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A comparative study on the cesium retention ability up to 1750 °C in Cs–Zr–Si–O, Cs–Al–Si–O, and Cs–Si–O
Ceramics International 45 (2019) 15754–15757
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Short communication
A comparative study on the cesium retention ability up to 1750 °C in Cs–Zr–Si–O, Cs–Al–Si–O, and Cs–Si–O
T
Sang-Chae Jeon∗, Dong-Joo Kim, Dong Seok Kim, Keon Sik Kim, Jong Hun Kim, Ji Hae Yoon, Jae Ho Yang Korea Atomic Energy Research Institute, Daedeok-daero 989-111, Yuseong-gu, Daejeon, 305-353, Republic of Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: Accident-tolerant fuel UO2 Fission gas release Cesium trapping Cesium silicates
As cesium-trapping agents in an accident-tolerant fuel (ATF) that reduces fission gas release (FGR), Al–Si–O and Zr–Si–O (resulting in Cs–Al–Si–O and Cs–Zr–Si–O) are considered promising due to their good cesium retention ability up to high temperatures. However, in previous research, there is a lack of experimental data to compare them and determine which material is superior. In this study, the cesium retention ability of these materials was assessed by observing their weight loss behaviors up to 1750 °C. Other cesium compounds, such as pollucite (CsAlSi2O6) and Cs–Si–O possibly formed in the real fuel environment were also compared. As a result, Cs–Al–Si–O (CsAlSiO4) exhibited the most favorable result. It released cesium at higher temperatures and showed lower loss rate: only 2.36% of captured cesium was released at 1750 °C when sufficiently homogenized at 1050 °C. The real nuclear fuel environment in the light water reactor (LWR) was taken into account for the interpretation of comparative results, demonstrating that Al–Si–O is the most advantageous trapping agent in terms of high-temperature cesium retention ability.
1. Introduction The tragic Fukushima accident highlighted the importance of nuclear fuel safety. This gave rise to an accident-tolerant fuel (ATF) concept that has attracted tremendous interest from many researchers both in academic and industrial circles [1–4]. The Korea Atomic Energy Research Institute (KAERI) is also developing safe UO2 fuel pellets in accordance with the ATF concept [5–8]. Hence, a ceramic microcell UO2 pellet, which is intended to reduce fission gas release (FGR) is now under development. The cell material acts as a chemical trap that captures the fission gases via chemical reactions [8]. Among various fission gases, cesium is one of major elements threatening fuel safety due to large amounts and radioactivity [9]. Trapping the cesium inside the fuel pellet reduces the rod internal pressure and suppresses or mitigates any other problems regarding its release [8]. These benefits help prevent accidents during the normal operation of nuclear plants. Also, in case of an accident, it can delay the release of cesium, leading to more coping time. To achieve this, the cesium should be retained within the ceramic cell walls up to as high a temperature as possible. In other words, cesium should be captured in a stable form, which has been proven to be profitable. From a similar viewpoint, Zr–Si–O and Al–Si–O compounds have
∗
been known as promising candidates. Bastide et al. [10] investigated the cesium retention ability of various oxides and found that more than 40% of cesium can be maintained in Cs–Zr–Si–O even at 2000 °C. Meanwhile, Junji et al. [11] elucidated a good cesium trapping behavior of the Al–Si–O compound with thermodynamic calculations and experimental confirmation. Gallagher et al. [12] also provided the excellent cesium retention ability in the Cs–Al–Si–O compounds up to 1400 °C. However, in the previous studies, any experimental evidence was not provided [10] or confirmed in limited temperature ranges [11,12], which mainly cover the normal operation temperature of nuclear fuel. In addition, the previous results focused on their high temperature stability without considering the real fuel environment under severe temperature gradient in a radial direction. Therefore, there is a lack of experimental data to compare the candidates at temperatures above the normal operation temperature for the desired purpose. In this study, the high-temperature stability of Cs–Al–Si–O (CsAlSiO4) and Cs–Zr–Si–O was compared by observing the cumulative weight loss behaviors up to 1750 °C, taking into account that their application as cesium-trapping agents in an ATF mitigates FGR. The wide measuring temperature allowed us to compare the candidates in not only normal operation but also an accidental environment. In addition, other cesium compounds, such as another form of Cs–Al–Si–O
Corresponding author. E-mail address: [email protected] (S.-C. Jeon).
https://doi.org/10.1016/j.ceramint.2019.05.038 Received 28 March 2019; Received in revised form 3 May 2019; Accepted 6 May 2019 Available online 07 May 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Ceramics International 45 (2019) 15754–15757
S.-C. Jeon, et al.
