Synthesis of pyrochlore-borosilicate glass-ceramics for immobilization of high-level nuclear waste

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Synthesis of pyrochlore-borosilicate glass-ceramics for immobilization of high-level nuclear waste Kangming Wua, Fu Wanga, Qilong Liaoa,∗, Hanzhen Zhua, Dongsheng Liua, Yongchang Zhub a State Key Laboratory of Environment-friendly Energy Materials, School of Materials Science and Engineering, Southwest University of Science and Technology, Mianyang, 621010, PR China b China Building Materials Academy, Beijing, 100024, PR China

A R T I C LE I N FO

A B S T R A C T

Keywords: Borosilicate glass-ceramic Pyrochlore Synthesis Normalized leaching rate

In this work, borosilicate based glass-ceramics with pyrochlore as crystalline phase for immobilization of highlevel nuclear wastes (HLWs) were successfully synthesized by a one-step heat-treatment method. X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDXS) demonstrate that the obtained glass-ceramics show a regularly and uniformly distributed single pyrochlore (Ca,Na) (Nb,Ti)2Nd0.67O6F crystalline phase. Moreover, the glass-ceramics prepared show LRNa, LRB, LRAl, LRSi, LRNd, LRTi and LRNb of about 6.8 × 10−3, 3.7 × 10−4, 1.5 × 10−2, 2.2 × 10−3, 3.0 × 10−5, 5.1 × 10−5 and 5.5 × 10−6 g m−2 d−1 respectively after 28 leaching days, which are comparable to stable glass-ceramics for HLW immobilization. The results of this study are expected to provide an experimental reference for the engineering synthesis of glass-ceramics for the immobilization of certain HLWs.

1. Introduction In the past half-century, with the development of nuclear power station and the retirement of military nuclear weapons, large amounts of high-level wastes (HLWs) were remained and asked for management. How to dispose of these HLWs safely and effectively has become a major problem [1–3]. Particularly, the isolation of plutonium and minor actinides (MA) with long half-life and high radioactivity from the environment of human beings has an important impact on the sustainable development of nuclear energy [4]. Up to now, the permanent geological disposal of HLWs is commonly accepted. For this method, firstly separating and immobilizing them in a stable matrix like glass, glass-ceramics and ceramics are generally required [5–8]. However, low loading capacity of plutonium and MA in glass matrix and selectivity of plutonium and MA in ceramics are unsatisfactory [9]. In recent years, synthesis of glass-ceramics for HLWs immobilization may potentially be applicable to immobilizing HLWs in industrial scale [7,10]. How to design the composition and synthesize corresponding glass-ceramic waste forms is the key point. Borosilicate glass is the widely used glass matrix for the immobilization of HLWs at present [4,11]. Borosilicate glass emerged as the most suitable matrix for containment of HLWs because of its ability to incorporate abundant elements of HLWs, as well as adequate leach resistance and radiation resistance [12]. In recent decades, pyrochlore ∗

has been exhibiting excellent accommodation of diversified radionuclides attracting many researchers extensive attention [13–15]. Pyrochlore compounds having the general formula A2B2O7, in which A and B sites often present 8- and 6-coordinations, represent a family of phases isostructural to the pyrochlore [13]. The wide variety of chemical substitution at the A, B and O sites are easily to occur if neutrality criterion of ionic radius and charge is to meet [14]. Moreover, the advantages of stable structural stability, excellent chemical durability and radiation resistance are often associated with pyrochlore [16–19]. Therefore, borosilicate based glass-ceramics with pyrochlore as the main crystalline phase show great potential application in HLWs immobilization. It is essential to explore the preparation method suitable for the engineering synthesis of the glass-ceramics. Academic studies showed that pyrochlore glass-ceramics mainly include niobate-, stannate-, titanate- and zirconate-pyrochlore glass-ceramics [20,21]. It was found that the synthesis of stannate-, titanate- and zirconate-pyrochlore glass-ceramics needs higher melt temperature than niobate-pyrochlore glass-ceramic [22,23]. Moreover, Ca2Nb2O7 phase played an important role in the simultaneously substitution of the compounds (Ca2Nb2O7)x(A2B2O7)1-x pyrochlore, providing (Ca2Nb2O7)x(A2B2O7)1-x pyrochlore with a stronger incorporation ability of HLWs and a lower melt temperature than A2B2O7 pyrochlore [24,25]. In the structure, Nd is usually chosen to be a nonradioactive surrogate for plutonium and minor actinides (MA) and expected to substitute for A3+ of

