1. Introduction

Various types of radioactive wastes are generated during the nuclear power plants (NPP) operation. The primary streams of these are dry active waste (DAW), boron concentrates, spent resin, and spent filters [1, 2]. Large amounts of DAW are generated from surface decontamination, maintenance activities, and conventional management activities.

Vitrification, one of the most promising solidification processes for various materials, has been applied to radioactive waste to improve its disposal stability and reduce the volume [3, 4]. Vitrification comprises thermal decomposition of radioactive waste and formation of glass material. In the thermal decomposition step, the primary organic species in DAW, including cellulose, are decomposed. The decomposed organic species converts to carbon-related gaseous materials, CO_X, and is treated in an off-gas treatment system. The inorganics from radioactive waste and initial glass frits react and result in a vitrified form [5]. Since the thermal decomposition of DAW reduces its volume significantly, the volume reduction factor of DAW vitrification is very high, while the melting of metal components is usually ~5 [6, 7]. The formation of glass takes place, simultaneously. The reaction between glass frits and inorganic species of radioactive waste transforms the material into a designed solidified form.

The solidification process, combining solid glass frits and/or semi-liquid materials from radioactive waste, results in a stable glass form [8]. The hazardous radioactive elements are incorporated and immobilized in a glass material. Since sufficient structural flexibility is required for the glass design, borosilicate glasses are often employed for vitrification of radioactive waste.

ZrO₂ and HfO₂ are generally considered to be network- formers in borosilicate-based glasses. It is generally understood that they usually increase the durability and viscosity of the resulting glasses. The Zr ions were mostly found to be six-fold coordinated and [ZrO₆] octahedra bonds with other network formers such as [SiO₄] tetrahedra via corner-sharing [9, 10, 11]. Similarly, it is expected that the addition of ZrO₂ in sodium-lithium borosilicate glasses polymerizes with glass networks by attracting alkali cations to compensate charge in six-fold coordinated Zr ion [12, 13].

The Korea hydro & nuclear power (KHNP) developed the optimal glass composition for the vitrification of DAW, spent resin, and mixtures of the two [14]. Since vitrification offers a high-volume reduction ratio, it is expected that disposal costs could be greatly reduced with such technology. The DG-2 glass composition was developed to vitrify DAW. The DG-2 glass exhibited good characteristics in terms of viscosity, electrical conductivity, chemical durability, and compressive strength.

During NPP maintenance, metal containing paper, clothes, wood, etc. are generated. For example, the decontamination of metal component surfaces will result in metal containing papers and clothes. The metal component in the waste could affect the structure of the resulting solidified material. Undesired metal species could affect the crystallization of glass material and result in crystalline glass-ceramic. It is generally understood that glass material could offer a sufficient amount of flexibility to a glass matrix that maintains its glassy property. The glass material, however, could transform to a crystalline material with specific components over a critical point.

In this study, a feasibility study of vitrification of DAW, which is containing metal particulates, is conducted in order to understand its applicability under various conditions. When considering the disposal of solidified forms, compressive strength and chemical durability are important factors. Since the chemical durability of a solidified form is strongly related to disposal stability, various evaluation methods such as Product Consistency Test (PCT) and leachability index have been suggested. PCT and leachability index are thus assessed in order to understand the chemical durability of the fabricated glasses.

2. Experiment

The raw chemicals were mixed sufficiently to achieve a homogeneous composition and reaction and melted at 1,150℃ in a clay crucible without a lid. The chemical composition for the glass is shown in Table 1. The chemicalcontaining crucible was placed in a MoSi₂ electric box furnace for 1 hour. After heating for 45 mins at 1,150℃, the melted glass was stirred with a fumed quartz rod to achieve a sufficient and homogeneous reaction. After stirring, this was placed in the MoSi₂ furnace again for an additional 15 mins. After thermal treatment for glass fabrication, the melted glass was poured into a graphite mold for the fabrication of a monolithic solidified form. The graphite mold was pre-heated at 500℃ to prevent any abrupt thermal shock and then cooled slowly.

The compressive strength of the fabricated monolithic cylindrical glass was measured as described in the KS F2405 (standard method for testing the compressive strength of concrete). According to the standard method, a glass was placed in the universal testing machine (UTM) and it was well aligned with the center of the testing equipment. The designed rate of loading should be constantly applied during the pressure-loading phase. This load was maintained until the load indicator started to decrease steadily, and the specimen displayed a well-defined fracture pattern. The load was applied until the sample reached complete fracture status. The dimensions of the monolithic cylindrical glass specimens used for the compressive strength was approximately around φ 30 mm × 60 mm.

