The low- and intermediate-level radioactive wastes discharged from domestic nuclear power plants (NPPs) are disposed of in a 200 L storage drum . According to the International Atomic Energy Agency, when a light water reactor (900-1,300 MWe) is decommissioned, approximately 6,200 t of low- and intermediate-level radioactive decommissioning wastes are generated in average. Based on this, approximately 14,000 drums have been predicted to be generated when the generated waste is packaged in 200 L drums [2, 3]. During NPP operations, various types of wastes (e.g., metal, concrete, concentrated waste solution, spent resin, spent filter, and sludge wastes) are generated. Sludge wastes are classified as corrosive, oil, concrete, and soil sludge . Corrosive sludge is generated by deposits of metallic substances generated by erosion of pipes and pumps constituting the secondary system on the upper side of the Tube Sheet (TS) and Tube Support Plate (TSP) on the secondary side of the steam generator . Selecting the optimal treatment method or finding the optimal operating conditions is not easy because the composition and the amount of the corrosive sludge are very diverse. In addition, the optimum mixing ratio of the corrosive sludge and the waste form materials varies depending on the chemical composition [6, 7]. Thus, research on the appropriate immobilization of corrosive sludge is necessary. Generally, the first step in decontaminating corrosive sludge is to remove radioactive materials, such as iron (Fe), nickel (Ni), chromium (Cr), and cobalt (Co), from the sludge through desorption, ion exchange, evaporation, and/or chemical precipitation . In addition, as a decontamination agent, ethylenediaminetetraacetic acid (EDTA) is used . If radioactivity is reduced within the regulatory limit based on the radioactive waste classification, the decontaminated corrosive sludge can be classified as extremely low-level waste and discharged into the environment under suitable management and monitoring; otherwise, it should be solidified by polymer, bitumen, asphalt, or cement waste form for permanent disposal [1,10]. However, in order to decontaminate the corrosive sludge, an additional decontamination agent must be used, and when ion exchange resin is used during decontamination process, organic waste is generated. Therefore, in this paper, a study was conducted to immediately solidify and dispose of corrosive sludge generated from nuclear power plants without additional processing.
The cement waste form is widely used to solidify radioactive waste because cement ingredients and technologies are well understood; and cement waste form exhibits excellent economic efficiency due to easy and simple process [7, 10, 11]. The Pacific Northwest National Laboratory and the Savannah River National Laboratory developed cast stone with 10wt% of Portland cement, 45wt% of blast furnace slag , and 45wt% of fly ash and salt stone with 8wt% of Portland cement, 47wt% of blast furnace slag, and 45wt% of fly ash, respectively [12, 13]. However, the cement waste form demonstrates drawbacks of high porosity, weak adhesion to substrate, low mechanical durability, high permeability, and weak resistance to acids, alkalis, and high sulfate content [14, 15]. In addition, some components, such as sulfate and borate, cause problems in the cement waste form . Sulfate can form a secondary mineral that can cause expansion and disintegration . Borate is also considered to cause the retardation of setting in the cement waste form .
Accordingly, research on geopolymer as an alternative waste form has been developed. Geopolymer is an inorganic binder hardened by the reaction of amorphous aluminosilicate materials (e.g., fly ash and metakaolin) and an alkali activator (e.g., NaOH and KOH) at room or low temperature . It exhibits a three-dimensional aluminosilicate framework synthesized by the reaction of solid aluminosilicate materials and alkaline solutions . The Si and Al released from raw materials can form a Si-O-Al framework, in which SiO4 and AlO4 tetrahedrals are linked to each other by sharing O2 molecules . The negatively charged and tetrahedrally coordinated Al atoms inside the network are charge-balanced by the alkali metal cations coming from the activator [16, 20]. In addition, geopolymer exhibits superior chemical and physical properties because of its unique structure . Therefore, compared to cement, geopolymer shows superior chemical resistance, long-term durability, high thermal properties, excellent compressive strength at an early age, and immobilization capability for toxic elements as a waste form . Recent research on radioactive waste solidification using geopolymer showed that the metakaolin-based geopolymer is a better binder of solidifying the high-sulfate waste materials generated by the hydrazine-based reductive metal ion decontamination process [14, 16]. A geopolymer study based on groundgranulated blast furnace slag reported a successful method of solidifying spent resin . In Slovakia, various sludge and spent resins discharged from the sedimentation tanks of the Slovakian NPP were solidified with a metakaolin-based geopolymer SIAL® matrix [22, 23]. In the United States, the DuraLith waste form was developed using fly ash, furnace slag, and metakaolin to solidify Hanford secondary wastes .