Corresponding author. E-mail address: [email protected] (Q. Liao).

https://doi.org/10.1016/j.ceramint.2019.11.071 Received 11 September 2019; Received in revised form 7 November 2019; Accepted 8 November 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Please cite this article as: Kangming Wu, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.11.071

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(Ca2Nb2O7)x(A2B2O7)1-x pyrochlore [26,27]. In addition, Ti is commonly used as the nucleating agent in the glass-ceramics and can be expected to substitute for B4+ of (Ca2Nb2O7)x(A2B2O7)1-x pyrochlore [40]. With these perspectives, Ca2Nb2O7 containing pyrochlore phase was selected as the ceramic phase and borosilicate glass was selected as the parent glassy phase in this work. For the preparation of pyrochlore glass-ceramics, melt-crystallization method and sintering method are the main methods [28–31]. However, melt-crystallization method is not conducive to the control of crystalline phase and composition, sintering method has complicated processes and high requirements for equipments [28,30,31]. In this work, borosilicate glass-ceramics with pyrochlore as the main crystalline phase were synthesized by melting the raw materials and immediately transferring the melts to a furnace with different initial temperatures (1100, 1050, 1000 or 900 °C), then cooling at a certain rate (5, 7.5, 10 or 15 °C·min−1) to annealing temperature and keeping at the annealing temperature for 2 h to remove inner stress. The glassceramic samples were obtained by cooling naturally in the furnace. The process, which is similar to traditional melt-quenching glass procedure, has advantages than the abovementioned two methods in the engineering synthesis of glass-ceramic waste forms. The most one could directly use the equipments for preparing glass waste forms in large scale. Moreover, the effects of different heat-treatment processes on the structure and properties of the glass-ceramics were systematically investigated and the optimum preparation process was discussed in detail.

Fig. 1. Preparation process of the borosilicate based glass-ceramics.

2. Experimental 2.1. Preparation of glass-ceramics ((Na,Ca)2Nb2O6F)0.75(Nd2Ti2O6F)0.25 and Na1.25Al0.25B0.2SiO3 were designed as the targeting crystalline phase and glassy phase respectively. Stoichiometric amounts of chemicals introduced by Na2SiO3, H3BO3, CaF2, Nb2O5 and Nd2O3 of analytical reagent grade were mixed by ball milling, then the thoroughly mixed batch was put in alumina crucibles and moved to a furnace, and heated with a heating rate of 5 °C·min−1 to 1300 °C and hold at this melt temperature for 1.5 h. Subsequently, the melt was quickly transferred to a new furnace with different initial temperatures (1100, 1050, 1000 or 900 °C), then cooled to an annealing temperature of 500 °C with cooling rates of 5, 7.5, 10 or 15 °C·min−1. After annealing at 500 °C for 2 h to eliminate the residual stress and naturally cooling in the furnace to room temperature, the glass-ceramic samples were obtained. The composition and processes of the glass-ceramics prepared is given in Table 1 and Fig. 1 respectively. Besides, there was a slightly different preparation method for comparison. The thoroughly mixed batch was heated at 1300 °C for 1.5 h in an alumina crucible with a heating rate of 5 °C·min−1, then directly cooled in the furnace naturally. And the cooling rate of the sample is shown in Fig. 2. All the preparation process parameters and corresponding sample labels are listed in Table 2.

Fig. 2. Experimental cooling rate of the sample cooled in furnace naturally.

Jade 6.5 software. Scanning electron microscopy (SEM, TM1000, Hitachi Limited, Japan) and energy dispersive X-ray spectroscopy (EDXS, TM4000, Hitachi Limited, Japan) were applied to obtain the chemical composition and microstructure of the glass-ceramics. All samples had been immersed in 10 mol% hydrofluoric acid for 15 s, a thin gold film was deposited on the samples before characterization. Moreover, X-ray fluorescence (XRF, Axios, PANalytical B.V., Holland) was also applied to analyze the chemical composition of the glassceramics. The chemical durability of the obtained glass-ceramics was evaluated by Product Consistency Test (PCT) in deionized water (pH = 7) [32]. The powders used were sieved between 100 and 200 meshes, a 1.5 g powder and 15 mL deionized water were put into a sealed polytetrafluoroethylene vessel. The leaching tests were performed at 90 °C ± 1 °C for the regular intervals (1, 3, 7, 14 and 28 days), once the leaching solution was removed and then 15 mL fresh deionized water was added to the vessel. The concentrations of Na, B, Si, Al, Ti, Nd and Nb element in leaching solutions were analyzed by an inductively coupled plasma optical emission spectroscopy (ICP-OES, iCAP6500, Thermo Fisher Corporation, USA). The normalized elemental leaching rate of element i (LRi, g·m−2·d−1) was calculated by the following formula [33]:

2.2. Characterization In order to identify potential crystalline phases of samples, powder X-ray diffractometer (DMAX1400, Rigaku Company, Japan) using CuKα radiation were performed, operating at 40 kV and 70 mA in the 2θ range of 10–80°, with an acquisition step of 0.02° and a scanning rate of 8° min−1. The collected X-ray diffraction (XRD) data were analyzed by Table 1 Composition of the borosilicate based glass-ceramics (wt.%). Composition

Na2SiO3

H3BO3

CaF2

Nb2O5

TiO2

Nd2O3

Al(OH)3

SiO2

Samples

36.80

5.98

8.30

22.11

4.80

14.00

3.00

5.00

2

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Table 2 Detailed preparation parameters of the borosilicate based glass-ceramics. Samplesa

Heating rate (°C·min−1)

Melted temperature (°C)

Holding Time (h)

Initial cooling temperature (°C)

Cooling rate (°C·min−1)

Annealing temperature (°C)

Annealing time (h)

P-900-7.5 P-1000-7.5 P-1100-7.5 P-CIF

5 5 5 5

1300 1300 1300 1300

1.5 1.5 1.5 1.5

7.5 7.5 7.5 -

500 500 500 -

2 2 2 -

P-1000-5 P-1000-10 P-1000-15 P-1050-5 P-1050-10 P-1050-15

5 5 5 5 5 5

1300 1300 1300 1300 1300 1300

1.5 1.5 1.5 1.5 1.5 1.5

900 1000 1100 Cooling in furnace naturally 1000 1000 1000 1050 1050 1050

5 10 15 5 10 15

500 500 500 500 500 500

2 2 2 2 2 2

a The samples were labeled by the initial cooling temperature and the cooling rate. For instance, P-900-7.5 represents that the initial cooling temperature adopted is 900 °C and the cooling rate is 7.5 °C·min−1. P-CIF is the sample obtained by directly cooling in the furnace naturally.

LRi =

Ci⋅V fi ⋅S⋅Δt

the glass-ceramics when glass melt is insignificantly undercooled. Usually, the decrease of crystallization temperature leads to an increasingly fine dendritic crystal [34,35]. That is to say, the lower the initial cooling temperature, the more obvious the dendritic crystal is. Besides, the crystalline size of samples varies with different initial cooling temperatures, this indicates that homogeneous nucleation usually occurs while crystallization [37–41]. According to the corresponding theory, the change of critical radius should be investigated to establish the glass-crystal phase transition, the radius of spherical particles is assumed to be r, then the following formula is the theoretical derivation of nuclei with the critical radius r* [40].

where Ci is the concentration of element i (g·L−1), fi is the mass fraction of the element in the glass-ceramic samples, Δt is the interval leaching duration in days (d), V is the volume of the leaching solution (L), S is the surface area of the samples (m2), the value of S/V is about 2000 m−1.

3. Results and discussion 3.1. Effect of initial cooling temperature

r ∗ =  −

The effects of different initial cooling temperatures were firstly discussed. SEM images of P-900-7.5, P-1000-7.5, P-1100-7.5 and P-CIF are showed in Fig. 3. It can be observed that P-1100-7.5 and P-CIF show crystalline phases in both dendritic and bulk shape, P-900-7.5 and P1000-7.5 show only dendritic crystalline phase. The size of dendritic crystalline phase is in the range of 2–30 μm and that of bulk crystalline phase is from 10 to 60 μm. Generally, the particle size of crystalline phases increases with increasing initial cooling temperature, in the sequence of P-900-7.5, P-1000-7.5, P-1100-7.5 and P-CIF. Moreover, Fig. 3 clearly illustrates the continuous and regular growths of dendritic crystalline phase while the disordered growth of bulk crystalline phase in samples. According to literatures [34,36,40], if glass melt is significantly undercooled, a fibrillar type of crystallization is common and normally referred as dendritic crystal, and bulk crystal usually occurs in

