Solidification of Radioactive Molten Salt Waste via Gel

Jan 13, 2007 - Based on the sol-gel process coupled with gel-networking and evaporative crys- .... vaporization of solvent, HCl or P2O5 from free H3PO...
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Environ. Sci. Technol. 2007, 41, 1345-1351

Stabilization/Solidification of Radioactive Molten Salt Waste via Gel-Route Pretreatment H W A N - S E O P A R K , * ,† I N - T A E K I M , † HWAN-YOUNG KIM,† SEUNG-KON RYU,‡ AND JOON-HYUNG KIM† Nuclear Fuel Cycle R&D Group, Korea Atomic Energy Research Institute, 150 Deokjin-dong, Yuseong -gu, Daejeon, 305-353, Korea and Department of Chemical Engineering, Chungnam National University, 220 Gung-dong, Yuseong-gu, Daejeon, 305-764, Korea

The volatilization of radionuclides during the stabilization/solidification of radioactive wastes at high temperatures is one of the major problems to be considered in choosing suitable wasteforms, process, material systems, etc. This paper reports a novel method to convert volatile wastes into nonvolatile compounds via a sol-gel process, which is different from the conventional method using metalalkoxides and organic solvents. The material system was designed with sodium silicate (Si) as a gelling agent, phosphoric acid (P) as a catalyst/stabilizer, aluminum nitrate (Al) as a property promoter, and H2O as a solvent. A novel structural model for the chemical conversion of molten salt waste, named RPRM (Reaction Product in Reaction Module), was established, and the waste could be solidified with glass matrix via a simple procedure. The leached fraction of Cs and Sr by a PCT leaching method was 0.72% and 0.014%, respectively. In conclusion, the RPRM model is to convert target wastes into stable and manageable products, not to obtain a specific crystalline product for each radionuclide. This paper suggested a new stabilization/ solidification method for salt wastes by establishing the gelforming material system and showing a practical example, not a new synthesis method of stable crystalline phase. This process, named “gel-route stabilization/solidification (GRSS)”, will be a prospective alternative with stable chemical process on the immobilization of salt wastes and various mixed radioactive waste for final disposal.

I. Introduction Molten salt waste, which is generated from the pyrochemical process to separate uranium and trans-uranium elements from spent nuclear fuel, has been interesting to researchers in the radioactive waste management. For its final disposal, direct immobilization into a suitable host matrix or indirect solidification by other chemical routes requires the control of chlorides and its volatility since molten salt wastes mainly consist of metal chlorides, about 90 wt % of LiCl electrolyte, 10 wt % of fission products (CsCl + SrCl2), and a minor quantity of actinides (1). This waste is difficult to apply to the * Corresponding author phone: 82 42 868 2054; fax: 82 42 868 2329; e-mail: [email protected]. † Korea Atomic Energy Research Institute. ‡ Chungnam National University. 10.1021/es0615472 CCC: $37.00 Published on Web 01/13/2007

 2007 American Chemical Society

conventional vitrification due to the high volatility of chloride compounds and the compatibility with silicate glasses (2). Phosphate glass as an alternative to borosilicate glass has been considered as a host matrix for chlorides and has high leach-resistance (3-5); Iron phosphate glasses with melting temperatures as low as 950 °C reduces the chance of Cs being volatilized during vitrification. Glass-bonded sodalite (Na6M2Al6Si6O24Cl2, 6-10) suggested by the Argonne National Laboratory (ANL), to the present, could be a practical solution to the immobilization of this waste, where waste form can be fabricated at about 915 °C, lower than the melting temperature of many borosilicate glasses (∼1150 °C). A wet dechlorination to oxides or a thermal conversion into borate glass was suggested to remove Cl from salt waste (11, 12), and it seemed that the preference of radionuclides for the intended chemical conversions or immobilizations described above could hardly be accomplished or failed, except the phosphate precipitation method suggested by Volkovich and his co-workers (13). This study considered the chemical conversion of metal chlorides into appropriate chemical forms as an intrinsic solution on eliminating Cl-induced disadvantage such as limitation on the host matrix or volatility, and was intended to establish the chemical system with a simple process and a preferential conversion of radionuclide into stable compounds. Based on the sol-gel process coupled with gel-networking and evaporative crystallization (see the approach concept in the Supporting Information), the structural chemical model (RPRM) described in Figure 1 was designed and developed by introducing additives to the reaction module and the reaction product. Using this model, our research group suggested a novel method to stabilize metal chlorides by a chemical conversion and also showed a practical example on the waste salt solidification.