Currently, studies on solidifying the corrosive sludge from Kori 1 NPP using the geopolymer waste form are still insufficient. Although solidification using the cement waste form has been studied, it exhibits various limits, including high porosity and low mechanical durability. Therefore, the corrosive sludge solidification process using the geopolymer waste form, which can compensate for the limitations of the cement waste form, must be investigated.
In this study, we solidify corrosive sludge, which is one of the wastes generated from Kori 1 NPP, using cement and geopolymer waste forms to realize effective waste disposal. In addition, we measure the compressive strength and analyze the chemical properties of the two waste forms according to the waste loading (%) of the corrosive sludge. The obtained results are then compared and used to evaluate which waste form is more suitable for corrosive sludge disposal.
2. Materials and methods
2.1 Preparation of the simulated corrosive sludge
The simulated corrosive sludge was made according to the chemical compositions provided by Korea Hydro & Nuclear Power (KHNP). It contained Fe, Ni, Cr, and Co, with Fe comprising majority of the ions (Table 1). Iron (Ⅲ) oxide (Fe2O3, Sigma-Aldrich, Germany), iron (Ⅱ, Ⅲ) oxide (Fe3O4, Sigma-Aldrich, Germany), nickel (Ⅱ) oxide (NiO, Sigma-Aldrich, Germany), chromium (Ⅲ) oxide (Cr2O3, Sigma-Aldrich, Germany), and cobalt (Ⅱ) oxide (CoO, Alfa Aesar, USA) were used to prepare the simulated corrosive sludge. One kilogram of chemical reagents was dissolved with 1 L of deionized water (DIW) in a 2 L beaker. The mixture was dried in a forced convection oven (OF-21E, Lab Companion, South Korea) at 105℃ for 24 h. Subsequently, the dried corrosive sludge was crushed and used for waste form development.
2.2 Formulation of the cement waste form
Ordinary Portland cement (OPC, Type I), blast furnace sludge (BFS), fly ash (FA, class F), and deionized water (DIW) were used as the raw materials for the formation of the cement waste forms. All materials, except the DIW, were from Sungshin Cement Co., Ltd. and used without any further purification process.
When the cement waste form was formed, the material proportion consisted of 20wt% of OPC, 40wt% of BFS, and 40wt% of FA at a 0.5 water: dry materials ratio. The corrosive sludge waste loading was varied from 0wt% to 60wt% of the total amount of dry materials.
To make the cement waste form, the OPC, BFS, FA, and simulated corrosive sludge were first mixed. Then, DIW was added to mix with dry ingredients using a Thinky mixer (ARE-310, Thinky Corporation, Japan). Mixing was performed for 2 min at 1,800 rpm. Subsequently, the mixtures were poured into polyethylene plastic molds with 29 mm diameter and 58 mm height. A vibrator (Denstar-500, Denstar, South Korea) was used for 5 min to remove the trapped air bubbles during the mixing process. The cement mixtures filled in the mold were stored in a tray under sufficient absolute moisture conditions of 100% for hydration and cured at room temperature for 28 days before demolding.
2.3 Formulation of the geopolymer waste forms
Metakaolin (MetaMax, BASF, Germany), potassium hydroxide (KOH, assay 93%, Daejung, South Korea), fumed silica (CAB-O-SIL, assay 99.99%, Cabot Corporation, Germany), simulated corrosive sludge, and DIW were used as the raw materials of the geopolymer waste form.
The geopolymer waste form was formed at a molar ratio of K2O:Al2O3:SiO2:H2O = 1:1:4:12 with 2.0 Si/Al ratio. The corrosive sludge was loaded from 0wt% to 40wt% of the total amount of geopolymer materials.
