Environ. Sci. Technol. 2007, 41, 7536-7542
Characteristics of Solidified Products Containing Radioactive Molten Salt Waste HWAN-SEO PARK,* IN-TAE KIM, YONG-ZUN CHO, HEE-CHUL EUN, AND JOON-HYUNG KIM Advanced Fuel Cycle Development, Korea Atomic Energy Research Institute, 150 Deokjin-dong, Yuseong -gu, Daejeon 305-353, Korea
The molten salt waste from a pyroprocess to recover uranium and transuranic elements is one of the problematic radioactive wastes to be solidified into a durable wasteform for its final disposal. By using a novel method, named as the GRSS (gel-route stabilization/solidification) method, a molten salt waste was treated to produce a unique wasteform. A borosilicate glass as a chemical binder dissolves the silicate compounds in the gel products to produce one amorphous phase while most of the phosphates are encapsulated by the vitrified phase. Also, Cs in the gel product is preferentially situated in the silicate phase, and it is vitrified into a glassy phase after a heat treatment. The Sr-containing phase is mainly phosphate compounds and encapsulated by the glassy phase. These phenomena could be identified by the static and dynamic leaching test that revealed a high leach resistance of radionuclides. The leach rates were about 10-3∼10-2g/m2‚day for Cs and 10-4∼10-3g/m2‚day for Sr, and the leached fractions of them were predicted to be 0.89% and 0.39% at 900 days, respectively. This paper describes the characteristics of a unique wasteform containing a molten salt waste and provides important information on a newly developed immobilization technology for salt wastes, the GRSS method.
I. Introduction Molten salt waste, which is generated from the pyrochemical process to separate uranium and transuranium elements from a spent nuclear fuel, has been of interest to researchers in the radioactive waste management field. In Korea, the electroreduction process, whose main purpose is to convert a spent oxide fuel into a metallic form in a molten lithium chloride (LiCl) salt bath, is under development (1). As a result, LiCl salt wastes are anticipated to be generated from an electrometallurgical processing (or pyroprocess) of the spent fuels. The spent LiCl waste is highly soluble in water and contains a relatively high amount of volatile radioactive elements such as Cs. This waste containing a small amount of fission products should be stabilized and solidified to reduce its environmental risk before its final disposal. However, it is difficult to directly apply them to the conventional stabilization/solidification method due to the high volatility of their chloride compounds and their compatibility with silicate glasses (2). As a solution, Argonne National Laboratory (ANL) suggested the conversion of metal * Corresponding author fax: 82-042-868-2329; e-mail: hspark72@ kaeri.re.kr. 7536
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chlorides into a sodalite for the immobilization of a chloride waste (3-7). Glass-bonded sodalite, which was fabricated at about 915 °C after mixing the salt-loaded zeolite and borosilicate glass powder, had a good leach resistance. Phosphate glass as an alternative to borosilicate glass has been considered as a host matrix for chlorides, and it has a high leach resistance (8-10). Iron phosphate glasses with melting temperatures as low as 950 °C reduce the chance of Cs being volatilized during a vitrification. A wet dechlorination to oxides or a thermal conversion into a borate glass was also investigated to remove the Clinduced disadvantage such as limitation as a host matrix or its volatility (11-13). As an alternative to the dechlorination, our research group established a gel-forming material system which consists of a gelling agent, a catalyst/stabilizer, and a property promoter. This reaction system could guarantee the conversion of radionuclides in a salt waste into stable compounds; there would be little vaporization of Cs up to 1100 °C, and the leached fractions of Cs and Sr in its solidified product were about 0.72% and 0.012%, respectively. As one of the immobilization technologies for a radioactive molten salt waste, it is named the GRSS (gel-route stabilization/ solidification) method (14). Previous study was focused on a “stabilization” of metal chlorides via an effective reaction system, but it did not fully investigate a “solidification” of the reaction products. With a unique wasteform containing molten salt waste, it should be characterized and qualified to see if it meets the requirements for the final disposal. Among the important factors to evaluate a new radioactive wasteform, its leach resistance is the most crucial one. In the ANL method, a borosilicate glass was used as a physical or chemical binder to produce a consolidated wasteform. The GRSS method also adopted this concept for the fabrication of a monolithic wasteform, where unknown chemical interactions between the glass and the reaction products from a gel-forming system might affect the properties of a wasteform, especially its chemical durability since there might be phenomena such as a “encapsulation by the glass” or a “dissolution into the glass”. The objective of this paper is to investigate the interaction of gel products with a series of glasses as a binder to produce a monolithic wasteform and compare the chemical durability of each wasteform by a static and dynamic leaching test method. This could provide important information on a unique wasteform containing a radioactive salt waste prepared by a novel method, the GRSS.