The fabricated glass crystal structure was analyzed with an X-ray diffraction pattern measurement using SmartLab, RIGAKU. The 2-theta range and scan speed were 10−90 degrees and 5 degrees/min, respectively.

The fabricated glass structure was analyzed using an Xray photoelectron spectrometer (Sigma Probe, Thermo VG Scientific, USA). XPS analysis provided the binding energy of the elements in the fabricated glass.

PCT was conducted following the method indicated in ASTM C 1285-97 [15]. The glass powder (diameter: 75 μm – 150 μm) was washed with flowing deionized water (DI) water and washed twice using ultrasonic with DI water to remove any undesired particles adhering to the glass surface. The washed glass was washed thrice again with ethanol to remove organics using ultrasonic cleaner. Then it dried in oven for 12 hours at 90℃. The prepared ground glass was mixed with DI water in a stainless-steel vessel. The ratio of the sample surface and leachate (DI water) was around 2,000 m⁻¹. The vessel was filled with the sample and DI water and stored at 90℃ for 7 days. The leachate was filtered after completion of PCT using a 0.45 μm syringe. The chemical composition of the resulting leachate was analyzed using inductively coupled plasma optical emission spectroscopy (ICP-OES) for B, Li, Na, and Si.

The ANS 16.1 leach test method was used to evaluate the leachability index of the fabricated glass [16]. The fabricated monolithic cylindrical glass was placed in the center of a container, and the glass was placed on a teflon holder which was designed to maximize the contact area of the glass with the leaching solution. The container was filled with the leaching solution (DI water). The volume (m³) of the leaching solution was 10 times the surface area (m²) of the glass. At each of the leaching intervals, the leaching solution was exchanged for fresh leachate. The cumulative leaching times for these experiments were 24, 48, 72, 96, and 120 hrs. The leaching solution was analyzed using inductively coupled plasma mass spectroscopy (ICPMS) for Cs, Sr, and Co.

The microstructure of the glass was investigated using a scanning electron microscope (SEM, REGULUS 8230 HITACHI). The SEM was operated at 3 kV to examine physical and chemical homogeneity. The glass surface was analyzed and images taken in backscattered-electron mode. Element mapping of the glass followed in an attempt to understand the composition and uniformity of the glass.

3. Results and Discussion

Photos and SEM images are shown in Fig. 1. Zr and Hf were added to the DG-2 glass composition to understand the structural flexibility of the DG-2 glass. The fabricated glass containing 2.5wt% of Zr was named DG-2_Zr-2.5. The amounts of Zr added to DG-2 were 2.5wt%, 5wt%, and 7.5wt%. Similarly, the amounts of Hf added to DG-2 were 2.5wt%, 5wt%, and 7.5wt%. The initial chemical compositions are found in Table 1. The photo and magnified SEM image of DG-2_Zr-7.5 is shown in Fig. 1(d). The mixture of milky and transparent glass is observed in fabricated glass. It seems that the limited solubility of ZrO₂ in DG-2 glass matrix hinders the formation of Zr incorporated DG-2 glass after certain concentration of Zr. It seems that the evidence of glass-ceramic formation is observed in the magnified image, found in inset of Fig. 1(d).

Fig. 1

Photo and SEM image of fabricated glass.

Fig. 2 presents the element mapping results. Homogeneity is an important factor when characterizing glass quality. Element mapping of the fabricated glass is conducted in order to investigate the distribution of elements. Fig. 2 indicates that the primary elements comprising the glass matrix and Zr and/or Hf were uniformly distributed.

Fig. 2

Element x-ray mapping of the fabricated glass: (a) DG-2, (b) DG-2_Zr-5.0, and (c) DG-2_Hf_5.0.