To make the geopolymer waste form, an alkali activator was first prepared by mixing with KOH, fumed silica, and DIW (Table 2). This activator was stirred at room temperature for 24 h before mixing with metakaolin and the simulated corrosive sludge. The activator, metakaolin, and simulated corrosive sludge were mixed using the Thinky mixer. Mixing was performed for 2 min at 1,800 rpm and 30 s at 2,100 rpm for defoaming. The mixtures were poured into polyethylene plastic molds with a same size to those used in the cement waste forms. A vibrator was also used for 5 min. Subsequently, the geopolymer mixtures filled in the mold were sealed by a cap. In addition, since the heat can accelerate the geopolymerization reaction, geopolymer mixtures were cured first at 60℃ for 1 day followed by additional 6 days at room temperature (25 ± 1℃).
2.4 Characteristic analysis
An X-ray fluorescence spectrometer (XRF S4-Pioneer, Bruker, Germany) was used to analyze the chemical composition of the simulated corrosive sludge.
A X-ray diffraction analysis was performed using an Xray diffractometer (MiniFlex600, Rigaku, Japan) to analyze the crystallographic properties of the simulated corrosive sludge and the waste forms. The diffraction patterns were obtained over a range of 3-90° with a step size of 0.02°. The scanning speed was 10° min−1 at 40 kV voltage and 15 mA current.
A compressive strength testing machine (ST-1001, Salt Inc., South Korea) was used to measure the compressive strength of the waste forms. The loading rate was 0.3 MPa·s-1. The compressive strengths of the waste forms were measured according to the American Society for Testing and Materials C39 method . The compressive strength was measured after curing of 28 days for cement waste forms and after curing of 7 days for geopolymer waste forms in triplicate.
Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR, Nicolet iS10, Thermo Fisher Scientific, USA) was used to analyze the chemical bondings of the simulated corrosive sludge and the final cured waste forms with the wavenumber range of 525-4,000 cm−1.
High-resolution field-emission scanning electron microscopy (FE-SEM, JSM-7800F, JEOL Ltd., Japan) was employed to analyze the morphology and microstructure of the waste forms. The samples were coated with Pt at 20 mA for 20 s to obtain high-resolution data. Then, an analysis was conducted at an acceleration voltage (Va) of 20 kV.
3. Results and discussion
3.1 Chemical properties of the simulated corrosive sludge
The XRF results of the simulated corrosive sludge are shown in Table 3. It contains 75.07% Fe2O3, 11.83% Cr2O3, 10.32% NiO, and 0.249% CoO, with Fe oxide accounting for the majority of ions. The corrosive sludge was caused by the large amount of iron. The XRF showed results that were similar to the chemical composition provided by the KHNP (Table 3).
The XRD result of simulated corrosive sludge is shown in Fig. 1. Hematite (Fe2O3, #00-033-0664), magnetite (Fe3O4, #01-071-6336), eskolaite (Cr2O3, #01-078-5444), and bunsenite (NiO, #01-071-1179) were observed in the simulated corrosive sludge. Although Co was used to prepare the simulated corrosive sludge, the XRD pattern of any Co crystalline phase was not detected because of its small amount. The XRD results showed that the dominant oxides were observed in the composition consistent with the XRF result.
The FT-IR results of the simulated corrosive sludge are shown in Fig. 2. The bands around 2,664 cm−1 and 1,537 cm−1 are normally attributed to the O-H stretching and HO- H bending vibration, respectively, indicating that the corrosive sludge contained water molecules [26, 27]. The band around 2,352 cm−1 exhibits the presence of CO2, which is a common impurity in IR spectra . The band located near 1,100 cm−1 is attributed to the Ni-O-Ni stretching mode  in the presence of bunsenite. Meanwhile, the band at 897 cm−1 is attributed to the Fe-O-H bending vibration , representing a bond with Fe in the corrosive sludge and the presence of OH−. The strong absorption band at 558 cm−1 is attributed to the Fe-O bonds in the bending mode , which is consistent with the simulated corrosive sludge mainly composed of Fe oxides.