(CsAlSi2O6) and Cs–Si–O that can be formed in the fuel environment, were also compared. To this end, a beneficial candidate was identified from a practical viewpoint considering the real fuel environment. 2. Experimental Samples of cesium compounds were prepared by chemical reactions using commercial powders: Al2Si2O5(OH)4 (natural kaolinite (CAS: 1318-74-7), Sigma-Aldrich, USA), ZrSiO4 (98 wt % purity, Alfa Aesar, USA), and Cs2CO3 (99.9 wt % purity, Sigma-Aldrich, USA). The Al2Si2O5(OH)4 and ZrSiO4 powders were separately mixed with a sufficient amount of Cs2CO3 powder by a turbula mixer for 30 min. Both powder mixtures were heat-treated to obtain relevant cesium compounds. Based on previous studies [10,13], the heat treatment was established as progressive steps for reaction, crystallization, and then homogenization. The reaction heat treatment was carried out at 630 °C for 5 h in a dry air atmosphere, giving rise to reaction with cesium. At this temperature, the carbonate was decomposed into volatile CO2 gas and melted Cs2O. The melt reacted with the Al2Si2O5(OH)4 and ZrSiO4 powders, resulting in Cs–Al–Si–O (hereafter CASO) and Cs–Zr–Si–O (CZSO) compounds, respectively. The reacted CASO and CZSO samples were heat treated for further crystallization at 800 °C for 3 days. For homogenization, they were heat treated again at higher temperatures: a single step at 1050 °C for 18 h in air for the CASO sample while multiple steps at 900 °C for 5 h, 1200 °C for 5 h, and 1600 °C for 5 h for the CZSO sample. For comparison, the pollucite sample (CsAlSi2O6, hereafter CASO-P) was prepared by reaction of the homogenized CASO sample with the addition of SiO2 powder (99.6 wt % purity, Sigma-Aldrich, USA) at 1050 °C for 10 h in air. The Cs–Si–O compound (CSO) was also prepared by the same mixing and reaction heat treatment. In addition, a commercial Cs2SiO3 powder (99.9% purity, Alfa Aesar, USA) was prepared. The crystal structure of the powder samples was examined by X-ray diffraction (XRD) analysis (Rigaku Mini-Flex, Japan). The diffraction patterns of the samples were identified and their crystallinity was obtained by using a MDI Jade 6 (Materials Data, Inc.). The cumulative weight loss values were calculated from thermo-gravimetric analysis (TG92, SETARAM, France). During TG measurement, each powder was heat treated in an Ar atmosphere at a flow rate of 20 mL/min and a heating rate of 15 °C/min. To quantify the volatile behavior, the cumulative weight loss of the samples was calculated from the TGA data, assuming that weight loss occurs only due to the volatilization of Cs2O [13]. The Gibbs free energy variation regarding reactions was obtained using HSC Chemistry 9 [14]. 3. Results and discussion Fig. 1 displays the XRD patterns of the CASO, CZSO, and CSO samples prepared by heat treatments. The heat treatments for the CASO and CZSO samples consisted of three steps, reaction, crystallization, and homogenization, while only reaction heat treatment was applied to the CSO sample, as denoted by R, C, and H in Fig. 1. From the CASO samples, similar XRD patterns were obtained, regardless of the heat treatment step, as shown in Fig. 1(a). Except the unknown peaks at ∼21 and 34° (denoted by downward arrows) in the reacted sample, most of the diffraction peaks matched well with cesium nepheline (CsAlSiO4). The unknown peaks were diminished after the reaction heat treatment, whereas the cesium nepheline phase was maintained after the homogenization heat treatment. This provides a temperature range at which cesium nepheline is stable. In this study, it lies between 630 (reaction) and 1200 °C (homogenization), which is similar with the LWR fuel temperature during normal operation (500–1200 °C [15]). Therefore, cesium trapping reaction by the Al–Si–O compound was found to be operative in nuclear fuel during normal operation. As the heat treatment proceeded to crystallization, and homogenization, the peaks gradually sharpened with increasing intensity and detailed
Fig. 1. XRD patterns of the (a) CASO, (b) CZSO, and (c) CSO samples after each step of heat treatment. R, C, H indicate reacted, crystallized, and homogenized samples, respectively.