II. Experimental Section II.1. Materials. Different from sol-gel process using metal alkoxide, water-soluble materials and H2O were used. Sodium silicate powder (36 wt % Na2O, 64 wt % SiO2, Junsei) as a gelling agent and phosphoric acid (85 wt % purity, Showa) as an acidic catalyst/stabilizer were considered as base gelforming material system (Si-P system). H3BO3, Al(NO3)3‚ 9H2O, and FeCl3‚6H2O from Showa Co. were added to the Si-P system for constructing Si-P-B, Si-P-Al, and SiP-Fe as expanded material systems. LiCl, CsCl, and SrCl2 (99% purity, Merck) with a composition of 90, 6.8, and 3.2 wt %, respectively, were used to simulate the waste salt. The gel-forming compositions were 0.4/0.6∼0.7/0.3 of Si/P ratio in a base system and 0.4/0.4/0.2 of Si/P/additive (Al, Fe or B) ratio in the expanded material system. For 0.1 mole of phosphoric acid, 1∼10 g of metal chlorides was loaded to the Si-P system and 10 g of salt waste with a real composition was also loaded to each expanded material system. II.2. Method. Each material was dissolved in deionized water at room temperature and mixed by strongly stirring for 5 min to obtain clear solution. The mixture was poured into a plastic bottle, tightly sealed, and placed into electric furnace. 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 prepared with expanded material systems were mixed with glass powder with a mixing ratio of 70(gel product):30(glass) by weight. This powder mixture was heat-treated at 900 °C or 1050 °C for 2 h without pressing. The experimental procedure is indicated at Figure S1 in the Supporting Information. VOL. 41, NO. 4, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Schematic structural model by a combined reaction, reaction 1-chemical conversion into phosphates and reaction 2-formation of gel network; at the gel state (top), it contains two phases, gel-networks capturing radionuclides and solvent phase containing stabilizer. This model gives two chemical routes for the chemical conversion of metal chlorides or radionuclides. After drying and heat-treatment, two phases would be separated if not react with each other. The structure (A in the bottom), named RPRM (reaction product in reaction module), is one of the proofs to confirm the suggested chemical model described above.

FIGURE 2. SEM images of samples with Si/P ) 1 after heat-treated at 600 °C for 2 h; Aging at 70 °C for (a) 3 days and (b) 7 days. II.3. Characterization. The leach-resistance of solidified products was evaluated by PCT-A leaching method (ASTM 1346

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Standard C 1285-02). The compositions of samples were analytically determined by an acidic dissolution of solids

(14) and analysis of the resulting solution by ICP-AES. The concentration in leachate was measured by AAS and ICPAES. Some of gel product and heat-treated samples 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). 29 Si, 27Al, and 31P MAS NMR spectra were recorded at ambient temperature with a Bruker DSX 400 multinuclear spectrometer. Spin speeds were 5.5 kHz for 29Si and 12 kHz for 27Al and 31 P. The resonance frequencies observed were 79.5 MHz for 29Si, 104.2 kHz for 27Al and 161.9 MHz for 31P. Chemical shifts were recorded with respect to TMS for 29Si, [Al(H2O)6]3+ for 27 Al, and H3PO4 for 31P.