3.2 Cement waste form
The compressive strength of the cement waste form at various OPC, BFS, and FA ratios was measured before the corrosive sludge loading on the cement waste form, to find the optimum ratio of the cement waste form (Table 4). Accordingly, BFS and FA were mixed with OPC because BFS demonstrates various advantages when used as a dry ingredient for the cement waste form formulation. It exhibits relatively constant chemical composition, low heat of hydration, high sulfate and acid resistance, better workability, higher ultimate strength, and fineness . Meanwhile, FA is a pozzolan that contains aluminous and siliceous materials that form cement in the presence of water. When mixed with lime and water, FA forms a compound similar to Portland cement. Therefore, when it is used in the cement waste form, it improves the workability, concrete strength and segregation, and makes pumping easier with enhanced flowability, and reduces the heat of hydration . The compressive strength was the highest when 20wt% OPC, 40wt% BFS, and 40wt% FA were mixed at a 0.5 water: dry ingredient ratio. This formulation is referred to herein as K-Stone.
The photographs of the cement waste forms with various sludge waste loadings after 28 days of curing are shown in Fig. 3. The compressive strength of the cement waste form without corrosive sludge was 24.4 MPa. The measured compressive strengths were inversely proportional to the waste loading (Fig. 4). Up to 50wt% of the corrosive sludge waste loading, the compressive strength of the cement waste forms was all over 3.45 MPa, thereby satisfying the compressive strength of the acceptance criteria for the waste form in radioactive waste disposal sites. However, the cement waste form showed a lower compressive strength (2.7 MPa) than 3.45 MPa at corrosive sludge waste loading of 60wt%. Therefore, the corrosive sludge was loaded to the cement waste forms of up to approximately 50wt%. The cement waste form was solidified through a hydration reaction . However, the corrosive sludge was an inactive component in the cement waste form, and it absorbed the free water used for the formulation of the cement waste form. The free water decreased as the corrosive sludge waste loading increased. The cementation reaction also decreased. Therefore, the compressive strength of the cement waste form decreased as the corrosive sludge waste loading increased.
The XRD results of the cement waste forms are shown in Fig. 5. The cement waste forms without the corrosive sludge showed calcite (CaCO3, #01-072-1937), quartz (SiO2, #01-089-1961), mullite (Al6Si2O13, #00-015-0776), and calcium silicate hydrate (Ca6Si3O12∙H2O, #00-011- 0507). Quartz and mullite are mainly the crystal phases of FA. Meanwhile, OPC was mainly composed of calcium silicate (Ca3SiO5, #00-055-0740) and portlandite (Ca(OH)2, #01-076-0571). Calcium silicate hydrate (C-S-H) and calcite were the crystal phases formed from the combination of raw materials. Subsequently, hematite (Fe2O3, #00-033-0664), magnetite (Fe3O4, #01-071-6336), eskolaite (Cr2O3, #01-078-5444), and bunsenite (NiO, #00- 071-1179) were observed after the corrosive sludge loading to the cement waste form. Peaks were detected from the corrosive sludge. The peak intensity increased in proportion to the corrosive sludge waste loading. In addition, ettringite (Ca6Al2(SO4)3(OH)12∙26H2O, #00-002-0059) was observed at 2θ = 9.13° when 10wt% corrosive sludge was loaded. The late formation of ettringite is a compound that has a major influence on the redcution of the compressive strength of the cement waste form [35-38]. Fe-ettringite is considered to be formed by Fe in the corrosive sludge, partially replacing Al in the cement waste form during the hydration reaction [39, 40]. The relative content of the raw materials decreased when the corrosive sludge waste loading increased; hence, it was not well observed at a waste loading exceeding 10wt%. The OPC, BFS, and FA contents forming the ettringite decreased when the corrosive sludge was loaded to the cement waste forms, consequently decreasing the amount of ettringite in the hydration reaction.
C-S-H is a type of amorphous microporous phase with high specific surface area and high-density hydrogen bonding. It could tightly bind heavy metals with strong chemical adsorption . The binder performance was significantly improved by the C-S-H phase . In addition, CaCO3(s) was formed by the reaction of CaO and C-S-H hydrate with CO2 in the atmosphere. CaCO3(s) exhibits low solubility in water, making it more chemically stable [43,44]:
The relative content of the raw materials decreased as the amount of corrosive sludge increased; hence, the amount of C-S-H, which is important for the strength of the cement waste form, decreased. Correspondingly, the amount of calcite also decreased. This is seen in the XRD results, where the intensity of the peak related to calcite decreased with the increasing waste loading (Fig. 5). The corrosive sludge amount caused the reduction of the compressive strength of the cement waste form.