splitting. The inset in Fig. 1(a) magnifies the tendency near 2θ of 52 to 53°. The peak sharpening in XRD may be attributed to stabilization of the crystal structure. The sharpening tendency of diffraction peaks generally indicates an increase in the degree of ordering in crystalline materials. This leads to the convergence of lattice parameters that might bring the sharpening tendency as shown in Fig. 1(a). Assuming this, the best high-temperature stability can be expected in
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homogenized sample. In addition, the crystallinity in the case of the R, C, and H samples is 88.53, 93.22, and 96.94%, respectively. On the other hand, in the CZSO samples, the XRD patterns significantly varied with repeated heat treatments as shown in Fig. 1(b). The inset in Fig. 1(b) reveals the tendency with enlarged XRD patterns at ∼ 45°. It is plausible that a cesium-trapping reaction occurs during the reaction heat treatment based on various XRD patterns with that of raw powders. However, the chemical form of the resultant material would not be CsZrSi3O8.5, which is known to have excellent hightemperature stability [10]. Only the homogenized CZSO sample seemed to be in the form of CsZrSi3O8.5. Despite the lack of identification, it is clear that higher temperatures are required to obtain the CsZrSi3O8.5 compound than in the CASO case. It should be noted that the homogenization temperature for the CZSO sample (1600 °C) exceeds the highest temperature in the LWR fuel environment (∼1200 °C) during normal operation. Therefore, the CsZrSi3O8.5 phase is hardly formed. Nonetheless, the reacted and crystallized CZSO samples need to be examined because they are feasible in the real fuel environment. Meanwhile, the CSO samples were prepared either from a reaction heat treatment or a commercial powder (denoted by ‘R’ and ‘Comm.’ in Fig. 1(c)). The reaction heat treatment conditions were the same at those applied to the CASO and CZSO samples, 630 °C for 5 h in air. Significant volatilization of cesium from the Cs–Si–O compound starts at about 800 °C, while there is no appreciable weight loss below 600 °C [16]. This limited the heat treatment temperature for the CSO sample. Instead, the commercial powder was prepared. The XRD result in Fig. 1(c) shows that the XRD patterns from both samples were similar, indicating the presence of the same Cs–Si–O compound. In addition, the pollucite structure (CsAlSi2O6) has been known to retain cesium over a wide temperature range [17,18]. For better cesium-trapping performance in LWR fuel, the resultant cesium compounds should be stable up to high temperature. Meanwhile, pollucite is excellent due to accurate stoichiometry [12]. Moreover, stoichiometric pollucite requires complicated preparation methods [18]. These are not practical in the fabrication of real fuel pellets, which may permit the formation of quasi-stoichiometric pollucite. Even if its high-temperature stability is expected to be somewhat inferior to the stoichiometric, it needs to be evaluated. In this study, pollucite was prepared in this study by a simple heat treatment to reflect real circumstances. Fig. 2 shows an XRD pattern of the CASO-P sample. Gibbs free energy variation regarding the formation reaction is presented in the inset in Fig. 2. The temperature range that is favorable for reaction is below 1100 °C. This covers broad region of the LWR fuel pellet except the centerline during normal operation [15]. The formation reaction of CASO-P was performed at 1050 °C for homogenization of the crystalline structure. The XRD result reveals that the CASO-P sample was a mixture
Fig. 2. XRD pattern of the CASO-P sample after reaction with SiO2 at 1050 °C for 10 h in air. The inset shows Gibbs free energy variation regarding the formation reaction of the pollucite.
Fig. 3. wt loss behaviors of the cesium compounds with respect to temperature. The inset shows cumulative weight loss ranges at 1750 °C.