Mechanical properties of the fabricated glass were investigated using the standard method, KS F2405. It is generally understood that destruction of glass increases its effective surface area and decreases mechanical support under disposal conditions. Since this increased effective surface area increases the possibility of negative environmental impacts, the durability of the vitrified form is considered to be an important characteristic. The measured compressive strength of the fabricated glass is shown in Fig. 3. The waste acceptance criteria (WAC) of Korea radioactive waste agency (KORAD) indicates that the compressive strength of the solidified form should be higher than 3.44 MPa when it is tested in accordance with KS 2405 and ASTM 2010 from the US nuclear regulatory commission (NRC)’s technical position on waste forms (US NRC 1991) [17, 18]. The results indicated that the fabricated glass exhibited good mechanical properties, far higher than 3.44 MPa. It was found that the compressive strength of DG-2_Zr-5.0 and DG-2_Hf-2.5 was higher than that of any other fabricated glass.

Fig. 3

Compressive strength of fabricated glass with respect to chemical composition.

The XRD patterns for the fabricated glasses are shown in Fig. 4. The results indicate that the fabricated glasses were all amorphous phase except for DG-2_Zr-7.5. The sharp peak observed in DG-2_Zr-7.5 was found to be a peak of ZrO₂. This means that phase separation of ZrO₂ appeared at a certain point between Zr-5 and Zr-7.5. Since phase separation of a certain element appear, the homogeneity of the glass cannot be assured.

Fig. 4

XRD analysis of fabricated glass with respect to chemical composition: (a) DG-2 and Zr added DG-2 glasses (b) DG-2 and Hf added DG-2 glasses.

The XPS spectra of the fabricated glasses, Zr added to DG-2 glass, are shown in Fig. 5. The Si 2p spectra, found around 102 eV, was similar in DG-2 and the Zr-added glasses [19, 20]. The O 1s spectra, found around 530–532 eV, was similar in DG-2 and the Zr-added glasses. The Zr 3d spectra, found around 182.5 and around 185 eV, was similar in DG-2 and the Zr-added glasses. The fabricated glasses consisted of various cations and their oxides. Since fabricated glasses are consisted complex oxides and their structure evolution with respect to the addition of Zr is not clearly observed in XPS spectra, it is a reasonable interpretation that Zr is blended well in DG-2 glass. This means that the DG-2 composition has sufficient structural flexibility with Zr cation and results in good vitrification processability in terms of Zr addition.

Fig. 5

XPS spectra of (a) Si, (b) O, and (c) Zr from fabricated glass with respect to chemical composition.

The XPS spectra of the fabricated glasses, Hf added in DG-2 glass, are shown in Fig. 6. The Si 2p spectra, found around 102 eV, is similar in DG-2 and Hf-added glasses [21, 22]. The O 1s spectra, found around 530–532 eV, is similar in DG-2 and Hf-added glasses. The Hf 4f spectra, found around 17 and around 19 eV, is similar in DG-2 and Hf-added glasses. The fabricated glasses consist of various cations and their oxides. Since fabricated glasses consist of complex oxides, and their structure evolution with respect to addition of Hf is not clearly observed in XPS spectra, it is a reasonable interpretation that Hf is blended well in DG-2 glass. This means that the DG-2 composition has sufficient structural flexibility with Hf cation and results in good vitrification processability in terms of Hf addition.

Fig. 6

XPS spectra of (a) Si, (b) O, and (c) Hf from fabricated glass with respect to chemical composition.

Chemical analysis of the leachates from 7-day PCT is shown in Fig. 7, calculated using analyzed glass composition. The 7-day PCT results indicate that the chemical durability of metal particles containing DAW is similar to the DG-2 glass. The primary elements, B, Li, Na, and Si, were observed in chemical analysis of the leachates.

Fig. 7

7-day PCT result of fabricated glasses: (a) DG-2 and Zr added DG-2 glasses (b) DG-2 and Hf added DG-2 glasses.

The normalized release (g/m²) was calculated using the following equation (1).

where C_i is the concentration of elements in the leachate (g/m³) and f_i is the mass fraction of elements in the glass. SA/V, which was 2,045 m⁻¹ for this calculation, is the ratio of the sample surface area to the leachate volume (m⁻¹). The results indicate that all the normalized release for B, Li, Na, and Si were lower than 2 g/m².

The calculated normalized release was similar for all the glasses. Lu et al. reported that the addition of Zr in boroaluminosilicate glass improves its chemical durability [23]. The DG-2 and Zr-added glasses, however, showed similar chemical durability. This may be attributed to complex cation composition and oxide formation. Zr is considered to be a network former, similar to Si in a glass matrix. The complex cation composition allows for structural flexibility and results in similar glass properties. The Hf-added glasses were similar to the Zr-added glasses. The Hf was also blended well into the glass matrix, resulting in good chemical durability.

