li+ to hydroxide, Li shows a relatively low leaching rate in the alkali region. Si, Al and B, the major elements of the waste forms, showed a high leaching rate in both the acidic region and the alkali region, but the free range from the effect of the pH was different among the
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elements. Al showed an almost constant leaching rate at a wide range of the pH 6–0, and Si showed a similar leaching rate at the pH range of 8–10 and B at the pH range of 6–8. Si showed a relatively high leaching rate in the acidic region, and B in the alkali region. The pH of the leaching solution out of the leaching by the ISO method using demineralized water (pH 7) was around 9, and in this region, Si and Al showed a very low leaching rate but B showed a high rate. In a water solution, Si and B exist mainly in the form of a hydroxo-oxo ion and Al in the form of a hydrate ion. Therefore, the dissolved Al3+ is highly likely to reprecipitate by a rise of the pH except when very high, but Si and B are less likely to recombine with a hydroxide ion unlike Al because they turn into oxo ions with a rise of the pH and they show a high leaching rate in all the regions except for the neutral region because of the low possibility for a precipitation into a hydroxide.
considering the high leaching rate of Cs, it is believed that Cs exists in a free state rather than being immobilized by inorganic matter. In the results of the long-term leaching test, the predicted 900 days’ leaching fraction of Cs was as high as 5.13% but that of Sr was very low at 0.24%. This suggests that the key to a salt waste solidification is raising the leach resistance of Cs. Also in the leaching experiment with a varying pH, the major nuclides such as Cs and Sr, and the Li of the major salt show different leaching characteristics. That is, Li and Cs showed a high leaching rate in both the acidic region and the alkali region, but Li showed a higher leaching rate in the acidic region. On the contrary, Cs showed a higher rate in the alkali region but it existed in an unstable state at any condition, whether alkali or acidic. Sr only showed a high leaching rate in the acidic condition of pH 4 and almost no change in the other conditions, suggesting that it exists in a stable state in most conditions.
4. Conclusions Cs showed a low leaching rate of 1.015 101 g/m2 d only at the mixing ratio (r = 0.1) at which all of the free salt, which is a highly water-soluble chloride, reacts with zeolite and turns into a non-water-soluble sodalite. Because alkali chloride is chemically very stable it does not cause a reaction that changes a component of the glass, and therefore the leach resistance of the waste forms solidified with soda glass is not much different from those solidified with borosilicate glass. However, waste forms solidified with soda glass have a larger amount of crystalline compounds and thus are less affected by a degradation of the waste forms by the heat generation nuclides. In the morphology analysis, the sodalite phase contained Cl, but did not likely to contain Cs. In addition,
Acknowledgment This work was funded by the National Mid- and Long-term Atomic Energy R&D Program supported by the Ministry of Science and Technology of Korea. References [1] M.A. Lewis, M.C. Hash, A.S. Hebden, W.L. Ebert, ANL-02/10 Argonne National Laboratory, 2002, p. 1. (a) G. Talebi, M. Sohrabi, S.J. Royaee, R.L. Keiski, M. Huuhtanen, H. Imamverdizadeh, J. Ind. Eng. Chem. 14 (5) (2008) 614. [2] W.L. Ebert, ANL-05/43 Argonne National Laboratory (2005) p. IV1. [3] J.G. Kim, J.H. Lee, I.T. Kim, E.H. Kim, J. Ind. Eng. Chem. 13 (2) (2007) 292.