of pollucite and cesium nepheline. The crystallinity in the case of the CASO-P sample is 63.12%, indicating the presence of an amorphous phase. Taking into account this, cesium nepheline remained possibly as a result of incomplete reaction that resulted in an unreacted amorphous silica phase. Fig. 3 shows the cumulative weight loss behaviors of the prepared cesium compounds in a temperature range between 400 and 1750 °C. The weight loss was calculated from TG data with an assumption that the volatile form in the cesium compounds was solely Cs2O. If so, weight loss could be an indication of the cesium retention ability. In both the CASO and CZSO samples, a pair of reacted and homogenized samples was prepared. This was intended to reflect the realistic fuel environment with a severe temperature gradient in a radial direction. From the viewpoint of temperature, the reacted samples can be considered cesium compounds formed at the lower-temperature region near the cladding, while the homogenized samples can be regarded as cesium compounds formed at the high-temperature region near the centerline. This approach provided the cumulative weight loss behaviors of the CASO and CZSO samples as seen in Fig. 3. In addition, the wide range of measuring temperature covered abnormal accident conditions in which the temperature rises to 1750 °C. Both the CASO and CZSO samples exhibited much lower weight loss in the homogenized samples than in the reacted samples. The homogenized CASO and CZSO samples hardly release volatile cesium up to 1200 °C, whereas the reacted samples lost significant amounts of volatile cesium ∼12% in the CASO sample and ∼23% in the CZSO sample at 1200 °C. In the CASO sample, the enhancement may be attributed to the stabilization of the crystal structure, which is in accordance with Fig. 1(a). The CZSO samples showed relatively poor cesium retention ability in both the homogenized and reacted cases. Moreover, the CZSO sample started to lose volatile cesium at a lower temperature compared to the CASO sample. Consequently, the cumulative weight loss range of the CZSO samples lies beyond that of the CASO samples: ∼8.07%–∼87.72% for the CZSO samples and 2.36%–34.75% for the CASO sample. From the reacted CZSO sample, most of the captured Cs2O was released at 1750 °C. The weight loss behavior of the CASO-P sample showed the volatile loss from ∼1300 °C, which is lower than the temperatures at which this occurred for the homogenized CASO and CZSO samples. Furthermore, it showed a higher cumulative weight loss value, reaching ∼20% at 1750 °C. Note that the weight loss might have originated from the unstable amorphous phase revealed in the XRD analysis. Therefore, the inferior thermal stability of the CASO-P sample should be distinguished from the general cases [17,18]. Nonetheless, the weight loss behavior of
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the CASO-P sample should be compared with other compounds because the CASO-P sample could be a compound that is feasible in an LWR fuel environment with a pollucite-like composition (Si/Al ≅ 2 (in mol)). Regardless of origin, any kind of volatile loss from the cesium silicates disturbs capturing of cesium. Considering that the CASO-P sample was of a homogenized form, meager benefit from the pollucite-like composition is expected in terms of high-temperature stability in this study. The poor result may be attributed to inaccurate stoichiometry or the unstable amorphous phase resulting from the simple preparation method, or both. With the same purpose, for comparison, the weight loss behaviors of the CSO samples were also observed and confirmed to be inferior to those of the CASO samples. The difference between the CSO samples might be related to the difference in the XRD analysis shown in Fig. 1(c). The inset in Fig. 3 summarizes the cumulative weight loss ranges at 1750 °C. It clarifies that the retention of cesium in the form of CASO is more favorable than in other forms from a practical viewpoint for ATF. 4. Conclusions The high-temperature cesium retention ability of Cs–Al–Si–O, Cs–Zr–Si–O, and Cs–Si–O was assessed by comparing their weight loss behaviors over a wide temperature region up to 1750 °C. To reflect the real fuel environment having a severe temperature gradient, the Cs–Al–Si–O and Cs–Zr–Si–O compounds were prepared at different temperatures. This concept provided the cesium retention ability of the cesium compounds as ranges. A comparison showed that the highest fraction of cesium was maintained in the CsAlSiO4 phase: only 2.36% of cesium was released at 1750 °C. The results demonstrate that Al–Si–O has benefits that make it suitable for application as a cesium-trapping agent in accident-tolerant fuel. Acknowledgements This work was supported by the National Research Foundation of Korea grant funded by the Korea government (MSIT: Ministry of Science and ICT) (No. 2017M2A8A5015056).
