Introduction

The high-level wastes (HLWs) stored in large underground steel tanks have been accumulating from reprocessed nuclear fuel [1]. Due to the long-term radiotoxic of radioisotopes existing in the HLWs, such storage is acceptable for few years and exerts an ever-increasing threat to the biosphere. Thus, these wastes must be immobilized in non-dispersible and leach-resistant matrix, stored and disposed off within suitable geological repositories [1,2,3]. For UMo HLWs containing a high concentration of molybdenum, borosilicate glassy matrices have been developed to immobilize molybdenum, other fission products and actinides. Researches on the solubility of Mo within borosilicate glass melts indicate that is relatively low (~ 1 wt% MoO3), which limits the amount of waste loading (15–20 wt%) [4, 5]. Moreover, borosilicate glass waste form accompanies the shortcoming of phase separation on melting and can crystallize simultaneously with poorly durable Na2MoO4 and more durable CaMoO4 (powellite) phases [6,7,8]. In this regard, significant efforts are being made to incorporate Mo, other fission products and actinides in a highly stable and durable ceramic matrix. The main ceramic phases considered for immobilization of theses nuclides are sodium zirconium phosphate (NZP) [9,10,11], monazite [12, 13], powellite [14, 15], etc.

Molybdenum is a multivalent element and can exist as + 4 or + 6 oxidation state in the compounds [16]. Tetravalent and hexavalent Mo can enter either octahedral (Oh) position L or tetrahedral (Td) position T of the NZP framework, likely NaMo2(PO4)3, AMo2P3O12 (A = K, Rb, Ti) and AMo2(PO4)3 (A = Ba, Sr, Ca) compounds [17,18,19]. However, the NZP structure for the immobilization of Mo (IV) is not stable under air atmosphere because tetravalent Mo can be easily oxidized [10]. An attempt has been made to introduce Mo as +6 oxidation state in monazite waste forms with La1−xCaxP1−yMoO4 (x = y = 0.1–0.9) compositions, which can crystallize in either monazite or scheelite structure depending on the level of Mo (VI) substitution [12]. The chemical durability of monazite waste form La0.4Nd0.1Y0.1Gd0.1Sm0.1Ce0.1Ca0.1P0.9Mo0.1O4 has been tested through dynamic MCC-5 method, and the leach rate of molybdenum is found to be in the order of 10−3–10−4 g m−2 d−1 [13].

Powellite (nominally, CaMoO4), which crystallizes in the scheelite-type tetragonal structure (space group I41/a, Z = 4), consists of CaO8 polyhedron and MoO4 tetrahedron sharing common vertices and can be visualized as the assembly of columns made up by –CaO8–MoO4–CaO8–MoO4– along c-axis [20, 21]. The polyhedral view of the powellite structure is shown in Fig. 1. Based on the structural flexibility, the powellite structure can accommodate considerable trivalent actinides. Until now to our knowledge, previous researches, which mainly employ the formation of powellite phase during glass composite materials processing, are investigated as a component in the multiphase waste forms [7, 15, 22,23,24,25,26]. However, limited work focuses on the relation between structure and property of single-phase powellite materials [15, 21, 27]. For instance, the chemical durability of powellite CaMoO4 has been investigated using the PCT-B. The present work shows low normalized release rates for Ba, Ca and Mo, suggesting high chemical durability [15]; however, little information is known regarding the leaching behavior of actinides incorporated in the powellite structure. A systematic investigation of structure and aqueous durability of powellite materials can provide useful information for understanding current and future single- or multiphase waste forms.

Figure 1
figure 1

Polyhedral view (left) and ab projection of the powellite-type structure (right). Magenta spheres are oxygen anions

Full size image

To close this gap, the immobilization of actinides in single-phase powellite ceramics and chemical durability of the corresponding waste forms are investigated in this paper. According to the isomorphism theory, Gd3+ is used to mimic trivalent curium (Cm). Thus, a series of ceramics Ca1−xLix/2Gdx/2MoO4 (0 ≤ x ≤ 1) are prepared by a solid-state reaction process. The effect of the nature of minor actinide surrogate (Gd) on the structure in the Ca1−xLix/2Gdx/2MoO4 system is undertaken, with leaching behaviors of Gd and Mo analyzed by PCT-B as well.