2γ Δgv

The interfacial energy that corresponds to the required energy for the formation of the new surface of the nucleus is represented by γ, Δgv is the free energy change per unit volume that is produced by the formation of nuclei. The nuclei smaller than r* are incapable of growing and eventually disintegrate, the crystalline particle size is correspondingly related to the critical nucleus size [37]. Generally, the higher initial cooling rate, the smaller critical nucleus size r* is. This is due to the overwhelming influence of the energy for the formation of the new surface of the nucleus [39]. Obviously, the higher initial cooling temperature of samples means the lower initial cooling rates of samples. Therefore, the above-mentioned sequence of crystalline size is obtained (P-CIF > P-1100-7.5 > P-1000-7.5 > P-900-7.5). Moreover, TiO2 and F are nucleating agents in this study. Nucleating agents always

Fig. 3. SEM images of P-900-7.5, P-1000-7.5, P-1100-7.5 and P-CIF. 3

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Fig. 4. XRD patterns of P-900-7.5, P-1000-7.5, P-1100-7.5 and P-CIF.

crystalline phase belonging to pyrochlore (Ca,Na)(Nb,Ti)2Nd0.67O6F, which is formed from the composition of pyrochlore, heated (Ca,Na)2(Nb,Ti)2O6F (JCPDS-PDF No. 17-0747) with the substitution of Nd3+ in CaNdNb2O7 crystalline phase (JCPDS-PDF No. 47-0037). Based on EDXS data and literatures, Ca2+ and Ti4+ site in the structure of (Ca,Na)2(Nb,Ti)2O6F is able to be occupied by 2Nd3+. Moreover, Ca2+ can be replaced by Nd3+ while Na+ as charge compensation, Ti4+ can be replaced by Nd3+ along with the formation of oxygen vacancies [24,25,30]. In addition, glassy phases of P-1100-7.5 and P-CIF samples show little differences in elemental composition. The constituents of glassy phase include Na, Al, Si, Ca, Ti, Nb, Nd and O element, and the average molar ratio of Na: Al: Si: Ca: Ti: Nb: Nd: O is about 25: 30: 55: 5: 2: 6: 3: 125. It indicates that the Nd in glassy phase is around 1.2 mol %. The Nd in pyrochlore is around 8.7 mol%, and the molar ratio of crystalline phase and glassy phase designed is about 58: 42. This indicates that the Nd is main incorporated in crystalline phase. The relative quantity of crystalline phases of P-900-7.5, P-1000-7.5, P-1100-7.5 and P-CIF can be compared using X-ray diffraction measurements [42]. As shown in Table 3, the influences of different initial cooling temperatures on the crystallization can be obtained from the analysis of diffraction peak height and area. Both the highest peak height (2θ = 29.75°) and peak area (2θ = 10°–65°) of samples have the same varying tendency as P-1000-7.5 > P-900-7.5 > P-11007.5 > P-CIF. This indicates that the optimum initial cooling temperature may be around 1000 °C.

accumulate in a specific microphase of the phase-separated glass, the higher initial nucleating temperature, the easier the specific microphase will change [40,41]. So, disordered growth of bulk crystal generally appears in P-CIF and P-1100-7.5 with more change of the nucleating specific microphase, while continuous and regular growths of dendritic crystal occur in P-1000-7.5 and P-900-7.5. Fig. 4 shows XRD patterns of P-900-7.5, P-1000-7.5, P-1100-7.5 and P-CIF. It can be observed that the diffraction peaks of crystalline phases are intensive and dispersive diffraction peak of amorphous phase is not obvious. It is preliminarily confirmed that pyrochlore, heated (Ca,Na)2(Nb,Ti)2O6F (JCPDS-PDF No. 17-0747) and calcium neodymium niobium oxide CaNdNb2O7 (JCPDS-PDF No. 47-0037) are the crystalline phases according to the standard XRD data. And the peak intensity of (Ca,Na)2(Nb,Ti)2O6F phase is almost the same as that of CaNdNb2O7 phase. The diffraction peaks of (Ca,Na)2(Nb,Ti)2O6F locate at 2θ = 14.64°, 28.36°, 29.75°, 34.40°, 45.33°, 49.46°, 58.94°, 61.83° and 72.77°, and the diffraction peaks of CaNdNb2O7 appear at 2θ = 14.64°, 28.36°, 29.75°, 34.40°, 37.71°, 45.33°, 49.46°, 58.94°, 61.83° and 72.77°. The diffraction peaks between 2θ = 36° and 48° are amplified (in the right side of Fig. 4). All these indicate that diffraction peaks of (Ca,Na)2(Nb,Ti)2O6F and CaNdNb2O7 almost locate at the same position. In order to obtain the accurate phase information, SEM equipped with EDXS was employed to investigate the composition of the crystalline phase. P-1100-7.5 and P-CIF were chosen in this investigation since the two samples contain both dendritic crystal and bulk crystal. Fig. 5 shows the corresponding SEM images and EDXS data. The EDXS data acquired from the crystalline phase in the area (1, 2, 3, etc.) or glassy phases in the SEM images, and the atomic percentages of crystalline phases are the average of the EDXS data of the selected areas. The results indicate that there should be only one crystalline phase in all samples. Although both dendritic crystal and bulk crystal appear in P-1100-7.5 and P-CIF samples, the two shapes of crystals show little differences in elemental composition. This indicates that dendritic crystal and bulk crystal are affected only by initial cooling temperatures in this study. Moreover, the constituents of dendritic crystal and bulk crystal include Nd, Ca, Na, Nb, Ti, O and F elements, and the average molar ratio of Nd: Ca: Na: Nb: Ti: O: F is about 4: 3: 3: 10: 2: 22: 2. Combing with XRD analysis, it is confirmed that all samples contain the