II. Experimental Procedures Materials. LiCl, CsCl, and SrCl2 (99% purity, Merck) with a composition of 90 wt %, 6.8 wt %, and 3.2 wt %, respectively, were used to simulate the waste salt. A gel-forming material system was selected according to previous results: sodium silicate powder (36 wt % Na2O, 64 wt % SiO2, Junsei) as a gelling agent, phosphoric acid (85 wt % purity, Showa) as an acidic catalyst/stabilizer, and Al(NO3)3‚9H2O as a property promoter. The molar ratios of Si/P/Al were 0.4/0.4/0.2 and 0.35/0.35/0.3, and the waste loadings were varied. The batch compositions are indicated in Table 1. Three kinds of glasses as chemical binders to form a monolithic wasteform were used, and they are also indicated in Table 2. Solidification Method. A simulated salt waste was first gelled by a selected material system to convert it into stable compounds by the following procedure. Each material was dissolved in the deionized water at room temperature and mixed with strong stirring for 5 min to obtain a clear solution. 10.1021/es0712524 CCC: $37.00
2007 American Chemical Society Published on Web 10/03/2007
TABLE 1. Batch Composition of Gel-Forming Systema Si/P/Al ) 0.4/0.4/0.2
metal chloride sodium silicate phosphoric acid aluminum nitrate a
LiCl CsCl SrCl2 SiO2 Na2O P2O5 Al2O3
Si/P/Al ) 0.35/0.35/0.3
W20
W25
W30
W35
W15
W20
W25
W30
18.00 1.36 0.64 14.40 8.10 17.04 6.12
22.50 1.70 0.80 14.40 8.10 17.04 6.12
27.00 2.04 0.96 14.40 8.10 17.04 6.12
31.50 2.38 1.12 14.40 8.10 17.04 6.12
13.50 1.02 0.48 12.60 7.09 14.91 9.18
18.00 1.36 0.64 12.60 7.09 14.91 9.18
22.50 1.70 0.80 12.60 7.09 14.91 9.18
27.00 2.04 0.96 12.60 7.09 14.91 9.18
Unit: gram.
TABLE 2. Glass Composition of Borosilicate and Aluminosilicate Glassa G1 G2 G3 G4 B0 B1 Al a
Na2O
Li2O
CaO
B2O3
Al2O3
SiO2
ZnO
remarks
6.65 7.74 9.17 13.70 11.90 12.03 11.80
2.39 2.88 -
1.38 1.60 1.89 2.82 4.88 8.28 -
19.64 22.84 27.04 40.40 16.94 15.58 -
7.61 8.84 10.46 15.63 5.93 7.14 19.50
64.81 58.98 51.44 27.45 54.94 50.46 68.70
3.02 3.63 -
referred glass
R7T7 glass
Unit: wt%.