The FT-IR results of the cement waste forms are shown in Fig. 6. The bands around 3,240 cm−1 and 1,640 cm−1 are normally attributed to the O-H stretching and H-O-H bending vibration, respectively. This shows that the cement waste form contained water molecules [28, 29] because the cement waste form was cured through a hydration reaction under sufficient moisture condition. The band around 1,412 cm−1 is attributed to the absorption peaks of C-S-H that exhibits a major influence on the increase of the compressive strength of the cement waste form. The band at 713 cm−1 is attributed to calcite . The band intensity decreased as the corrosive sludge waste loading increased, causing a compressive strength reduction. Meanwhile, the band at 875 cm−1 is attributed to the stretching vibration of the Si-O bonds . As the amount of corrosive sludge increased, the band intensity also decreased because of the decrease in the raw material content (e.g., OPC, BFS, and FA), and this result is consistent with the XRD results.
The SEM images of the cement waste forms are shown in Fig. 7. Unlike in (a), a needle-like ettringite was observed in (b). The hydration reaction of OPC involves four types of hydration components: alite (C3S), belite (C2S), aluminate (C3A), and ferrite (C4AF) . C3S constitutes approximately 50 to 70% of OPC by mass, and the hydration reaction with water produces a C-S-H compound [35- 38]. In addition, C3A is the fastest hydration compound among the cement compounds. It reacts with gypsum to form ettringite. We considered Fe-ettringite to be formed by Fe in the corrosive sludge waste, partially replacing Al in the cement waste form during the hydration reaction [39, 40]. The late formulation of ettringite is a factor inhibiting the compressive strength (e.g., cracking) by inducing an expansion inside the cement waste form [46, 47]. Therefore, the compressive strength was reduced when the corrosive sludge was loaded. However, ettringite was not well observed when the waste loading increased because the content of the raw materials (e.g., OPC, BFS, and FA) decreased as the corrosive sludge increased. Ettringite was not visible at 60wt% waste loading because of a very low content of raw materials. Therefore, when more than 10% of the corrosive sludge was loaded, Fe-ettringite was generated, and the compressive strength decreased. In addition, the amount of Fe-ettringite decreased as the corrosive sludge waste loading increased. However, the production of the C-S-H compound, which exhibits a major influence on strength, also decreased, resulting in the reduction of the compressive strength.
3.3 Geopolymer waste form
Before the corrosive sludge loading, geopolymer waste forms were formulated by changing the Si/Al ratios to 1.6, 1.8, and 2.0 to measure the compressive strength and to optimize the formation condition (Table 5). The compressive strength was high when Si/Al was approximately 1.8 to 2.0. Unlike coal ash (fly ash, bottom ash, and pond ash) based geopolymer, the compressive strength of metakaolinbased geopolymer is highest when the Si/Al ratio is close to 2.0 . Therefore, 2.0 Si/Al was selected as the optimal ratio of the geopolymer waste form.
The photographs of the geopolymer waste forms with various sludge waste loadings after 7 days of curing are shown in Fig. 8. The Fe content increased as the amount of corrosive sludge increased; hence, the color of the geopolymer waste forms gradually became much darker. The geopolymer waste form loading of more than 40wt% of corrosive sludge was not solidified because the ingredients were not mixed (Fig. 8). The corrosive sludge was loaded into the geopolymer waste form from 0 to 40wt%. The compressive strength of the geopolymer waste forms increased from 23.9 MPa to 31.9 MPa (Fig. 9). The amount of water decreased as the corrosive sludge increased because the molar ratio was same to K2O:Al2O3:SiO2:H2O = 1:1:4:12 in a quantity other than waste at the same total weight. Therefore, the liquid/solid ratio decreased. The corrosive sludge absorbed water; thus, the activator concentration increased as the amount of water in the activator decreased. Therefore, the geopolymerization increased, and the geopolymer waste structure became more compact. As a result, the compressive strength of the geopolymer waste form increased .