3.2. Effect of cooling rate Based on the analysis about the effects of initial cooling temperatures, the optimum initial cooling temperature may be around 1000 °C, and the morphology of pyrochlore crystalline phase varies greatly from initial cooling temperatures of 1000 °C–1100 °C. Therefore, in the subsequent experiments, the melt was quickly transferred to a new furnace of 1000 °C or 1050 °C, then cooled to the annealing temperature of 500 °C with a cooling rate of 5, 10 or 15 °C·min−1, as the P-1000-5, P1000-10, P-1000-15, P-1050-5, P-1050-10 and P-1050-15 samples listed in Table 2. Powder XRD data of these samples are shown in Fig. 6. Three characterized diffraction peaks corresponding to pyrochlore at 2θ = 29.75°, 34.40° and 49.46° are observed in the XRD patterns. 4

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Fig. 5. SEM images and EDXS data of P-1100-7.5 and P-CIF.

cooling temperatures almost locate at the same position. This indicates that the crystalline phases of all samples belong to ordered pyrochlore. Moreover, no significantly differences in peak intensity among these samples are observed, indicating that the influence of cooling rate on the crystalline phase is insignificant. More detailed information about the influence of cooling rate is analyzed by using SEM. Fig. 7 shows SEM images of P-1000-5, P-1000-10, P-1000-15, P1050-5, P-1050-10 and P-1050-15. It is found that all the samples contain single and homogeneous pyrochlore crystalline phase with dendritic shape. The sizes of crystalline phase of P-1000-5, P-1000-10 and P-1000-15 are from about 2.5, 2.0 and 1 μm to about 9.0, 8.5 and 8.0 μm respectively, and those of P-1050-5, P-1050-10 and P-1050-15 are from about 2.0, 1.5 and 1.2 μm to 8.0, 7.5 and 7.5 μm respectively. This results show that the crystalline size is significantly affected by

Table 3 Peak height (2θ = 29.75°) and peak area (2θ = 10°–65°) of P-900-7.5, P-10007.5, P-1100-7.5 and P-CIF. Samples

P-900-7.5

P-1000-7.5

P-1100-7.5

P-CIF

Peak height (2θ = 29.75°) Peak area (2θ = 10°–65°)

3026 61892

3091 63157

2707 55009

1940 45510

Moreover, the diffraction peaks corresponding to ordered pyrochlore at 2θ = 14.64°, 28.36°, 37.71° and 45.33° are detected, the corresponding miller indices are (111), (311), (331) and (511) respectively [24,31]. The diffraction peaks between 2θ = 36° and 48° are amplified (in the right side of Fig. 6). Besides, the diffraction peaks of samples prepared with different cooling rates and samples prepared with different initial 5

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The data given in Table 4 shows the chemical analysis of fracture sections of P-1050-10 sample. It can be observed that SiO2, Nb2O5, Na2O, Al2O3, Nd2O3, Nd2O3, CaO and TiO2 are major oxides while other impurity oxides are present in trace amounts, B2O3 is absent because it cannot be detected by X-ray fluorescence. This indicates that the chemical composition of the final material is basically consistent with the designed batch composition. 3.3. Crystallinity Different calculation methods can be used to estimate crystallinity in the crystal characteristics [43–46]. In this work, the crystallinities of all samples were analyzed by Jade 6.5 software and determined on the basis of the following formula [43].