The mixture was poured into a plastic bottle and placed into an electric furnace after being tightly sealed. Gelation was carried out at 70 °C for different aging times, 1∼7 days, and the products were dried at 110 °C for 2∼3 days. The gel products were mixed with the selected the glass powders with the different mixing ratios. The mixture with particles less than 150 µm in size were heat-treated at 1000∼1200 °C for 2 h in an inert gas phase. Used borosilicate glasses in this study were selected by considering the process temperature; as a component to lower the process temperature, B2O3 in the glass (G1, G2, G3, and G4) based on the ANL method and Na2O in the R7T7 glass (B0 and B1) were considered. Aluminosilicate glass (A1) with a high melting temperature was also applied for a solidification to evaluate its effect on the leach resistance. As a binding material for a monolithic wasteform, a glass must have a low working point for processability while it needs high silica (means high working temperature) for the durability of a wasteform. Characterization. The solidified products were characterized by X-ray powder diffraction (XRD, Rikaku, Cu KR radiation). Microstructure of each sample was obtained by scanning electron microscopy (SEM-EDX, Hitachi S 800). Also, the thermal stability of mixtures before heat treatment was evaluated by thermogravimetric analysis (TGA, SEIKO 6300). These analyses were implemented to select a useful condition for the fabrication of a reliable wasteform. Leach Test. Basically, the leach resistance of the solidified products was evaluated by the PCT-A leaching method (ASTM Standard C 1285-02) at 90 °C. In addition to this method as a static equilibrium leaching test, the ISO method (15) was carried out to evaluate the dynamic leaching behavior of the radionuclides and glass components at 70 °C. The concentrations in the leachant were determined by using atomic absorption spectrometry (AAS) and induced couple plasma spectrometry (ICP-AES).
III. Results and Discussion Characterization of the Solidified Products. Using a SiP-Al system, metal chlorides could be converted into metal aluminosilicates, metal aluminophosphates, and metal phosphates. At a real composition of salt wastes, a series of solidified products prepared in this study contained Li3PO4 and NaCl as crystalline phases, depending on the waste
loadings (see Figure S1 in Supporting Information). For the case of the excess over the optimum salt loading in the SiP-Al system, the excess metal chlorides in the salt waste are expressed as NaCl since Li, Cs, and Sr are more reactive in the reaction system than Na. Figure 1 shows the XRD patterns of the solidified products prepared with the borosilicate glasses with the different compositions (G1∼G4). NaCl in the solidified products gradually disappeared to G4, possibly because the silicate glass accommodated a small content of the metal chlorides. The weight loss in 600∼1100 °C by using the TGA analysis, as shown in Figure 2, could support the condition of the optimum waste loading, W20. The glass composition was based on the one used in the ANL method. G1, G2, and G3 were set up for the binding effec,t and G4 was set up for a perfect vitrification. Figure 3 shows the backscattered and the element-mapping images of the solidified products with different glass compositions, indicating two distinctive phases: the Si-rich phase and the P-rich phase. It should be noted that Al in the solidified products is uniformly distributed into the Si- and P-rich phases. As shown in Figure 3, the P-rich phase might be assigned to Li3PO4 or aluminophosphate, and the Si-rich phase contains glass and metal aluminosilicate as one of the gel products. Different from other solidified products, the distribution of the elements in the solidified product with G4 glass shows a uniform distribution. Considering that this composition of G4 is an extreme case to confirm a compatibility between the glass and gel product, the homogeneity of this solidified product would indicate that borosilicate glass can strongly interact with the gel products, namely, a strong chemical binding. These results revealed the information on the consolidation mechanism, as shown in Figure 4. There would be three kinds of chemical forms in the gel-products: Si-rich phase, Al-containing intermediate phase, and P-rich phase. The Si-rich phase is dissolved in a given borosilicate glass to produce one phase while the P-rich phase is encapsulated by the glassy phase, where a fraction of the P-rich phase might be dissolved into the glass phase. By these phenomena, a monolithic wasteform could be obtained. The XRD patterns of a series of solidified products prepared by using B0, B1, and Al glass powder with various mixing ratios at different temperatures are indicated in Figure S2 in the Supporting Information. Similar to those by G1∼G3, VOL. 41, NO. 21, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. XRD patterns of a series of solidified products with different glass compositions (b: Li3PO4, 0: NaCl). the solidified products by B0 and B1 contain Li3PO4 as a crystalline phase while some solidified products by Al glass contain LiAlSi2O6 that distinctively appeared with a decrease of the mixing ratio of the glass. From these results, it could be inferred that the borosilicate glasses encapsulated the phosphate phases while the given aluminosilicate glass might dissolve them into one phase. These phenomena could be confirmed by the shape of the bulk wasteforms (see the photograph of the wasteform at Figure S3 in Supporting Information). With the naked eye, the wasteforms by the Al glass look to have a good shape. Leach Resistance under a Static Leaching Test. Figures 5 and 6 indicated the leaching results obtained from the PCT-A method on a series of solidified products prepared at different conditions. Leach rate (LR) was calculated by the following equation.