Experimental

Preparation of samples

Ceramics with the general formula Ca1−xLix/2Gdx/2MoO4 (0 ≤ x ≤ 1) are synthesized by conventional solid-state reaction of the starting reactants, CaCO3 (Macklin and purity > 99%), Gd2O3 (Aladdin and purity > 99%), Li2CO3 (Aladdin and purity > 99%) and Mo2O3 (Aladdin and purity > 99%). The corresponding stoichiometric compositions of powders are milled on the planetary milling machine (Nanjing Machine Factory, China) at 300 rpm for 4 h using anhydrous ethanol as the milling media and calcined at 500 °C for 3 h. The obtained slurries are dried at 90 °C for 24 h, calcined in air at 500 °C for 3 h and milled for 4 h afterward. Subsequently, the homogeneous powders are dried, granulated with 5 wt% PVA and pressed into cylinders (15 mm in diameter and 4–5 mm in height) under a uniaxial pressure of 200 MPa. Finally, these cylinders are sintered from 525 to 950 °C for 3 h in the ambient atmosphere.

X-ray diffraction (XRD)

The crystalline structures of the sintered products are investigated by X-ray diffraction analysis with Cu Kα radiation (PANalytical X′pert PRO diffractometer). The XRD data are collected over the 2θ ranging from 10° to 120° with steps of 0.02°. The structural parameters are analyzed with the Rietveld refinement method using the FullProf package [28].

HRTEM, SEM and Raman spectroscopy

The high-resolution transmission electron microscopy (HRTEM) is performed using a Tecnai G2 F20 S-TWIN FEI transmission electron microscope operating at 200 kV. Microstructures of platinum coated samples are observed using a Hitachi S-3700 N scanning electron microscopy (SEM). Raman spectra at room temperature are collected with a microspectrometer DXR SmartRaman (Thermo Fisher) excited by an argon laser at 514.5 nm.

Dissolution study

The chemical durability of ceramics is examined by the ASTM C1285-14 product consistency test method B (PCT-B) [29]. In the test method, samples are crushed and sieved to isolate the size fraction of the USA (Standard ASTM (100–200 mesh)). Three grams of cleaned particles is placed in a 60-ml polytetrafluoroethylene (PTFE) vessel. An amount of deionized water equal to 30 ml is added, and the PTFE vessel is sealed. The PTFE vessel is placed in a constant temperature device at 90 °C. After different leaching times, the concentrations of the released Gd and Mo are measured with inductively coupled plasma mass spectrometry (ICP-MS) using XSENIES apparatus. From ICP-MS results, normalized elemental mass losses of gadolinium and molybdenum are determined by the equation derived from PCT-B [29]. To analyze the differences between microstructures at longer leaching durations, the deionized water in the testing vessel is renewed at every 28-day test.