X=

ΣIc × 100% ΣIc + ΣIa

where X is the crystallinity of samples, ΣIc is the intensity of total crystalline diffraction, and ΣIa is the scattering integral intensity of amorphous portion. Based on the calculation of Jade 6.5 software [43,45], the obtained crystallinity results are obtained and listed in Table 5. From Table 5, it can be seen that the crystallinity of samples is in the sequence of P-1000-7.5 ≈ P-1000-10 ≈ P-1050-10 ≥ P-10005 ≥ P-1000-15 ≥ P-1050-15 ≥ P-1050-5 ≥ P-900-7.5 ≥ P-11007.5 ≥ P-CIF. Comparing the crystallinity of P-900-7.5, P-1000-7.5, P1100-7.5 and P-CIF, it can be obtained that the crystallinity of P-10007.5 is the highest, indicating that the crystallinity is affected by initial cooling temperature. Moreover, the crystallinity of P-1050-10 is higher than P-1000-5, P-1000-10, P-1000-15, P-1050-5 and P-1050-15. This result indicates that high crystallinity is more likely obtained when initial cooling temperature of 1050 °C and cooling rate of 10 °C·min−1 are adopted. Based on previous analysis and the crystallinity data, it further demonstrates that the optimum preparation condition belongs to the synthesis process for preparing P-1050-10 sample.

Fig. 6. XRD patterns of P-1000-5, P-1000-10, P-1000-15, P-1050-5, P-1050-10 and P-1050-15.

cooling rate. Generally, lower cooling rate induces larger crystalline size of pyrochlore. Besides, the narrow size distribution of crystalline phase is obtained in P-1050-10 sample. It indicates that the corresponding preparation process is favorable to the uniform growth of fine dendritic crystal [35]. On the whole, the preparation process for P1050-10 may be the optimum process for the synthesis of borosilicate glass-ceramics with the pyrochlore as crystalline phase. Fig. 8 shows SEM and EDXS data of P-1050-5, P-1050-10 and P1050-15. The EDXS data acquired from the crystalline phase (1, 2, 3, etc.) in the SEM images, the atomic percentages are the average of the EDXS data of selected areas. Although these crystalline particles crystallized at different cooling rates, the differences of elemental composition among P-1050-5, P-1050-10 and P-1050-15 as well as P-1100-7.5 and P-CIF are insignificant. Combing with XRD analysis, the samples prepared with different cooling rates and initial cooling temperatures have little effect on the crystalline composition. It further indicates that these crystalline phases belong to pyrochlore (Ca,Na) (Nb,Ti)2Nd0.67O6F.

3.4. Normalized elemental leaching rate In order to evaluate the chemical durability of the glass-ceramic samples, normalized leaching rates of elements were detected. Fig. 9 shows the normalized leaching rates of Na (LRNa), B (LRB), Si (LRSi), Al

Fig. 7. SEM images of P-1000-5, P-1000-10, P-1000-15, P-1050-5, P-1050-10 and P-1050-15. 6

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Fig. 8. SEM images and EDXS data of P-1050-5, P-1050-10 and P-1050-15.

Table 4 Chemical composition of the fracture sections of P-1050-10 sample. Chemical composition

Weight (wt%)

SiO2 Nb2O5 Na2O Al2O3 Nd2O3 CaO TiO2 F RuO2 PdO MgO SO3 K2O MnO P2O5 ZnO ZrO2

24.71 19.27 17.75 13.65 12.09 6.39 4.87 0.71 0.20 0.11 0.08 0.05 0.04 0.03 0.02 0.01 0.01

Table 5 Pyrochlore crystallinity of samples with different preparation process parameters and their major effect factors. Samples

Crystallinity (error value)

Amorphous phase 2θ(°)

Amorphous phase FWHM(°)

Residual Error of Fit

P-900-7.5 P-1000-7.5 P-1100-7.5 P-CIF P-1000-5 P-1000-10 P-1000-15 P-1050-5 P-1050-10 P-1050-15

53.51(1.68)% 58.58(2.19)% 53.38(1.79)% 50.93(2.28)% 57.11(2.50)% 58.09(2.05)% 56.33(1.86)% 54.75(2.04)% 58.84(1.98)% 55.43(2.08)%