LRi )
mi fiSt
(1)
where mi is a leached amount of i element in a solution (g), fi is a weight fraction of i element in a solid, S is a surface area of a test sample (0.0299 m2/g), and t is the test duration (day). As shown in Figure 5, the leach rates of Cs and Sr were 10-3∼10-2 g/m2‚day and 10-4∼10-3 g/m2‚day, respectively. The leach rate of Li increased with the Si/Na ratio. It is wellknown that an increase of Si/Na in a borosilicate glass changes 7538
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its thermal property, glass transition point, softening point, working point, and melting point, which might affect its interaction with a gel product at a given temperature. The solidified products by G1 glass with a higher thermal property showed a relatively higher leach rate when compared with the solidified products with G2 glass. Assuming that the leached fraction is an unstable fraction in the solidified products under the PCT test condition, the given temperature to acquire a chemical interaction with an unstable phase is relatively insufficient for the G1 glass. For this reason, the leach rate of Cs or Sr in the solidified products by G1 glass is higher than those by G2 glass. However, at Al ) 0.3, their leach rates showed a small change with the Si/Na ratio. In the GRSS reaction system, Si-Al-P, there are three kinds of reaction routes to produce metal aluminosilicate (M2OAl2O3-SiO2), metal aluminophosphate (M2O-Al2O3-P2O5), and metal phosphate (MO or M2O-P2O5), and they might exist as an amorphous or crystalline phase. Among them, M2O-Al2O3-P2O5 (glassy phase) is known to be more stable than alkali-phosphate glass. An increase of the Al mole fraction can form a higher fraction of Si-O-Al or Al-O-P to produce aluminophosphates and aluminosilicates. Therefore, a small change of the leach rate with the Si/Na ratio could be caused by a decrease of a fraction of an unstable phase or an increase of the stability of phosphate-related compounds.
FIGURE 2. Thermogravimetric analysis of the mixture of gel product and glass powder.
For a vitrified product at Si/Na ) 1 as an extreme case, the change of leach rate of the Cs was greater than that of Li, but the leach rate of Sr was slightly changed. This result would indicate the effect of glass on an immobilization of each radionuclide. A fraction of Li exists mainly as Li3PO4 which would be encapsulated by a given glass. Therefore, its leach resistance is greatly affected by the durability of glass. For Cs and Sr, the small change of the leach rate at Si/Na ) 3∼5 shows that they exist as very stable products, showing a relatively low effect of glass. Also, from a greater change of the leach rate of Cs at Si/Na ) 1, a Cs-containing phase in the gel products might be more soluble to a glass phase than a Sr-containing phase. These phases are uniformly distributed at a size below µm, as shown in Figure 3. At a real composition of the waste with a high Li/Cs and Li/Sr ratio, we cannot establish the preference of Cs for the reaction with aluminosilicate or phosphates from only the XRD analysis. However, the leaching behavior shows additional information. Under the assumption that Li is more reactive than Cs, most of the Cs should be located at a P-rich phase and encapsulated by a Si-rich phase, thus the leaching behavior must be greatly affected by the glass composition. The Cs-containing phase is very stable and hardly changed with the glass compositions. Therefore, the Cs-containing phase would be mainly aluminosilicate compounds, meaning a preference of Cs for Si-O-Al. Figure 6 shows the effect of the mixing ratio of glass on the leach resistance. It is well-known that an alkali oxide or boron oxide can be used to lower the melting temperature of a borosilicate glass. B0 and B1 have a relatively high alkali
FIGURE 3. Element-mapping image of the solidified products with different glass compositions (W20). VOL. 41, NO. 21, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 4. Schematic solidification model for a mixture of gelproducts and borosilicate glass. (This is based on the RPRM (reaction product in reaction module) model in previous work, ref 14).