III. Result and Discussion III.1. RPRM for Chemical Conversion. III.1.1.Chemical Behavior in Si-P System. In sol-gel methods using TEOS and organic solvent for synthesis of phosphosilicate compound, hetero-linkage, Si-O-P, can be formed at a high content of P, while siloxane matrix can trap phosphorus monomers at a low content of P (15-16). The Si-P system in this study is intended to produce phosphate compound. Therefore, Si-O-P linkage is not favorable for the chemical conversion. In the Si-P system, clear hydrogel was obtained with different gelation time, 10 s ∼ 4 h, depending on the Si/P molar ratio. The pH of the initial solution in this study was below 1, and this could indicate the end group coupling to produce ≡Si-O-Si≡ (17). The gel products with a different Si/P ratio contain only sodium hydrogen phosphates as a crystalline compound (Figure S2 in the Supporting Information). Considering low possibility on the presence of HPO42or PO43- form in a solution at low pH, the change of Na/P ratio of phosphates proves the evaporative crystallization, not precipitation. The Si-related compound was not detected, and Si would exist as an amorphous phase with Si-O-Si or Si-O-P bond. This chemical behavior appeared nearly similar to the system with metal chlorides. As the input of metal/phosphate (M/P) ratio increases, LiCl was converted into lithium phosphates, from LiH2PO4 to Li3PO4. SrCl2 was also converted into SrHPO4. For CsCl, Cs-containing phase was not detected (Figure S3 in the Supporting Information). It should be noticed that all the gel products contained NaCl, and other metal chlorides were not shown by XRD analysis. From these results, the chemical system can be simply expressed as follows: [Na+, SiO44-, H3PO4]sol + MCl (aq)f(Si-OP, Si-OSi)net + M-H3PO4 (s) + NaCl(s)+ HCl(g), where subscript net means network former of gel, and the Si-O-Si chain would be a major network in the Si-P system. This system has two chemical processes, gelation by silicate and subsequent evaporative crystallization to metal phosphates during a drying step. Also, Li, Cs, and Sr are more reactive to phosphate anion than Na. If sufficient phosphates exist in this system, all chlorides could be vaporized as HCl during the drying step. In the Si-P system, the Si/P molar ratio suitable to the process described above was from 0.67 to 2.33. III.1.2. Microstrucure. The microstructure of gel product by SEM analysis shows a series of pore with various shape and size, depending on the Si/P ratio (Figure S4 in the Supporting Information). Increase of the Si/P ratio lowered pore size and enhanced the regularity of pore shape. One possible mechanism on the formation of pore might be the vaporization of solvent, HCl or P2O5 from free H3PO4 with temperature. The microstructure of gel product prepared by longer aging time was indicated in Figure 2. Different from the microstructure in Figure 2(a), SiO2 particles with about 100 nm in size was distinctively shown in Figure 2(b); phosphates are situated between the particles. With longer aging time, the size of sol increased and the connection of Si-O-Si sol was enhanced. The increase of Si/P makes the

FIGURE 3. Change of microstructure with the increase of loadings of CsCl at constant amount of Si and P. content of Si-O-Si sol high, the rigidity of the phase constructing pore was enhanced, and then the shape becomes more regular and spherical. Similar to the effect of the aging condition on the connection between sols, the existence of small content of salt during gelation changed the gel structure (Figure S5 in the Supporting Information). The pore had a highly ordered structure, 2∼3 µm larger in size than those in Figure 2. This can confirm the suggested reaction module in RPRM model. Figure 3 shows the microstructure change with CsCl loading. The pores and VOL. 41, NO. 4, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 4. Possible realized structural model for chemical conversion in sodium silicate, H3PO4 and metal chloride system; Si-rich particle is expressed as template or reaction module, and P-rich phase occupies the template, depending on the content of metal chlorides. surfaces were gradually occupied and covered with the phosphate phase. This makes pores seem small or even absent at some points. The XRD analysis (Figure S6 in the Supporting Information) of product from Si-P system with CsCl after a heat-treatment at different temperatures shows that an amorphous Si-O-Si gel network was developed to crystalline SiO2 (cristobalite) and NaCl disappeared above 600 °C; this indicated that the phase between SiO2 particles, shown in Figures 2 and 3, is Na2O-P2O5 or Na2O-Cs2O-P2O5 glass which can be formed at 650∼700 °C (18). From these results, the proposed RPRM model can be realized, as shown in Figure 4. The Si-O-Si network functioning as a reaction module is changed into crystalline SiO2 particles that agglomerate to produce templates. A decrease or increase of Si/P changes the density of network, where the reaction module might be detected in µm scale. Longer aging time and salt addition can form a more distinctive reaction module. The surface structure in Figure 4 appeared like honey in the honeycomb, and the highlighted phase in the inner part is situated in the reaction module. The suggested RPRM model was intended to develop a desirable chemical conversion into stable compounds, not to obtain an ordered reaction module. All the results by SEM analysis showed that the RPRM concept is valid in the gelation with sodium silicate and phosphoric acid, regardless of the size of reaction module or Si/P ratio. The Si-O-Si network can be always formed by catalyzing acidic agent, H3PO4, and most of the phosphate ions react with target metal cations to produce phosphate compounds. III.1.3. Thermal Stability. In the Si-P system, metal chlorides can convert into phosphate compounds, where Cs would be in the glassy phase. Figure 5 shows the TG/DTA curves of gel products with different CsCl loading. The weight loss showed four steps: (a) below 200 °C for vaporization of free H2O or residual HCl, (b) 200∼600 °C for dehydration of Si-OH, or P-OH (c) 600∼1100 °C for vaporization of metal chlorides and (d) above 1200 °C for vaporization of NaCl. Depending on the atmospheric condition, CsNO3, Cs2O, and CsCl can be vaporized in the 400∼900 °C (19). In the case of the presence of free CsCl that does not react with phosphates, 1348