Contents lists available at ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Study on the leaching behavior of salt waste forms prepared by using zeolite and lime glass Hwan-Young Kim *, Jeong-Guk Kim, In-Tae Kim, Hwan-Seo Park Korea Atomic Energy Research Institute, Yuseong, Daejon 305-353, Republic of Korea
A R T I C L E I N F O
Article history: Received 12 February 2008 Accepted 17 November 2008 Keywords: Free salt Zeolite Waste form Electrolytic reduction process Lime glass Borosilicate glass
A B S T R A C T
When the reaction of salt and zeolite was used to minimize the free salt in waste forms (r = 0.1), Cs showed the lowest leaching rate, 1.015 101 g/m2 d. Because alkali chloride is chemically stable, the reaction that alkali elements become components of glass does not happen and thus the leach resistance of the waste form solidified with soda glass was not much different from that solidified with borosilicate glass. The crystalline phase containing Cl was sodalite, but the tendency that Cs exists prior to sodalite phase was not confirmed. From a result of a long-term leaching, the predicted leaching fraction of Cs in 900 days was as high as 5.13%, but that of Sr was as low as 0.24%. The leaching experiment with a varying pH showed the major nuclides such as Cs, Sr, and Li in salt waste had different leaching characteristics each other. ß 2009 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
1. Introduction Radioactive salt waste generated from the electrolytic reduction process, which reduces spent nuclear fuel electrolytically, has some alkali, alkaline earth, and rare earth nuclides that have been reduced formerly to metals by lithium metal in the electrolytic reduction process and converted to chlorides in the molten salt of LiCl or that are easier to exist as chlorides out of a metal conversion in a molten LiCl salt. Of course, besides these nuclides, there are also some daughter nuclides generated from the beta decay of the nuclides contained in a salt. Although these daughter nuclides are highly likely to exist as non-radioactive chemical species due to their short half lives, they also affect the leach resistance of waste form. So we need to consider their effect on waste forms. In addition, along with nuclides, a large amount of LiCl used as a molten salt medium is generated as a waste. Because they are highly water-soluble and too difficult to form a highly integrated waste form, they cannot be disposed as they are. Thus, two-step method to fabricate a final waste form for salt waste, which is composed of conversion into a water-insoluble form and then transformation into a monolithic form, is generally suggested. US ANL (Argonne National Laboratory) actually showed an enhancement for the leach resistance of salt waste form by adsorbing the radionuclides with zeolite and immobilizing the salt in zeolite as a chemical species of product other than a free salt [1,1a,2]. Such result
* Corresponding author. E-mail address: [email protected] (H.-Y. Kim).
was obtained for a LiCl–KCl eutectic salt waste, while an application for a LiCl salt waste was recently accomplished by Kim et al. [3]. When LiCl salt containing Cs, Sr, or Ba reacts with zeolite 4A at 1188 K, a sodalite, which is stable even in water, is formed and a free salt is immobilized within the sodalite, as shown in follow reaction. Na12 ½AlO2 Þ12 ðSiO2 Þ12 nH2 O þ 4LiCl ! 2Na6 Li2 ½AlO2 Þ6 ðSiO2 Þ6 Cl2 In this reaction, 1 mol zeolite reacts with 4 mol LiCl and produces 2 mol sodalite, and the weight ratio of sodalite to LiCl is 100:9.95, which shows an 10 times increase of product compared to its original salt waste. In this work, we examined the leaching characteristic of waste forms solidified SLZ (salt-loaded zeolite) with lime glass (SINIL) or borosilicate glass (R7T7) used in solidification of high-level waste (HLW) in France, respectively. In addition, we also attempted to characterize the waste forms by XRD (X-ray diffractometer) and SEM (scanning electron microscope). Also the leaching characteristic of the waste forms solidified with a reduced amount of zeolite A was compared with the results above in order to establish the possibility of reducing the quantity of the waste forms. 2. Experimental 2.1. Chemicals The molten LiCl salt was prepared by heating commercial LiCl powder (Aldrich, 99+%) to 923 K. The simulated salt waste was prepared by a physical mixing of LiCl, CsCl (Aldrich, 99.9%), SrCl2 (Aldrich, 99.9%), and BaCl2 (Aldrich, 99.9%) powders in an Ar glove
1226-086X/$ – see front matter ß 2009 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jiec.2008.11.009
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box and then a heating under Ar gas flow conditions. A granulartype zeolite A [Aldrich, Molecular Sieves, 4A, Na12(AlSiO4)12] with an 8–12 mesh size was used as an immobilization medium. Zeolite A was dehydrated before use by heating it with a purge of nearly pure nitrogen (99.999+%); it was stored in an Ar glove box. 2.2. Apparatus 2.2.1. Dehydrator The as-received zeolite A contained up to 20 wt% of water. If the water in the zeolite is not extracted during the salt-loading process, the radioactive nuclides and chlorides might not be fully loaded onto the zeolite or they might become hydroxides. Dry nitrogen gas was passed through a packed zeolite column, which was heated to 820 K by a cylindrical furnace. After a dehydration, the water content of the zeolite was below 0.5 wt%. 2.2.2. Blender Salt-loaded zeolite (SLZ) samples were prepared by a V-type blender. A rotating V-type blender located within an electric furnace was used to mix the zeolite A and LiCl salt at r = 0.4, 0.25, and 0.1. The blender was made of stainless steel (SUS-316) and it was connected to a vacuum pump via a needle valve and could be maintained at 923 5 K. 2.2.3. Furnace A muffle furnace was used for fabrication of a final waste form sample at 1193 K.