24.815 26.855 26.055 26.534 25.974 24.960 25.140 24.782 24.865 25.131

43.318 45.439 46.527 45.503 46.261 45.022 45.356 44.860 43.349 45.593

9.37% 8.99% 9.18% 9.79% 8.46% 9.47% 8.89% 9.00% 8.94% 8.88%

sharply decrease with immersion time within the initial leaching days and then the decreasing rates slow down gradually. LRNa, LRB, LRSi, LRAl, LRNd, LRTi and LRNb range from low values of about 5.8 × 10−3, 2.6 × 10−4, 2.1 × 10−3, 1.0 × 10−2, 3.0 × 10−5, 4.8 × 10−5 and 5.5 × 10−6 g m−2 d−1 to high values of about 9.9 × 10−3, 3.7 × 10−4, 4.8 × 10−3, 2.5 × 10−2, 1.8 × 10−4, 3.8 × 10−4 and −5 −2 −1 5.0 × 10 g m d respectively after 28 leaching days. The results indicate that the preparation parameters of glass-ceramics have great influence on their chemical durability. Fig. 9(a) and (b), (c) and (d) also

(LRAl), Nd (LRNd), Ti (LRTi) and Nb (LRNb) for all samples after immersion in 90 °C deionized water for 1, 3, 7, 14 and 28 days. In general, the normalized leaching rates of the samples decrease with the increasing immersion time. The normalized leaching rates of all samples 7

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Fig. 9. Normalized leaching rates of (a) Na, (b) B, (d) Si, (f) Al, (h) Nd, (j) Ti and (l) Nb of P-1000-5, P-1000-10, P-1000-15, P-1050-5, P-1050-10 and P-1050-15, (c) Si, (e) Al, (g) Nd, (i) Ti and (k) Nb of P-900-7.5, P-1000-7.5, P-1100-7.5 and P-CIF.

are about 6.8 × 10−3, 3.7 × 10−4, 1.5 × 10−2, 2.2 × 10−3, 3.0 × 10−5, 5.1 × 10−5 and 5.5 × 10−6 g m−2 d−1 respectively after 28 leaching days. Usually, the more stable the crystalline phase, the lower the normalized leaching rate is. The low normalized leaching rates of P-1050-10 also further confirm the above-mentioned analysis about the optimum preparation process for preparing the sample. In the pyrochlore-borosilicate glass-ceramic samples, the normalized leaching rate of neodymium (Nd), a nonradioactive surrogate for plutonium and minor actinides (MA), is lower than or comparable to the reported data [1,47]. The normalized leaching rates of the major elements in samples such as Si, Al, B and Na range from about 2.6 × 10−4 to 2.5 × 10−2 g m−2 d−1 after 28 leaching days, which are lower than or comparable to the values of the typical borosilicate glasses applying to HLWs immobilization (10−4−10−1 g m−2 d−1 after 28 leaching days) [29,48,49]. In addition, the overall chemical

show that LRNa, LRB, LRSi insignificantly varies after 7 days, demonstrating that different preparation parameters have little influences on the chemical durability of borosilicate glassy phase. From Fig. 9 (e), (g), (i) and (k), among the glass-ceramics prepared with different initial cooling temperatures, P-1000-7.5 and P-900-7.5 show lower normalized leaching rate. That may be due to the non-homogeneous growth of crystalline phase leading to worse chemical durability in P-1100-7.5 and P-CIF (as shown in SEM images). Fig. 9 (f), (h), (j) and (l) show the LRAl, LRNd, LRTi and LRNb of P-1000-5, P-1000-10, P-1000-15, P-10505, P-1050-10 and P-1050-15 respectively, and P-1050-10 remains the lowest normalized leaching rate. This indicates that the initial cooling temperature of 1050 °C and the cooling rate of 10 °C·min−1 is conducive to the good chemical durability of final glass-ceramics. In addition, P1050-10 has the lowest leaching rates among all samples, LRNa, LRB, LRAl, LRSi, LRNd, LRTi and LRNb in the leaching solutions of P-1050-10 8

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durability of the glass-ceramics samples is good [19,47]. Besides, chemical substitutions at the A, B and O sites in pyrochlore crystalline phase are easily to occur and some elements such as Si, Al and Na can be stably retained in the borosilicate glassy phase [50]. Therefore, the studied pyrochlore-borosilicate glass-ceramics are potential hosts for the disposal of HLWs containing high concentrations of plutonium and MA.