References [1] X. Wu, T. Kozlowski, J.D. Hales, Neutronics and fuel performance evaluation of accident tolerant FeCrAl cladding under normal operation conditions, Ann. Nucl. Energy 85 (2015) 763–775. [2] J.G. Stone, R. Schleicher, C.P. Deck, G.M. Jacobsen, H.E. Khalifa, C.A. Back, Stress analysis and probabilistic assessment of multi-layer SiC-based accident tolerant nuclear fuel cladding, J. Nucl. Mater. 466 (2015) 682–697. [3] C.P. Deck, G.M. Jacobsen, J. Sheeder, O. Gutierrez, J. Zhang, J.G. Stone, H.E. Khalifa, C.A. Back, Characterization of SiC-SiC composites for accident tolerant fuel cladding, J. Nucl. Mater. 466 (2015) 667–681. [4] K.D. Johnson, A.M. Raftery, D.A. Lopes, J. Wallenius, Fabrication and microstructural analysis of UN-U3Si2 composites for accident tolerant fuel applications, J. Nucl. Mater. 477 (2016) 18–23. [5] Y.H. Koo, J.H. Yang, J.Y. Park, K.S. Kim, H.G. Kim, D.J. Kim, Y.I. Jung, K.W. Song, KAERI's development of LWR accident-tolerant fuel, Nucl. Technol. 186 (2014) 295–304. [6] D.J. Kim, Y.W. Rhee, J.H. Kim, K.S. Kim, J.S. Oh, J.H. Yang, Y.H. Koo, K.W. Song, Fabrication of microcell UO2-Mo pellet with enhanced thermal conductivity, J. Nucl. Mater. 462 (2015) 289–295. [7] H.G. Kim, J.H. Yang, W.J. Kim, Y.H. Koo, Development status of accidenttolerant fuel for light water reactors in Korea, Nucl. Eng. Technol. 48 (2016) 1–15. [8] D.J. Kim, K.S. Kim, D.S. Kim, J.S. Oh, J.H. Kim, J.H. Yang, Y.H. Koo, Development status of microcell UO2 pellet for accident-tolerant fuel, Nucl. Eng. Technol. 50 (2018) 253–258. [9] R.J.M. Konings, T. Wiss, O. Beneš, Predicting material release during a nuclear reactor accident, Nat. Mater. 14 (2015) 247–252. [10] B. Bastide, L. Buisse, B. Morel, Moirans, M. Allibert, Nuclear fuel elements comprising a trap for fission products based on oxide, US patent, 5,268,947 (1993). [11] M. Junji, T. Yoshihiro, K. Kazuyuki, U. Katsumi, Y. Ryoichi, H. Mutsumi, Fundamentals of GNF Al-Si-O additive fuel, TopFuel, American Nuclear Society, Paris, France, 2009 September 6–10. [12] S.A. Gallagher, G.J. McCarthy, High tempeature thermal stability of CsAlSiO4 and CsAlSi2O6, Mater. Res. Bull. 17 (1982) 88–94. [13] S.A. Gallagher, G.J. McCarthy, Preparation and x-ray characterization of CsAlSiO4, Mater. Res. Bull. 12 (1977) 1183–1190. [14] A. Roine, Outokumpu HSC Chemistry 9 for Windows, Pori Finland, 2002. [15] R.J.M. Konings, Comprehensive nuclear materials, in: D.D. Baron, L. Hallstadius (Eds.), Fuel Performance of Light Water Reactors (Uranium Oxide and MOX), vol 2, Elsevier Ltd., 2012, p. 493. [16] L.N. Grossman, Interactions in the system Cs(g,l)–SiO2–Al2O3, Rev. Int. hautes Temper. Refract. 16 (1979) 255–261. [17] S. Komarneni, W.B. White, Stability of pollucite in hydrothermal fluids, Mater. Res. Soc. Symp. Proc. 38 (1981) 387–396. [18] K. Yanagisawa, M. Nishioka, N. Yamasaki, Immobilization of cesium into pollucite structure by hydrothermal hot-pressing, J. Nucl. Sci. Technol. 24 (1987) 51–60.
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Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Short communication
A comparative study on the cesium retention ability up to 1750 °C in Cs–Zr–Si–O, Cs–Al–Si–O, and Cs–Si–O
T
Sang-Chae Jeon∗, Dong-Joo Kim, Dong Seok Kim, Keon Sik Kim, Jong Hun Kim, Ji Hae Yoon, Jae Ho Yang Korea Atomic Energy Research Institute, Daedeok-daero 989-111, Yuseong-gu, Daejeon, 305-353, Republic of Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: Accident-tolerant fuel UO2 Fission gas release Cesium trapping Cesium silicates
As cesium-trapping agents in an accident-tolerant fuel (ATF) that reduces fission gas release (FGR), Al–Si–O and Zr–Si–O (resulting in Cs–Al–Si–O and Cs–Zr–Si–O) are considered promising due to their good cesium retention ability up to high temperatures. However, in previous research, there is a lack of experimental data to compare them and determine which material is superior. In this study, the cesium retention ability of these materials was assessed by observing their weight loss behaviors up to 1750 °C. Other cesium compounds, such as pollucite (CsAlSi2O6) and Cs–Si–O possibly formed in the real fuel environment were also compared. As a result, Cs–Al–Si–O (CsAlSiO4) exhibited the most favorable result. It released cesium at higher temperatures and showed lower loss rate: only 2.36% of captured cesium was released at 1750 °C when sufficiently homogenized at 1050 °C. The real nuclear fuel environment in the light water reactor (LWR) was taken into account for the interpretation of comparative results, demonstrating that Al–Si–O is the most advantageous trapping agent in terms of high-temperature cesium retention ability.