Table 1 The compositions of the used glasses. Component
Mixed ratio (wt%) Lime glass (SIN)
Borosilicate glass (R7T7)
SiO2 Na2O K2O CaO ZnO MgO BaO SrO Al2O3 Fe2O3 Li2O TiO2 B2O3
55.0 0.5 – 21.8 – 0.5 – – 14.5 0.2 – 0.5 7.5
53.3 11.5 – 4.7 2.9 – – – 8.8 – 2.3 – 16.4
(Micrometrics ASAP 2400, USA). An international standards organization (ISO) method is an evaluation method for the leach resistance of the waste form under long-term controlled conditions. The test was accomplished at 343 K for more than 365 days. The materials characterization center test no. 1 (MCC-1), which is accomplished at 343 K for more than 7 days, can be used to characterize a corrosion mechanism. The both ratios of the volumes of demineralized water for ISO and buffer solution for MCC-1 to surface areas of waste form samples are both 10. 3. Results and discussion
2.3. Procedure
3.1. Leaching characteristic according to mixing ratio of salt to zeolite
2.3.1. SLZ preparation The SLZ was formed during blending granular zeolite A and a mixture of salt powders were loaded into V-type blender. The mixture of salt powders was prepared by adding of 5.32 wt% CsCl, 2.89 wt% SrCl2, and 4.96 wt% BaCl2 to a lithium compound (97 wt% LiCl and 3 wt% Li2O). The blender was sealed and placed in the furnace, which could heat the whole vessel to 923 K. Before a heating, the blending vessel was evacuated using a mechanical vacuum pump. The vessel was rotated at 20 rpm under a heating rate of 5 K/min. Once the furnace temperature had reached 923 K, vacuum conditions were re-established prior to 20-h isothermal blending with a rotation. After being cooled, the blended zeolite was stored in an Ar-atmosphere glove box.
A mixing ratio of salt to zeolite, r (=salt/zeolite) had a significant influence on a leaching characteristic. The leaching rate of Cs at r = 0.1 was 1.015 101 g/m2 d, but the leaching rates at r = 0.25 and 0.4 were 1.148 and 1.430 g/m2 d, respectively, increasing by around 10 times. Such effect was the same on the leaching rate of Sr. The leaching rate of Sr at r = 0.1 was 2.089 102 g/m2 d, showing a high leach resistance, but the leaching rates at r = 0.25 and 0.4 were 9.221 101 and 2.026 101 g/m2 d, respectively, increasing significantly. Even if we consider that the amount of Sr added to the waste form was smaller than that of Cs, the result shows that Sr is leached away less than Cs. These results of Cs and Sr show that an optimum mixing ratio exists to form a good waste form with a high leach resistance. When LiCl was incorporated with zeolite of 10 times more weight, the leaching rate approached a considerably low value, as discussed in a theoretical review, because most of the LiCl reacted with zeolite and transformed to a component of sodalite having very low solubility in water. Therefore, if the mixing ratio of salt to zeolite increases, a free salt, not-reacted excessive LiCl, would also increase significantly, resulting in an increase of the leaching rate of radionuclides, such as Cs and Sr.