FIGURE 6. Leach rate of the solidified products with different mixing ratios of the glasses.
FIGURE 5. Leach rate of the solidified products with different glasses. oxide content while the G1∼G3 glass had more boron oxide; the melting temperature of B0 was lower than that of G1. So, B0 and B1 glass were chosen to maximize the chemical binding effect on the leach resistance to evaluate the mixing ratio of glass. As described in the XRD analysis, the solidified product fabricated with Al glass contained NaCl and we could not detect Li3PO4 which was in the solidified products prepared by the borosilicate glass. The Al glass had a high melting temperature, but its compatibility with the phosphate in the gel products might be relatively high. As shown in Figure 6, the leach rate of Li slightly decreased with the mixing ratio of the glass and was not really changed with the kind of glass. For Cs and Sr of the solidified products with borosilicate glass, their leach rates increased with the 7540
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mixing ratio of the glass while that for Sr with Al glass decreased. An increase of the mixing ratio of a glass means that its functions as a chemical binder, its dissolution and encapsulation of an unstable phase, might be strengthened. Considering that the leach fraction of an element under the PCT test condition could be considered as a fraction of an unstable phase, little change of Li means that an unstable Li-containing phase was not chemically interacted with the glass, namely, the glass functioned as a physical binder for Li. The change of the leach rate of Cs and Sr revealed that the radionuclides-containing unstable phases must be chemically dissolved into the glass phase. However, the magnitude of a change of the leach rate was not great, and also the effect of the glass binder on the leach resistance was not significant; the gel-forming system guarantees a chemical conversion into stable products. So, the optimum mixing ratio of the glass might be determined by whether the bulk shape of the solidified products reveals a monolithic wasteform or not. Considering a leach resistance, the final volume, and shape of a wasteform, we concluded that the optimum condition for the mixing ratio was about 0.7∼1.0. At this condition, the leach rates of Li, Cs, and Sr were 10-2∼10-1, 10-3∼10-2, and 10-4∼10-3 g/m2‚day, respectively. It should be noted that the Al glass revealed a very good shape of a wasteform but its leach resistance of the radionuclides was
relatively low, possibly due to the same reason as the case of the G1 glass, the thermal property of a given glass. Leaching Behavior under a Dynamic Leaching Test. To evaluate the leaching behaviors, three wasteforms were fabricated at the optimized conditions: (1) the mol fraction of Al in the gel-forming system was 0.2 and 0.3, (2) the waste loading was W20 and W25, (3) the mixing ratio of glass was about 0.9, and (4) the processing temperature was about 1100 °C for 2 h. Generally, it is well accepted that the leaching of an element from a solid phase is a result of a complex mechanism combined with a dissolution-precipitation, diffusion, sorption-desorption, etc. The leaching mechanism of the components from a glassy waste form can be classified into a wash-off, diffusion, and dissolution at an interface. The following time-dependent terms to describe the important rate-limiting leaching mechanisms were used to calculate the cumulative fraction leached (CFL) and then to establish the leaching behavior of the materials as a function of the time (16-17).