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FIGURE 5. TG/DTA analysis of the products with different loadings of CsCl on Si-P system. its vaporization would begin above 800 °C. The weight loss at 600∼1100 °C was 1∼2 wt % and this might be attributed to the vaporization of volatile P2O5 or Cs. However, the weight fraction of CsCl, based on the batch composition, is over 30 wt % at Cs/P ) 0.6. Therefore, the weight loss at 600∼1100 °C by Cs would be quite low. III.2. Stabilization/Solidification of Salt Waste. III.2.1. Additive Effect. At the Si-P system, Si-O-Si chains cannot chemically capture radionuclides but function as a reaction module. Possible alkali phosphate glass as a reaction product is known to be less durable (20). Addition of Al2O3 or Fe2O3 to alkali phosphate glass increases durability (21-23). Considering prospective HWL glass, a possible reaction module might be Si-O-B or Si-O-Al, which can chemically incorporate cations as a charge compensator for excess negative charge attributed to the formation of BO45- or AlO45-. Also, it is more desirable to change the reaction products, phosphates, from M2O-P2O5 to M2O-Al2O3-P2O5 or M2OFe2O3-P2O5. From these possible modifications, Si-P-B, SiP-Al and Si-P-Fe system can be established. Figure 6 shows the comparison of the elemental concentration in leachate, Ei expanded system/Ei SiP (R) by the PCT method. All the elements in the expanded material system

FIGURE 6. Comparison of each material system on the immobilization ability. Ei expanded: concentration of i element in leachant for expanded material system, Ei SiP: concentration of i element in leachant for Si-P system. have higher leach-resistance than those in the Si-P system, except one case, Li in a Si-P-Fe system. The order of stabilization effect was Sr > Cs > Na > Li on metal cations and Si-P-Al > Si-P-B > Si-P-Fe on material systems. With the same amount of phosphate in batch composition, the change of R would mean that the two purposes on additives can be realized. With only this result, it may not be sufficient to determine whether the decrease of R is mainly due to the change of reaction module or not. However, considering that the decrease of R on Cs and Sr as minor elements in real salt wastes is much greater than that on Li and Na, they have higher preference on chemical conversion into stable compounds than Li or Na. III.2.2. Chemical Behavior in the Si-P-Al System. Figure 7 indicated the XRD patterns of gel product with different Si/P/Al molar ratio after heat-treatment at 1000 °C for 2 h. Different from the Si-P system, metal chlorides was converted into metal aluminosilicates at Al mol fraction of above 0.3; Li2Al2Si3O10 for Li, CsAlSi2O6 for Cs, and Sr2Al2SiO7 for Sr. Below that mol fraction, the Li or Cs-containing crystalline phase was not detected while Sr exist as Sr5(PO4)6Cl2 and Sr2P2O7. At a real composition of salt waste, only Li3PO4 was detected (Figure S7 in the Supporting Information). These results indicate that the Si-P-Al system preferentially converts metal chlorides into metal aluminosilicate, depending on the content of Si-O-Al generated in a gel state, and then, excess metal cations over required charge compensators for Si-O-Al are changed into metal phosphate by reacting with free phosphate anions during the gelation/ drying step. Generally, metal aluminosilicate or alkali earth phosphate has a high thermal stability and low solubility in water. Therefore, even though under the worst case that Li would be more favorable to Si-O-Al or phosphate than Cs or Sr, all the metal chlorides can be formed as stable chemical forms if free phosphates are sufficient in the gel. From the results, Cs and Sr must be preferentially stabilized into aluminosilicate or phosphates, depending on the content of Si-O-Al and free phosphates anions. The MAS NMR spectra of each nuclear are given in Figure 8. For 29Si NMR spectra, two distinctive peaks, -103 ppm and -112 ppm, were observed. Resonances around -90∼110 ppm are ascribed to tetrahedral silicon atoms bound via oxygen to aluminum atoms with 0∼4, Si(4Al), Si(3Al), Si(2Al), Si(1Al), and Si(0Al) (24-25). From these chemical shifts, the silicon would exist as Si(2Al) or Si(0Al). It is noticed that the number of Al bound to Si atom was not changed with Al fraction and temperature, while the relative fraction of Si(2Al) increased. This shows the characteristic of networking at initial step during gelation, where mono-linkage of SiO44is dominant rather than hetero-linkage between SiO44- and AlO45-. Consequently, the coupling to produce Si-O-Al bond