2.3.2. Solidification The granular SLZ crushed to be fine powder below 100 mesh, and then mixed with glass powder of 100–200 mesh. The waste form samples were obtained by heating the mixture in an electric furnace at 1193 K for 72 h. The glass frits used in this experiment were soda glass (SINIL, Korea) and borosilicate glass (simulated R7T7), which was used to treat HLW in France, shown in Table 1. The glass powder were added up to 25(glass was 1/3 of SLZ) and 40 wt% of final waste form. The final waste form had 3.5–6% less weight than before solidification because of LiCl volatilization. Solidification was carried out with an injection of argon gas to minimize an oxidation of carbon vessel filled with the mixture of SLZ and glass powder. 2.3.3. Leaching test The leaching characteristics were tested by three different methods. A product consistency test (PCT) is an evaluation method for the chemical durability of a waste form. 30 mL demineralized water was used for 3 g waste form sample, which was crushed in 100–200 mesh. The PCT was performed at 90 8C for more than 7 days. The surface area was measured through a BET analysis
3.2. The crystallinity of the waste forms obtained according to the type and amount of glass frit Table 2 shows the XRD analysis (by Bruker D8 Advance with TOPAS V3 software) results crystallized substances for the final waste forms, which were fabricated by an incorporation of SLZ with r = 0.1 and two different glass frits. In the soda glass frit (SIN), the contents of SiO2 and Al2O3 are high and CaO exists as a substitute for the alkali and boron. Compared to borosilicate glass (R7T7), its thermal characteristic (glass point and mp) is relatively high and thus its characteristic of a reaction with SLZ at a given manufacturing temperature 1188 K is different. Due to the difference solubility of the sodalite in glasses at the same temperature, waste forms incorporated with borosilicatie glass are more amorphous than
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Sample
Result of XRD
respectively. These phenomena is considered due to that the matrix of waste form is changed more dense structure, because the effects of dissolution in glass and binding the crystals increase when the content of glass increase.
SIN-25
Sodalite 46.69% Albite (NaAlS3O8) 53.31%
3.4. Morphology of the Cs
SIN-40
Sodalite 28.49% Anorthite (CaAl2S2O8) 71.59%
R7T7-25
Sodalite 91.65% Halite 8.35%
R7T7-40
–
Table 2 The results of the XRD analysis of the crystallized substance in the waste forms.
those with soda glass, more transparent and compact waste forms from borosilicate glass were obtained. In the waste forms incorporated with soda glass (40 wt%), there is sodalite as well as crystallized calcium aluminum silicate that appears through a annealing of metallic aluminum silicate over Tg (723–823 K). This phenomenon shows that the glass plays the role of a binder that enables a physical binding without a perfect solubility to the sodalite. The soda glass seems to be more favorable for the salt waste containing lots of heat generation nuclides such as Cs and Sr because it helps to form a high crystalline product. 3.3. Compressive strength of the waste forms according to amount of glass
Fig. 1 shows the SEM photographs (by EPMA, JEOL JAX-8600) of the waste form (SIN-25) incorporated with soda glass. It shows the morphology between the glass medium and the sodalite phase. Because the elements of the added glass matrix are almost identical to the major elements of sodalite, we examined the phase through a mapping for the elements only existing in the glass and those in the sodalite. As seen in the figure, the part where Ca exists is clearly distinguished from the part where Cl exists, and they are mixed with each other at a level of 100 mm. They seem to be distributed at a similar level to the particle size of each phase during the incorporation. The sodalite phase contained Cl, but did not likely to contain Cs. Because the amount of Cs was small, its existence could not be confirmed through the SEM photograph. The glass and sodalite phases, two major components of waste form, were not only separated physically by the glass matrix but also combined with each other chemically by reaction at interface, thus clear phase boundary by a phase separation was not observed. Therefore, the used glass is considered as a usable binder. 3.5. Short-term leaching characteristic
The compressive strength and density of waste form with 25 wt% soda glass were 146.8 MPa and 2040 kg/m3, respectively, while those of 40 wt% soda glass were 230.9 MPa and 2260 kg/m3,
As a short-term evaluation for a hydrochemical stability of the waste form, the leaching rates, measured by PCT-A at 363 K for 7
Fig. 1. Photographs of the SEM and element mapping of the SIN-25 waste forms.