The leachability index is calculated to understand the chemical durability of the fabricated glasses. The ANS 16.1 method was adopted to evaluate the diffusivity and surface release from vitrified form. The effective diffusivity is calculated using equation (2).

where D is effective diffusivity (cm²·s⁻¹) during test interval and m is the slope of derived by linear regression of cumulative fraction of element released, considering volume and geometric surface area of the glass, through test interval and cumulative reaction time over test interval.

The leachability index is calculated using equation (3).

where L_i is the leachability index of element, β is a defined constant (1.0 cm²·s⁻¹), and D_e,i is the effective diffusivity (cm²·s⁻¹) of element from the test data during the test interval.

The leachability index, derived from the ANS 16.1 test, evaluates diffusive release behavior of contaminant, which were Cs, Co, and Sr in this composition, with respect to the time. Since the leachability index offers the characteristics related to the chemical durability of the element of interests, the leachability index is generally considered as an effective tool to evaluates the disposal safety under the repository condition [16, 17]. According to the waste acceptance criteria, the solidified waste form is considered to be satisfied when the leachability index is equal to or greater than 6. The calculated leachability index for Cs, Sr, and Co for DG-2_Zr-5 and DG-2_Hf-5 were 16.3, 16.6, 17.2, 16.3, 16.6, 19.2, respectively, as shown in Table 2. The results indicate that the fabricated glass, which is applicable to vitrification of DAW, which is containing Zr and/or Hf particulates, is suitable for the disposal in the repository, in terms of chemical durability.

It is known that the addition of ZrO₂ to borosilicate glass enhances initial leaching characteristics but diminishes long-term leaching performance [24]. In this study, the leaching characteristics of the prepared glasses were evaluated using two leaching methods, PCT and ANS 16.1. For the PCT method, the surface area of the glass exposed to the leachate is substantial (SA/V ~2,000 m⁻¹), with leaching conducted at a high temperature of 90°C for 7 days. Conversely, for the ANS 16.1 method, the exposed glass surface area is minimal (SA/V 10 m⁻¹), and leaching occurs over 5 days at room temperature. Consequently, it can be inferred that the accelerated experiment using PCT represents long-term leaching characteristics, whereas the more moderate ANS 16.1 method reflects initial leaching characteristics. The ANS 16.1 results indicate that the effective diffusivities of Cs, Sr, and Co ions in glasses containing ZrO₂ and HfO₂ are approximately 10 times lower than in DG-2 glass. This result is consistent with previous research demonstrating that the addition of ZrO₂ to borosilicate glasses reduces the initial dissolution rate of the elements [24]. Similarly, the initial dissolution rate was also reduced in glasses containing HfO₂, suggesting that Hf, the same group 4 element as Zr, exists in a comparable structural role in borosilicate glasses. The 7-day PCT results alone were insufficient to definitively analyze the effect of ZrO₂ and HfO₂ introduction on the long-term dissolution rate of borosilicate glass. Nevertheless, the leaching resistance of DG-2 glass sufficiently satisfied the Hanford Site solid waste acceptance criteria of less than 2 g/m² of normalized elemental release evaluated by the PCT method, both with and without ZrO₂ and HfO₂ addition [25]. Therefore, DG-2 glass demonstrates sufficient miscibility to incorporate metal ions such as Zr and Hf.

4. Conclusion

This feasibility study of vitrification for DAW, which is containing metal particulates, was conducted in order to understand applicability under various conditions. The designed glasses were fabricated well at 1,150 degrees. Microstructure analysis of the fabricated glasses using SEM indicated that their surfaces were smooth. They exhibited good mechanical properties, with compressive strength over 50 MPa. XRD analysis indicated that the fabricated glasses were amorphous phase, except for DG-2_Zr-7.5. Chemical durability was investigated using PCT and leachability index calculation. The 7-day PCT normalized Li, B, Na, and Si release were far lower than 2.0 g/m² which is the Hanford contract limit for low activity waste borosilicate glass. The leachability index of the fabricated glasses was over 6. Results indicated that the DAW containing Zr and Hf was well vitrificated and exhibited good characteristics when considering the generic requirements of solidified forms.