1. Introduction The tragic Fukushima accident highlighted the importance of nuclear fuel safety. This gave rise to an accident-tolerant fuel (ATF) concept that has attracted tremendous interest from many researchers both in academic and industrial circles [1–4]. The Korea Atomic Energy Research Institute (KAERI) is also developing safe UO2 fuel pellets in accordance with the ATF concept [5–8]. Hence, a ceramic microcell UO2 pellet, which is intended to reduce fission gas release (FGR) is now under development. The cell material acts as a chemical trap that captures the fission gases via chemical reactions [8]. Among various fission gases, cesium is one of major elements threatening fuel safety due to large amounts and radioactivity [9]. Trapping the cesium inside the fuel pellet reduces the rod internal pressure and suppresses or mitigates any other problems regarding its release [8]. These benefits help prevent accidents during the normal operation of nuclear plants. Also, in case of an accident, it can delay the release of cesium, leading to more coping time. To achieve this, the cesium should be retained within the ceramic cell walls up to as high a temperature as possible. In other words, cesium should be captured in a stable form, which has been proven to be profitable. From a similar viewpoint, Zr–Si–O and Al–Si–O compounds have
∗
been known as promising candidates. Bastide et al. [10] investigated the cesium retention ability of various oxides and found that more than 40% of cesium can be maintained in Cs–Zr–Si–O even at 2000 °C. Meanwhile, Junji et al. [11] elucidated a good cesium trapping behavior of the Al–Si–O compound with thermodynamic calculations and experimental confirmation. Gallagher et al. [12] also provided the excellent cesium retention ability in the Cs–Al–Si–O compounds up to 1400 °C. However, in the previous studies, any experimental evidence was not provided [10] or confirmed in limited temperature ranges [11,12], which mainly cover the normal operation temperature of nuclear fuel. In addition, the previous results focused on their high temperature stability without considering the real fuel environment under severe temperature gradient in a radial direction. Therefore, there is a lack of experimental data to compare the candidates at temperatures above the normal operation temperature for the desired purpose. In this study, the high-temperature stability of Cs–Al–Si–O (CsAlSiO4) and Cs–Zr–Si–O was compared by observing the cumulative weight loss behaviors up to 1750 °C, taking into account that their application as cesium-trapping agents in an ATF mitigates FGR. The wide measuring temperature allowed us to compare the candidates in not only normal operation but also an accidental environment. In addition, other cesium compounds, such as another form of Cs–Al–Si–O
Corresponding author. E-mail address: [email protected] (S.-C. Jeon).
https://doi.org/10.1016/j.ceramint.2019.05.038 Received 28 March 2019; Received in revised form 3 May 2019; Accepted 6 May 2019 Available online 07 May 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Ceramics International 45 (2019) 15754–15757
S.-C. Jeon, et al.
(CsAlSi2O6) and Cs–Si–O that can be formed in the fuel environment, were also compared. To this end, a beneficial candidate was identified from a practical viewpoint considering the real fuel environment. 2. Experimental Samples of cesium compounds were prepared by chemical reactions using commercial powders: Al2Si2O5(OH)4 (natural kaolinite (CAS: 1318-74-7), Sigma-Aldrich, USA), ZrSiO4 (98 wt % purity, Alfa Aesar, USA), and Cs2CO3 (99.9 wt % purity, Sigma-Aldrich, USA). The Al2Si2O5(OH)4 and ZrSiO4 powders were separately mixed with a sufficient amount of Cs2CO3 powder by a turbula mixer for 30 min. Both powder mixtures were heat-treated to obtain relevant cesium compounds. Based on previous studies [10,13], the heat treatment was established as progressive steps for reaction, crystallization, and then homogenization. The reaction heat treatment was carried out at 630 °C for 5 h in a dry air atmosphere, giving rise to reaction with cesium. At this temperature, the carbonate was decomposed into volatile CO2 gas and melted Cs2O. The melt reacted with the Al2Si2O5(OH)4 and ZrSiO4 powders, resulting in Cs–Al–Si–O (hereafter CASO) and Cs–Zr–Si–O (CZSO) compounds, respectively. The reacted CASO and CZSO samples were heat treated for further crystallization at 800 °C for 3 days. For homogenization, they were heat treated again at higher temperatures: a single step at 1050 °C for 18 h in air for the CASO sample while multiple steps at 900 °C for 5 h, 1200 °C for 5 h, and 1600 °C for 5 h for the CZSO sample. For comparison, the pollucite sample (CsAlSi2O6, hereafter CASO-P) was prepared by reaction of the homogenized CASO sample with the addition of SiO2 powder (99.6 wt % purity, Sigma-Aldrich, USA) at 1050 °C for 10 h in air. The Cs–Si–O compound (CSO) was also prepared by the same mixing and reaction heat treatment. In addition, a commercial Cs2SiO3 powder (99.9% purity, Alfa Aesar, USA) was prepared. The crystal structure of the powder samples was examined by X-ray diffraction (XRD) analysis (Rigaku Mini-Flex, Japan). The diffraction patterns of the samples were identified and their crystallinity was obtained by using a MDI Jade 6 (Materials Data, Inc.). The cumulative weight loss values were calculated from thermo-gravimetric analysis (TG92, SETARAM, France). During TG measurement, each powder was heat treated in an Ar atmosphere at a flow rate of 20 mL/min and a heating rate of 15 °C/min. To quantify the volatile behavior, the cumulative weight loss of the samples was calculated from the TGA data, assuming that weight loss occurs only due to the volatilization of Cs2O [13]. The Gibbs free energy variation regarding reactions was obtained using HSC Chemistry 9 [14]. 3. Results and discussion Fig. 1 displays the XRD patterns of the CASO, CZSO, and CSO samples prepared by heat treatments. The heat treatments for the CASO and CZSO samples consisted of three steps, reaction, crystallization, and homogenization, while only reaction heat treatment was applied to the CSO sample, as denoted by R, C, and H in Fig. 1. From the CASO samples, similar XRD patterns were obtained, regardless of the heat treatment step, as shown in Fig. 1(a). Except the unknown peaks at ∼21 and 34° (denoted by downward arrows) in the reacted sample, most of the diffraction peaks matched well with cesium nepheline (CsAlSiO4). The unknown peaks were diminished after the reaction heat treatment, whereas the cesium nepheline phase was maintained after the homogenization heat treatment. This provides a temperature range at which cesium nepheline is stable. In this study, it lies between 630 (reaction) and 1200 °C (homogenization), which is similar with the LWR fuel temperature during normal operation (500–1200 °C [15]). Therefore, cesium trapping reaction by the Al–Si–O compound was found to be operative in nuclear fuel during normal operation. As the heat treatment proceeded to crystallization, and homogenization, the peaks gradually sharpened with increasing intensity and detailed
Fig. 1. XRD patterns of the (a) CASO, (b) CZSO, and (c) CSO samples after each step of heat treatment. R, C, H indicate reacted, crystallized, and homogenized samples, respectively.