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Table 3 The leaching rate of the waste forms solidified with varying compositions and amounts of glass. Sample
Leaching rate of Cs
Leaching rate of Sr
Leaching rate of Ba
SIN-25 SIN-40 R7T7-25 R7T7-40
3.892 101 4.013 101 2.389 101 2.1722 103
7.826 104 1.445 103 2.555 103 2.231 102
<4.540 104 5.423 104 4.860 104 <5.899 104
days, are showed in Table 3. Both leaching rates of Sr and Ba were 104 to 103 g/m2 d, that of Cs, however, was 101 g/m2 d, except waste form with 40 wt% borosilicate glass (=103 g/m2 d). Such a high leach resistance of a borosilicate glass-rich waste form may come from a change to glass without any crystalline substance. On the other hand, the leaching rates for Sr and Ba were both very low, which suggests that these nuclides are easily immobilized with stable chemicals having low solubility in water during the incorporation. 3.6. Long-term leaching characteristic Fig. 2 shows the leaching fractions of the waste forms incorporated with soda glass (SIN-25) for a year by the ISO method, which is one of the long-term leaching methods. Leaching fraction was high by the order of boron, the alkali elements, the major elements of the waste forms and the alkaline earth elements. The leaching fractions for the major nuclides Cs, Sr and Ba were 2.83%, 0.11% and 0.06%, respectively. As similar to the PCT method, the leaching fractions of the alkaline earth elements such as Ba and Sr were very low too. Based on the data from the experiments above, a 900 days’ longterm leaching characteristic, by using a semi-empirical equation, can be predicted. The model equation is expressed as follows. CFL ¼ k1 ð1 expðk2 tÞÞ þ k3 t 0:5 þ k4 t Fig. 3 shows the predicted long-term behavior of the major elements of the waste forms and the coefficient of each term of the equation above. The predicted 900 days’ leaching fraction is 5.13% for Cs and 0.24% for Sr. The 900 days’ leaching fractions of Si, Al and Ca, which are major elements of the waste forms, are expected to
Fig. 2. Leaching fractions of the elements obtained by the ISO method.
be 2.85%, 1.94% and 1.00%, respectively. Considering k3 and k4, which shows a diffusion and a dissolution, respectively, it is believed that most of the elements existing in the waste forms except for B and K are leached out through a diffusion mechanism for a long period of time. 3.7. Leaching characteristic according to the pH in the MCC-1P method Fig. 4 shows the leaching behavior according to the pH in the MCC-1 leaching method at 343 K. Buffer solution used at each pH showed almost no change in the pH after the experiment, and the given pH value can be taken as an equilibrium pH. Each element showed a different leaching behavior in acidic, neutral and alkali conditions, but in general the leaching rate was lowest in the neutral region. When the leaching behavior was examined using data from a 28 days’ leaching for a sufficient equilibrium time, Li, Cs and Sr showed different behaviors according to the pH. Li showed a high leaching rate in the acidic or alkali regions, and although this tendency was also observed in the other elements, the leaching rate of Li was higher in the acidic region than in the alkali region. Different from Li, Cs showed a higher leaching rate in the alkali region, and Sr showed a high leaching rate at pH 4 and almost no change in the other conditions.
Fig. 3. Results of predicting the leaching behavior obtained using a semi-empirical equation from the results of an experiment by the ISO method.
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Fig. 4. Leaching behaviors of the waste forms according to the pH using the MCC-1P method. Normalized leach rate (g/m2).