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4. Conclusions In this study, borosilicate glass-ceramics with pyrochlore as crystalline phase for nuclear waste immobilization were synthesized by a one-step heat-treatment method. A single pyrochlore phase (Ca,Na) (Nb,Ti)2Nd0.67O6F with regular and uniform distribution in the borosilicate based glass-ceramics is generally obtained. The effects of different preparation processes on the crystallinity, structure, morphology and chemical durability of the prepared glass-ceramics were investigated. The optimum preparation process is that: heating the thoroughly mixed batch at a heating rate of 5 °C·min−1 to 1300 °C and keeping at this temperature for 1.5 h; Subsequently, quickly transferring the obtained melt to a furnace having the temperature of 1050 °C and cooling to the annealing temperature of 500 °C with a cooling rate of 10 °C·min−1; Finally, annealing at 500 °C for 2 h and cooling in furnace naturally to room temperature. The borosilicate based glassceramic with pyrochlore (Ca,Na)(Nb,Ti)2Nd0.67O6F as crystalline phase prepared by this process shows the highest crystallinity, as well as the lowest normalized leaching rates which are comparable to stable glassceramics for HLW immobilization. The conclusions indicate that the studied pyrochlore-borosilicate glass-ceramics are potential hosts for the immobilization of certain HLWs and suggest a possibility of synthesizing glass-ceramic waste forms by the one-step heat-treatment method. Acknowledgements This work was supported by the China Industrial Technology Development Program [grant number 2017-1407], by the National Natural Science Foundation of China [grant number 51702268], and by Southwest University of Science and Technology [grant number 18LZX302, 18LZXT01 and 18fksy0212]. References [1] R.C. Ewing, W.J. Weber, J. Lian, Nuclear waste disposal–pyrochlore (A2B2O7): nuclear waste form for the immobilization of plutonium and “minor” actinides, J. Appl. Phys. 95 (11) (2004) 5949–5971, https://doi.org/10.1063/1.1707213. [2] J.S. McCloy, A. Goel, Glass-ceramics for nuclear-waste immobilization, MRS Bull. 42 (3) (2017) 233–240, https://doi.org/10.1557/mrs.2017.8. [3] M.M. Khan, M.R. Islam, Zero Waste Engineering: A New Era of Sustainable Technology Development, John Wiley & Sons, Inc., Hoboken, NJ, 2016. [4] D. Caurant, O. Majerus, P. Loiseau, I. Bardez, N. Baffier, J.L. Dussossoy, Crystallization of neodymium-rich phases in silicate glasses developed for nuclear waste immobilization, J. Nucl. Mater. 354 (1−3) (2006) 143–162, https://doi.org/ 10.1016/j.jnucmat.2006.03.014. [5] I.W. Donald, B.L. Metcalfe, R.J. Taylor, The immobilization of high level radioactive wastes using ceramics and glasses, J. Mater. Sci. 32 (22) (1997) 5851–5887, https://doi.org/10.1023/a:1018646507438. [6] W. Huang, D.E. Day, C.S. Ray, C.W. Kim, High temperature properties of an iron phosphate melt containing high chrome nuclear waste, J. Nucl. Mater. 346 (2−3) (2005) 298–305, https://doi.org/10.1016/j.jnucmat.2005.07.004. [7] R.C. Ewing, Ceramic matrices for plutonium disposition, Prog. Nucl. Energy 49 (8) (2007) 635–643, https://doi.org/10.1016/j.pnucene.2007.02.003. [8] M. Karabulut, E. Melnik, R. Stefan, G.K. Marasinghe, C.S. Ray, C.R. Kurkjian, Mechanical and structural properties of phosphate glasses, J. Non-Cryst. Solids 288 (1−3) (2001) 8–17, https://doi.org/10.1016/S0022-3093(01)00615-9. [9] P. Loiseau, D. Caurant, O. Majerus, N. Baffier, C. Fillet, Crystallization study of (TiO2, ZrO2)-rich SiO2-Al2O3-CaO glasses Part II Surface and internal crystallization processes investigated by differential thermal analysis (DTA), J. Mater. Sci. 38 (4) (2003) 853–864, https://doi.org/10.1023/A:1021825418336. [10] D. Enke, F. Janowski, W. Schwieger, Porous glasses in the 21st century–a short review, Microporous Mesoporous Mater. 60 (1−3) (2003) 19–30, https://doi.org/ 10.1016/S1387-1811(03)00329-9.

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