Results and discussion

XRD and HRTEM analyses

To investigate the optimal condition for the preparation of powellite ceramics with Ca1-xLix/2Gdx/2MoO4 (0 ≤ x ≤ 1) compositions, the typical composition Ca0.5Li0.25Gd0.25MoO4 is heated at several temperatures ranging from 525 to 950 °C. Analytic results of the corresponding XRD patterns are shown in Fig. 2. At 525 °C, the crystalline phases of the sample are CaMoO4 (JCPDS file no. 85-0546), Gd2O3 (JCPDS file no. 88-2165) and Li2Mo4O13 (JCPDS file no. 25-0494). Between 525 and 550 °C, the intensities of the peaks of Gd2O3 and Li2Mo4O13 phases decrease, while those of CaMoO4 phase increase. This indicates that the isomorphism of powellite forms during the increase in the sintering temperature. At 600 °C, the XRD patterns are indexed to the single-phase powellite with Ca0.5Li0.25Gd0.25MoO4 composition, due to the disappearance of the XRD lines from CaMoO4, Gd2O3 and Li2Mo4O13 phases. Above 600 °C, the crystallinity of the powellite ceramic Ca0.5Li0.25Gd0.25MoO4 is widely improved, which is determined by the average full width at the half maximum (FWHM) of the main XRD lines, corresponding to the 011, 112, 004, 020, 211, 114, 024, 020 and 116 peaks. The average FWHM of these peaks, which are obtained at 600, 650, 700, 750, 800, 850 and 900 °C, are found to be 0.251, 0.212, 0.203, 0.193, 0143, 0.145 and 0.140 °, respectively (Fig. 3). From the FWHM, it can be deduced that the optimized temperature for the synthesis of the powellite-type Ca1−xLix/2Gdx/2MoO4 (0 ≤ x ≤ 1) solid solution is over 800 °C.

Figure 2
figure 2

XRD analysis of Gd2O3–CaCO3–Mo2O3–Li2CO3 mixture calcined at several temperatures

Full size image
Figure 3
figure 3

Average FWHM obtained from main XRD lines of Ca0.5Li0.25Gd0.25MoO4 composition versus the sintering temperature

Full size image

To examine the effect of the minor actinide surrogate (Gd) content on the structure in the Ca1−xLix/2Gdx/2MoO4 system, the Ca1−xLix/2Gdx/2MoO4 (0 ≤ x ≤ 1) compositions are sintered at 850 °C and corresponding powder XRD patterns are shown in Fig. 4. The XRD investigation shows that Ca1−xLix/2Gdx/2MoO4 (0 ≤ x ≤ 1) compositions crystallize in the scheelite-type structure, and intense reflections of powellite phases observed in the range from 10° to 65° are indexed to (011), (112), (004), (020), (211), (114), (024), (200), (116), (312) and (224), respectively. Meanwhile, Rietveld refinement is performed on all of these compounds. Since Li element has a very low atomic number relative to the other elements in the powellite phases and a conventional Cu anode is used to collect the diffraction patterns, structural model including or without Li is used to evaluate the confidence of the refinement result. The agreement factors for the refined model including Li with representative composition Ca0.6Li0.2Gd0.2MoO4 are: Rp = 14.4%, Rwp = 9.52%, RB = 2.38% and RF = 2.49, S = 1.77; those for the initial model without Li are Rp = 14.9%, Rwp = 10.02%, RB = 2.67%, RF = 2.74% and S = 1.89. Compared with these values, it is found that the refinement of the structural model with Li atoms is credible due to its smaller agreement factors. As a result, final refinement plots of Ca0.6Li0.2Gd0.2MoO4 composition are presented in Fig. 5, and the corresponding atomic parameters are shown in Table 1. It is clear that the couple (Gd3+, Li+) enters the eightfold coordinated Ca site of the scheelite-type structure (Table 1). The analysis of the unit cell parameters given in Table 2 indicates that a noticeable decrease occurs in the compositions 0 ≤ x ≤ 1. The result can be attributed to the increase in x with the decrease in average cationic radii in the eightfold coordination of Ca site. According to the Shannon’s data [30], the equivalent radii of Ca site for the Ca1−xLix/2Gdx/2MoO4 ceramics are \( r = 1.12 - 0.1335x \) (Ion radius of Gd3+, Li+ and Ca2+ is 1.053 Å, 0.92 Å and 1.12 Å, respectively). In addition, the value of a/c increases with the increase in the x value.