splitting. The inset in Fig. 1(a) magnifies the tendency near 2θ of 52 to 53°. The peak sharpening in XRD may be attributed to stabilization of the crystal structure. The sharpening tendency of diffraction peaks generally indicates an increase in the degree of ordering in crystalline materials. This leads to the convergence of lattice parameters that might bring the sharpening tendency as shown in Fig. 1(a). Assuming this, the best high-temperature stability can be expected in
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homogenized sample. In addition, the crystallinity in the case of the R, C, and H samples is 88.53, 93.22, and 96.94%, respectively. On the other hand, in the CZSO samples, the XRD patterns significantly varied with repeated heat treatments as shown in Fig. 1(b). The inset in Fig. 1(b) reveals the tendency with enlarged XRD patterns at ∼ 45°. It is plausible that a cesium-trapping reaction occurs during the reaction heat treatment based on various XRD patterns with that of raw powders. However, the chemical form of the resultant material would not be CsZrSi3O8.5, which is known to have excellent hightemperature stability [10]. Only the homogenized CZSO sample seemed to be in the form of CsZrSi3O8.5. Despite the lack of identification, it is clear that higher temperatures are required to obtain the CsZrSi3O8.5 compound than in the CASO case. It should be noted that the homogenization temperature for the CZSO sample (1600 °C) exceeds the highest temperature in the LWR fuel environment (∼1200 °C) during normal operation. Therefore, the CsZrSi3O8.5 phase is hardly formed. Nonetheless, the reacted and crystallized CZSO samples need to be examined because they are feasible in the real fuel environment. Meanwhile, the CSO samples were prepared either from a reaction heat treatment or a commercial powder (denoted by ‘R’ and ‘Comm.’ in Fig. 1(c)). The reaction heat treatment conditions were the same at those applied to the CASO and CZSO samples, 630 °C for 5 h in air. Significant volatilization of cesium from the Cs–Si–O compound starts at about 800 °C, while there is no appreciable weight loss below 600 °C [16]. This limited the heat treatment temperature for the CSO sample. Instead, the commercial powder was prepared. The XRD result in Fig. 1(c) shows that the XRD patterns from both samples were similar, indicating the presence of the same Cs–Si–O compound. In addition, the pollucite structure (CsAlSi2O6) has been known to retain cesium over a wide temperature range [17,18]. For better cesium-trapping performance in LWR fuel, the resultant cesium compounds should be stable up to high temperature. Meanwhile, pollucite is excellent due to accurate stoichiometry [12]. Moreover, stoichiometric pollucite requires complicated preparation methods [18]. These are not practical in the fabrication of real fuel pellets, which may permit the formation of quasi-stoichiometric pollucite. Even if its high-temperature stability is expected to be somewhat inferior to the stoichiometric, it needs to be evaluated. In this study, pollucite was prepared in this study by a simple heat treatment to reflect real circumstances. Fig. 2 shows an XRD pattern of the CASO-P sample. Gibbs free energy variation regarding the formation reaction is presented in the inset in Fig. 2. The temperature range that is favorable for reaction is below 1100 °C. This covers broad region of the LWR fuel pellet except the centerline during normal operation [15]. The formation reaction of CASO-P was performed at 1050 °C for homogenization of the crystalline structure. The XRD result reveals that the CASO-P sample was a mixture
Fig. 2. XRD pattern of the CASO-P sample after reaction with SiO2 at 1050 °C for 10 h in air. The inset shows Gibbs free energy variation regarding the formation reaction of the pollucite.