CFL ) k1[1 - exp(-k2t)] + k3t0.5 + k4t
(2)
Each term in eq 2 describes a wash-off, leaching by a diffusion, and a matrix dissolution. This equation provides us with information on the leaching behavior of each element and enables us to predict a long-term leaching fraction with the experimental data obtained with a reasonable test duration. As time (t) increases, the diffusion and dissolution terms become the main leaching behaviors. By comparing k3 and k4, the leaching behavior of each element can be evaluated. Figure 7 shows the leaching results of the main glass components (Si, Al, and B) and radionuclides (Cs and Sr). The CFL of each element was slightly changed with the fabrication conditions, but its trend was similar to each other. Different from the CFL of the other elements, that of Sr was hardly changed with time. Phosphates in the gel products were encapsulated by the glass matrix, where the leaching of the elements in the phosphate was controlled by the dissolution of the glass matrix if the wasteform was nonporous. So, the elements encapsulated by the glass matrix were leached by a wash-off at the initial test duration and then diffused through the path formed by the glass matrix dissolution. The trend of the CFL of Sr can support this phenomena; namely, Sr was preferentially situated in the phosphate phase and encapsulated by the glass matrix. However, the leaching behavior of Cs differed from that of Sr and was similar to that of the glass components. Silicate compounds in the gel products were dissolved well into a given glass powder to produce a glassy phase after a heat treatment. This might be the reason for the similarity of the CFL trend with time. These phenomena can be confirmed by evaluating the k values obtained by eq 2. As shown in Figure 6, k3 of Sr is much larger than k4 and a “diffusion” is considered to be a dominant mechanism while, for Cs, the difference in the order of k3 and k4 was not great and the “dissolution” affected the long-term leaching behavior. In the case of silicon as a main glass component, a “dissolution and diffusion” are dominant leaching mechanisms. By using the obtained parameters from the experimental data, the CFLs of Cs and Sr can be predicted with eq 2, and the values are 0.89% and 0.39% at 900 days, respectively. Characteristics of the Solidified Products. The solidified products had reasonable physical properties: 2.3∼2.35 g/cm3 of a bulk density, 4.69∼4.72 GPa of a micro-hardness, 528∼ 537 °C of a glass transition temperature, and about 10-7(low temperature)∼10-5(high temperature)/°C of an thermal expansion coefficient. As a unique wasteform containing molten salt wastes, its absolute leach rate had about an order of 10-2 g/m2‚day. A borosilicate glass as a chemical binder dissolves the silicate compounds in the gel products to
FIGURE 7. Dynamic leaching result of the solidified products.
FIGURE 8. Dynamic leaching result of glass-bonded zeolite containing LiCl salt waste. produce one amorphous phase while most of the phosphates are encapsulated by the vitrified phase. Also, Cs in the gel product is preferentially situated in the silicate phase and vitrified into a glassy phase after a heat treatment. The Srcontaining phase is mainly a phosphates compound and encapsulated by the glassy phase. These phenomena could be identified by the static and dynamic leaching test that revealed a high leach resistance of the radionuclides; the leach rates are about 10-3∼10-2g/m2‚day for Cs and 10-4∼10-3g/m2‚day for Sr, and the leached fractions of them are 0.89% and 0.39% at 900 days, respectively. Figure 8 shows a dynamic leaching test result for the glass-bonded sodalite containing LiCl salt waste, not the LiCl-KCl salt waste used VOL. 41, NO. 21, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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in the ANL experiments. The fabrication procedure of this wasteform was similar to the ANL method; LiCl salt waste/ zeolite 4A/glass ) 1:10/3.67 on a weight basis, mixing salt with fresh zeolite at 650 °C and subsequently a mixing with glass and a heat treatment at 915 °C for 48 h. Its PCT test result was about 10-1 g/m2‚day for Cs and 10-4/m2‚day for Sr, respectively. For Cs, the leach rate was about 10 times higher than that by the GRSS method. The CFLs of Cs and Sr at 900 days, as shown in Figure 7, were predicted to be 5.13% and 0.23%, respectively. From the viewpoint of a disposal efficiency, the glass-bonded sodalite by the ANL method can treat the salt waste for about 7% on a weight basis while for the solidified products it is about 16∼19%. The leaching results for the GRSS method could be comparable to the results for the ANL method for immobilizing the LiCl-KCl salt waste. These experimental results provide basic information on a unique wasteform prepared by the GRSS method to qualify it for a final disposal. The long-term leaching test will be continued to obtain more accurate information on the leaching behavior of a final wasteform, and further researches are required to qualify a wasteform containing a radioactive molten salt. By these works, the GRSS method could be an alternative method for the solidification of radioactive molten salt wastes.
Acknowledgments This project has been carried out under the Nuclear R&D program by the Ministry of Science and Technology in Korea.
Supporting Information Available Information on XRD analysis and photograph of a series of solidified products. This material is available free of charge via the internet at http://pubs.acs.org.
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Received for review May 27, 2007. Revised manuscript received August 19, 2007. Accepted August 27, 2007. ES0712524