Compared to the cement waste forms, the geopolymer waste forms showed a high compressive strength at all corrosive sludge waste loadings and exceeded the repository acceptance criterion (> 3.45 MPa). A high compressive strength can be obtained despite the short curing time at the same corrosive sludge waste loading.
The XRD results of the geopolymer waste forms are shown in Fig 10. An impurity of anatase (TiO2, #00-004- 0477) was found in the geopolymer waste form without corrosive sludge . Hematite (Fe2O3, #00-033-0664), magnetite (Fe3O4, #01-071-6336), eskolaite (Cr2O3, #01- 078-5444), and bunsenite (NiO, #00-071-1179) were observed when corrosive sludge was loaded to the geopolymer waste form. Peaks were detected from the corrosive sludge and intensity of these peaks increased in proportion to the corrosive sludge waste loading. The XRD pattern representing the amorphous phase was centered at 2θ of 28° and intensity of this peak decreased because the amount of metakaolin decreased as the corrosive sludge waste loading increased. In addition, the XRD pattern intensity was high because the materials constituting the waste were crystalline materials and metals. In contrast, the anatase peak appeared to be relatively low.
The FT-IR results of the geopolymer waste forms are shown in Fig. 11. The bands around 3,377 cm−1 and 1,640 cm−1 are related to the water molecule [28, 29, 50]. This is considered as the effect of moisture in the atmosphere or water in the activator used to cure the geopolymer waste form. The band around at 593 cm−1 is attributed to the Si- O-Al band , whereas that around 981 cm−1 is attributed to the Si-O-T (T=Si or Al) stretching band, which is the main band of the geopolymer . The Si and Al contents decreased as the corrosive sludge waste loading increased; thus, the band intensity also decreased. With the increased corrosive sludge waste loading on the geopolymer waste forms, the bands related to the corrosive sludge were incorporated into the geopolymer, and no distinct band was associated with the corrosive sludge.
The SEM images of the geopolymer waste forms are shown in Fig. 12. They exhibited a uniform, dense, and finer microstructure as the corrosive sludge waste loading increased because the inactive corrosive sludge absorbed water to compact the geopolymer structure as the corrosive sludge waste loading increased. This was considered as a major factor in the compressive strength increase.
Compared to the cement waste form in Fig. 7, the geopolymer waste form exhibited small pores and flat and dense structures, which caused its high compressive strength at the same corrosive sludge waste loading.
This study investigated cement and geopolymer waste forms to solidify the corrosive sludge generated from NPPs and compared their properties. Compressive strength, XRF, XRD, FT-IR, and SEM were used to monitor the physical stability, chemical composition, crystallographic properties, crystal phases, chemical bonds, and morphology of the waste forms according to the corrosive sludge waste loading, respectively.
The compressive strength of cement waste forms was inversely proportional to the corrosive sludge waste loading. Herein, the cement waste form was solidified through a hydration reaction, but the corrosive sludge absorbed free water and inhibited the hydration reaction during the cement waste form curing. In addition, the raw material content of the cement waste form (i.e., OPC, BFS, and FA) decreased when the corrosive sludge waste loading increased. Therefore, the C-S-H which resulted in a major influence on the compressive strength, also decreased.
On the contrary, the compressive strength of the geopolymer waste forms was proportional to the corrosive sludge waste loading. The water content in the geopolymer waste forms decreased when the corrosive sludge waste loading increased, making the geopolymer structure more compact. However, the geopolymer formulation was not successful with higher waste loading (> 50%) because the ingredients were not mixed. Comparing the geopolymer waste forms to the cement waste forms, we found that the former demonstrated a higher compressive strength than the latter at the same corrosive sludge waste loading. In terms of the compressive strength, the geopolymer waste form can be used as a promising candidate for the safe and effective solidification of corrosive sludge and any other iron-rich radioactive wastes generated from NPPs.
Research on the evaluation of whether the waste forms meet the acceptance criteria of the final disposal site must be conducted in the future by performing thermal cycling, leaching, and radiation tests.