Fig. 3. wt loss behaviors of the cesium compounds with respect to temperature. The inset shows cumulative weight loss ranges at 1750 °C.
of pollucite and cesium nepheline. The crystallinity in the case of the CASO-P sample is 63.12%, indicating the presence of an amorphous phase. Taking into account this, cesium nepheline remained possibly as a result of incomplete reaction that resulted in an unreacted amorphous silica phase. Fig. 3 shows the cumulative weight loss behaviors of the prepared cesium compounds in a temperature range between 400 and 1750 °C. The weight loss was calculated from TG data with an assumption that the volatile form in the cesium compounds was solely Cs2O. If so, weight loss could be an indication of the cesium retention ability. In both the CASO and CZSO samples, a pair of reacted and homogenized samples was prepared. This was intended to reflect the realistic fuel environment with a severe temperature gradient in a radial direction. From the viewpoint of temperature, the reacted samples can be considered cesium compounds formed at the lower-temperature region near the cladding, while the homogenized samples can be regarded as cesium compounds formed at the high-temperature region near the centerline. This approach provided the cumulative weight loss behaviors of the CASO and CZSO samples as seen in Fig. 3. In addition, the wide range of measuring temperature covered abnormal accident conditions in which the temperature rises to 1750 °C. Both the CASO and CZSO samples exhibited much lower weight loss in the homogenized samples than in the reacted samples. The homogenized CASO and CZSO samples hardly release volatile cesium up to 1200 °C, whereas the reacted samples lost significant amounts of volatile cesium ∼12% in the CASO sample and ∼23% in the CZSO sample at 1200 °C. In the CASO sample, the enhancement may be attributed to the stabilization of the crystal structure, which is in accordance with Fig. 1(a). The CZSO samples showed relatively poor cesium retention ability in both the homogenized and reacted cases. Moreover, the CZSO sample started to lose volatile cesium at a lower temperature compared to the CASO sample. Consequently, the cumulative weight loss range of the CZSO samples lies beyond that of the CASO samples: ∼8.07%–∼87.72% for the CZSO samples and 2.36%–34.75% for the CASO sample. From the reacted CZSO sample, most of the captured Cs2O was released at 1750 °C. The weight loss behavior of the CASO-P sample showed the volatile loss from ∼1300 °C, which is lower than the temperatures at which this occurred for the homogenized CASO and CZSO samples. Furthermore, it showed a higher cumulative weight loss value, reaching ∼20% at 1750 °C. Note that the weight loss might have originated from the unstable amorphous phase revealed in the XRD analysis. Therefore, the inferior thermal stability of the CASO-P sample should be distinguished from the general cases [17,18]. Nonetheless, the weight loss behavior of
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the CASO-P sample should be compared with other compounds because the CASO-P sample could be a compound that is feasible in an LWR fuel environment with a pollucite-like composition (Si/Al ≅ 2 (in mol)). Regardless of origin, any kind of volatile loss from the cesium silicates disturbs capturing of cesium. Considering that the CASO-P sample was of a homogenized form, meager benefit from the pollucite-like composition is expected in terms of high-temperature stability in this study. The poor result may be attributed to inaccurate stoichiometry or the unstable amorphous phase resulting from the simple preparation method, or both. With the same purpose, for comparison, the weight loss behaviors of the CSO samples were also observed and confirmed to be inferior to those of the CASO samples. The difference between the CSO samples might be related to the difference in the XRD analysis shown in Fig. 1(c). The inset in Fig. 3 summarizes the cumulative weight loss ranges at 1750 °C. It clarifies that the retention of cesium in the form of CASO is more favorable than in other forms from a practical viewpoint for ATF. 4. Conclusions The high-temperature cesium retention ability of Cs–Al–Si–O, Cs–Zr–Si–O, and Cs–Si–O was assessed by comparing their weight loss behaviors over a wide temperature region up to 1750 °C. To reflect the real fuel environment having a severe temperature gradient, the Cs–Al–Si–O and Cs–Zr–Si–O compounds were prepared at different temperatures. This concept provided the cesium retention ability of the cesium compounds as ranges. A comparison showed that the highest fraction of cesium was maintained in the CsAlSiO4 phase: only 2.36% of cesium was released at 1750 °C. The results demonstrate that Al–Si–O has benefits that make it suitable for application as a cesium-trapping agent in accident-tolerant fuel. Acknowledgements This work was supported by the National Research Foundation of Korea grant funded by the Korea government (MSIT: Ministry of Science and ICT) (No. 2017M2A8A5015056).
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