FIGURE 7. XRD patterns of each product with different mol fraction of Al in Si-P-Al system. takes place at the silanol group on silicate chains fully formed under low pH. This can explain the reason on the change of Si(2Al) fraction. The 27Al NMR spectra show large resonances centered at 36 ppm, -10 ppm, and -16 ppm, where the chemical shifts, 36 ppm, is ascribed to tetrahedrally coordinated aluminum (26-27). Low chemical shifts, -10 ∼ -16 ppm, are attributed to pentahedrally and octahedrally coordinated aluminum. The fraction of a peak at 36 ppm increases with the Al fraction, indicating linkage of AlO4 to Si or P. Other peaks at -10 ∼ -16 ppm indicate the Al fraction not participated in gelVOL. 41, NO. 4, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 8. MAS NMR spectra of products prepared by Si-P-Al system with different Al mol fraction at different temperatures; (a) 29Si, (b) 27Al and (c) 31P.

FIGURE 9. Description of chemical conversion of metal chlorides in Si-P-Al system from XRD and MAS NMR analysis.

TABLE 1. Batch Composition of Gel Product Before and After Heat-Treatment at 1000 °C for 2 h SiO2 P2O5 Al2O3 Na2O Li2O Cs2O SrO

before heat-treatment

after heat-treatment

28.9 35.9 12.8 10.5 10.0 1.37 0.57

29.3 36.5 11.6 10.6 10.2 1.34 0.60

networks. For the 31P NMR spectra, sharp peaks of sample with Al ) 0.4 at -29 ppm is assigned to Q4 tetrahedral P site with neighboring AlO4, P(4Al). A large peak of sample with Al ) 0.2 at -18 ppm is attributed to Q2 P site. Approach to 6.4 ppm of chemical shift is an increase of P(ONa) in tetrahedral P site (28-30). These results can indicate that the products from the SiP-Al system have three domains as described in Figure 9. Al as an additive interacts with the Si-O-Si reaction module to produce a fraction of Si-O-Al in gel-network and also changes the reaction products into aluminum phosphates or M2O-Al2O3-P2O5 glassy phase. At a low Al fraction, the change of reaction product is dominant but gel-network is greatly changed at high Al fraction. The existence of Si-OAl and P-O-Al phases means the incorporation of metal cations of gel-network and increase in durability of reaction products, respectively. On the basis of RPRM model, the 1350

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FIGURE 10. Leached fraction of solidified products prepared by different conditions. *the leached fraction: a mean value of three data with below standard deviation of 10% chemical properties of the reaction module and the reaction product can be enhanced by using additives. The Si-P-Al system can have an effective reaction module and reaction products for the chemical conversion of Cs and Sr and it guarantees radionuclides to change into hydrothermally and thermally stable compounds, regardless of chemical forms. III.2.3. Chemical Durability. Table 1 shows the change of compositions before and after a heat-treatment at 1000 °C for 2 h, where analysis error was about 5∼10%. Judging from the result, there would be little weight loss due to the vaporization of Cs. With this thermal stability, the consolidation of gel products can be carried out at 900∼1050 °C. Figure 10 shows the comparison of leached fraction of each solidified product tested by the PCT-A method. Leached fraction of a solidified product without glass can be considered as an unstable fraction under a PCT test condition, where leached fractions of Li, Cs, and Sr were 23.2, 6.2, and 0.04 wt %, respectively. This indicated that 93.8% of Cs and 99.94% of Sr can be immobilized by gelation and heat-treatment without glass. Leached fraction of Li and Cs by consolidation with glass was lowered by 5∼10 times over those by only heat-treatment, while that of Sr showed little difference, regardless of solidification conditions. 93.8% of Cs in salt waste can be stabilized even in the existence of high Li content, which could be another proofs on the validity of given material system for the stabilization of Cs. At the stabilization-of-radionuclides point of view, it is noteworthy

that the Si-P-Al system can chemically convert alkali earth elements into high stable compounds, regardless of phosphate or aluminosilicate. A borosilicate glass (R7T7) as an arbitrary choice for consolidating gel products can significantly lower the leached fraction of Li and Cs, probably because of the encapsulation effect and the dissolution of unstable phase into glass. This study did not intend to indicate an effective glass composition but suggested how to fabricate a monolithic wasteform with obtained gel products. As suggested in this paper, gelation can be carried out with water-soluble materials, not with organic reagents. It is very meaningful when treating concentrated liquid or watersoluble wastes because such gel products are very manageable in a high-temperature process for vitrification or ceramization

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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 approach concept, the experimental procedure, XRD analysis of material system, microstructure of products and composition 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 June 30, 2006. Revised manuscript received December 3, 2006. Accepted December 4, 2006. ES0615472

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