At a high pH, leached alkaline earth metal elements such as Sr are converted to a hydroxide with a very low solubility and thus can be reprecipitated, and consequently they may show a low leaching rate in an alkali region. But as in Fig. 4, a very high leaching rate was observed at pH 4 and this suggests that most of the Sr is immobilized in a very stable matrix and it is hardly possible to exist in an unstable form (free salt). For Cs, pH 6 and 8 are the transition points of the leaching rate. Cs shows a high leaching rate in the acidic region because of the dissolution of the matrix as the immobilizer of Cs and also in the alkali region because of the desorption of Cs by the chemical change of the matrix due to the
existence of competing ions (Na+ and K+ from buffer solution) and because of the dissolution of the matrix material composed of Si and Al that are a amphoteric elements. Li shows a higher leaching rate in the acidic region than in the alkali region, and this is also because of a dissolution of the immobilizer of Li by an acid such as Cs. Because, with the rise of the pH, there is the possibility of a reprecipitation such as the conversion of Li+ to hydroxide, Li shows a relatively low leaching rate in the alkali region. Si, Al and B, the major elements of the waste forms, showed a high leaching rate in both the acidic region and the alkali region, but the free range from the effect of the pH was different among the
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elements. Al showed an almost constant leaching rate at a wide range of the pH 6–0, and Si showed a similar leaching rate at the pH range of 8–10 and B at the pH range of 6–8. Si showed a relatively high leaching rate in the acidic region, and B in the alkali region. The pH of the leaching solution out of the leaching by the ISO method using demineralized water (pH 7) was around 9, and in this region, Si and Al showed a very low leaching rate but B showed a high rate. In a water solution, Si and B exist mainly in the form of a hydroxo-oxo ion and Al in the form of a hydrate ion. Therefore, the dissolved Al3+ is highly likely to reprecipitate by a rise of the pH except when very high, but Si and B are less likely to recombine with a hydroxide ion unlike Al because they turn into oxo ions with a rise of the pH and they show a high leaching rate in all the regions except for the neutral region because of the low possibility for a precipitation into a hydroxide.
considering the high leaching rate of Cs, it is believed that Cs exists in a free state rather than being immobilized by inorganic matter. In the results of the long-term leaching test, the predicted 900 days’ leaching fraction of Cs was as high as 5.13% but that of Sr was very low at 0.24%. This suggests that the key to a salt waste solidification is raising the leach resistance of Cs. Also in the leaching experiment with a varying pH, the major nuclides such as Cs and Sr, and the Li of the major salt show different leaching characteristics. That is, Li and Cs showed a high leaching rate in both the acidic region and the alkali region, but Li showed a higher leaching rate in the acidic region. On the contrary, Cs showed a higher rate in the alkali region but it existed in an unstable state at any condition, whether alkali or acidic. Sr only showed a high leaching rate in the acidic condition of pH 4 and almost no change in the other conditions, suggesting that it exists in a stable state in most conditions.
4. Conclusions Cs showed a low leaching rate of 1.015 101 g/m2 d only at the mixing ratio (r = 0.1) at which all of the free salt, which is a highly water-soluble chloride, reacts with zeolite and turns into a non-water-soluble sodalite. Because alkali chloride is chemically very stable it does not cause a reaction that changes a component of the glass, and therefore the leach resistance of the waste forms solidified with soda glass is not much different from those solidified with borosilicate glass. However, waste forms solidified with soda glass have a larger amount of crystalline compounds and thus are less affected by a degradation of the waste forms by the heat generation nuclides. In the morphology analysis, the sodalite phase contained Cl, but did not likely to contain Cs. In addition,
Acknowledgment This work was funded by the National Mid- and Long-term Atomic Energy R&D Program supported by the Ministry of Science and Technology of Korea. References [1] M.A. Lewis, M.C. Hash, A.S. Hebden, W.L. Ebert, ANL-02/10 Argonne National Laboratory, 2002, p. 1. (a) G. Talebi, M. Sohrabi, S.J. Royaee, R.L. Keiski, M. Huuhtanen, H. Imamverdizadeh, J. Ind. Eng. Chem. 14 (5) (2008) 614. [2] W.L. Ebert, ANL-05/43 Argonne National Laboratory (2005) p. IV1. [3] J.G. Kim, J.H. Lee, I.T. Kim, E.H. Kim, J. Ind. Eng. Chem. 13 (2) (2007) 292.