Figure 4
figure 4

Representative XRD patterns of Ca1−xLix/2Gdx/2MoO4 (0 ≤ x ≤ 1) powellite solid solutions

Full size image
Figure 5
figure 5

Rietveld refinement plots of Ca0.6Li0.2Gd0.2MoO4 composition showing the observed (o), calculated (solid line), difference pattern (lower) and the positions of allowed Bragg reflections (vertical lines)

Full size image
Table 1 Structural parameters of the Ca0.6Li0.2Gd0.2MoO4 ceramic
Full size table
Table 2 Summary of phases identified in Ca1−xLix/2Gdx/2MoO4 (0 ≤ x ≤ 1) ceramics and their unit parameters
Full size table

To further understand the resulting phases of the Ca1−xLix/2Gdx/2MoO4 compositions, the samples are characterized by HRTEM. For the sake of briefness, only HRTEM result of powellite ceramic with Ca0.6Li0.2Gd0.2MoO4 composition is presented, as shown in Fig. 6. The observed zone of Ca0.6Li0.2Gd0.2MoO4 particles displays clear lattice fringes. Analyses of interplanar spacing of (022) and (020) planes along [400] axis are found to be 0.237 and 0.261 nm, respectively, which are in good agreement with Rietveld refinement results.

Figure 6
figure 6

HRTEM image of Ca0.6Li0.2Gd0.2MoO4 particles along the [400] zone with (022) and (020) planes

Full size image

Raman spectra study

Previous group theory calculations have suggested that powellite (CaMoO4) crystal has 26 vibrations of species (\( \varGamma = 3A_{g} + 5A_{u} + 5B_{g} + 3B_{u} + 5E_{g} + 5E_{u} \)), where 3Ag, 5Bg and 5Eg are 13 Raman active vibrations, 4Au and 4Eu are eight infrared active vibrations and 3Bu are three silent vibrations [31,32,33]. Raman active modes in the structure also can be divided into internal and external vibrational modes. The internal modes are associated with the oscillations inside the [MoO4]2− molecular group with a stationary mass center. The external modes, which are composed of rotation and translation modes, are related to the motion of A2+ cation and the rigid molecular units [32].

The spectroscopic characterizations of some representative Ca1-xLix/2Gdx/2MoO4 (0 ≤ x ≤ 0.9) compositions through Raman experiments in the frequency range of 100–1200 cm−1 are shown in Fig. 7. The internal mode frequencies range from 305 to 900 cm−1, whereas external mode frequencies are below 305 cm−1. The high-frequency vibration Raman line around 879 cm−1 corresponds to the symmetric stretching vibration v1(Ag). Peaks observed at 795 and 848 cm−1 are associate with the stretching motions which can be assigned as v3(Eg) and v3(Bg), respectively. Raman lines centered at 392 [v4(BgEg)] and 407 [v4(BgEg)] cm−1 belong to symmetric bending vibration. The Raman frequency located at 325 cm−1 presents the asymmetric bending mode v2(AgBg). Further, the Raman frequency located at 114 and 206 cm−1 is associated with the rotational and translational modes. For all the internal vibrational modes, Raman bands broaden with the increase in the level of Gd and Li substitution, while the change in the intensity reverses. These phenomena may result from the distortion of the MoO4 tetrahedra [33, 34]. When the proportion of Gd3+ and Li1+ ions on the Ca site increases, the MoO4 may appear more distorted on the local scale. As a result, the regular change in the internal vibration modes can be observed. Compared with the internal modes, the evolution of the external modes versus x values follows the same trend. This interesting result is related to the distortion of MoO4 tetrahedra and disordered arrangements of Gd3+ and Li+ [20, 33, 35].

Figure 7
figure 7

Representative Raman spectroscopy of Ca1−xLix/2Gdx/2MoO4 (0 ≤ x ≤ 0.9) ceramics

Full size image

Durability assessment

To assess the leaching behaviors of minor actinide surrogate (Gd) and Mo in powellite ceramics, PCT-B is performed on the selected composition Ca0.5Li0.25Gd0.25MoO4. The normalized releases of Gd and Mo for different leaching times are presented in Table 3, and corresponding plots are shown in Fig. 8. As shown in Fig. 8, the initial rapid releases of Gd and Mo occur during the leaching test. For 3-h leaching, the NLGd and NLMo reach the maximum values of 5.28 × 10−2 and 2.87 × 10−2 g m−2, respectively (Table 3). With the increase in leaching time, the normalized releases decrease and attain a near-equilibrium condition after 7 days (Fig. 8). For leaching times ranging from 7 to 28 days, the NLGd and NLMo are shown in the order of ~ 10−4 and ~ 10−4 g m−2, respectively (Table 3). These results indicate that powellite ceramics exhibit the great retention of Gd and Mo.

Table 3 Normalized releases of Gd and Mo from Ca0.5Li0.25Gd0.25MoO4 ceramic for different time durations
Full size table
Figure 8
figure 8

Evolution of Gd- and Mo-normalized releases of the Ca0.5Li0.25Gd0.25MoO4 ceramic versus leaching duration

Full size image

Based on the above description, the leaching behaviors can be explained by the interfacial dissolution–reprecipitation mechanism [36,37,38,39]. In this situation, the whole leaching process can be divided into three stages. The first stage is from 0 to 3 h, in which Gd(III) and Mo (VI) from superficial surfaces are quickly dissolved in water and form hydrolyzed species on the reaction interfaces subsequently. The second stage is from 3 h to 7 days. In this leaching period, the hydrolyzed species consistently form precipitation phases and cover on the surface of the Ca0.5Li0.25Gd0.25MoO4 particles which reduce the NLGd and NLMo. For the final stage with longer leaching times ranging from 7 to 28 days, the inner immobilized Gd and Mo elements hardly escape from the matrix due to the long pathways. Meanwhile, the formation and reprecipitation of hydrolyzed species reach the dynamic equilibrium leading to the invariant magnitude of NLGd and NLMo. These results are further verified by the microstructure of the Ca0.5Li0.25Gd0.25MoO4 ceramic after leaching 90 days, as shown in Fig. 9. The clear surface of the sample without leaching test is observed, whereas small particles observed on the surface after leaching 90 days suggest that the formation of precipitate phases occurs during the dissolution test. The observations of SEM are consistent with the leaching behaviors of minor actinide surrogate (Gd) and Mo. Unfortunately, such precipitates cannot be determined by EDS due to the contents below the instrumental detection limits. All the results indicate that powellite ceramics can be used as a promising candidate waste form for the conditioning of minor actinide and Mo.

Figure 9
figure 9

SEM images of Ca0.5Li0.25Gd0.25MoO4 ceramic a before leaching and b after 90 days of leaching in initially pure water at 90 °C

Full size image

Conclusions

The aim of this work was to investigate the structure and leaching behavior in the Ca1−xLix/2Gdx/2MoO4 (0 ≤ x ≤ 1) system. The main conclusions drawn from this paper are the following:

  1. (i)

    The preparation of the powellite-type Ca1−xLix/2Gdx/2MoO4 (0 ≤ x ≤ 1) solid solution requires sintering temperature over 800 °C. For all the Ca1−xLix/2Gdx/2MoO4 ceramics, the couple (Gd3+, Li+) is incorporated in the eightfold coordinated Ca site of the scheelite-type structure. The evolution of cell parameters decreases with the increase in x. The change in the peaks of Raman spectra is relevant to the distortion of MoO4 tetrahedra and the disordered arrangements of Gd3+ and Li+.

  2. (ii)

    The ceramic with Ca0.5Li0.25Gd0.25MoO4 composition exhibits the great retention of Gd and Mo; 7-day NLGd and NLMo are found to be in the order of ~ 10−4 and ~ 10−4 g m−2, respectively. The leaching behaviors of minor actinide surrogate (Gd) and Mo in the powellite ceramics are attributed to the interfacial dissolution–reprecipitation mechanism.