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Environmentally benign and novel management route for radioactive corrosion products by hydroxyapatite
Journal of Nuclear Materials 507 (2018) 218e225
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Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Environmentally benign and novel management route for radioactive corrosion products by hydroxyapatite Sajid Iqbal, Muhmood ul Hassan, Ho Jin Ryu, Jong-Il Yun* Department of Nuclear and Quantum Engineering, KAIST, Daejeon 34141, Republic of Korea
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
The direct solidification of the captured corrosion products by HA has been investigated. No organic binders were used for solidification of M-HA matrix. The sintered matrix showed good densification and mechanical properties. ASTM PCT test approved the chemical durability of sintered matrix.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 January 2018 Received in revised form 4 May 2018 Accepted 6 May 2018 Available online 7 May 2018
A novel and environmentally benign route for the direct solidification of captured corrosion products (Co, Cr, Fe, Ni, Cu, Zn, and Mn) generated from the primary coolant system of nuclear power plants is introduced in this study. Synthesized calcium hydroxyapatite was used to remove individual and mixtures of corrosion products from aqueous solutions. The results show more than a 95% removal of these corrosion products for both cases. The direct solidification of adsorbed corrosion products was done by pressure-less conventional sintering. The sintered matrix revealed a good hardness (3.70 ± 0.60 GPa) and a relative sintered density > 98% after heat treatment at 1150 C. The measured compressive strength (207.3 ± 9.5 MPa) was significantly higher than the established waste immobilization criteria of the US (3.5 MPa) and Russia (4.9 MPa). The corrosion products consolidated matrix had a normalized leaching rate ranging from 3.4#102 to 3.1#106 g/m2/day. Moreover, no additional chemical treatments, additives (gypsum, slaked lime, sodium silicate, etc.), and sophisticated equipment were needed for the adsorption and solidification process. Therefore, the proposed waste management route has no adverse effects on the ecosystem and can be highly efficient to immobilize adsorbed waste and to reduce the secondary waste volume. © 2018 Elsevier B.V. All rights reserved.
Keywords: Waste immobilization Nano-ceramic Hydroxyapatite Radioactive corrosion products
1. Introduction Corrosion products, known by Chalk River Unidentified Deposits (CRUD), such as 58Co, 60Co, 51Cr, 64Cu, 54Mn, and 59Fe are the main
* Corresponding author. E-mail address: [email protected] (J.-I. Yun). https://doi.org/10.1016/j.jnucmat.2018.05.016 0022-3115/© 2018 Elsevier B.V. All rights reserved.
sources of radiation build-up inside nuclear reactor vessel and pose serious problems in routine overhaul maintenance work. CRUD are released into the primary coolant due to the degradation of mechanical parts of the primary coolant system in nuclear power plants. The CRUD are transported to the reactor core and get activated under the high neutron flux. These are called radioactive CRUD which consequently increase the radiation level. CRUD can
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deposit on the surface of fuel rods and cause a reduction in the thermal conductivity. In certain cases, they may also damage the fuel rods due to stress corrosion cracking phenomena [1e3]. Therefore, it is necessary to use efficient adsorbents to maintain the coolant chemistry and to remove any metal ions that form due to the corrosion of structural materials of the nuclear power plant. Many natural wastes, organic matter, and synthesized adsorbents have been successfully used to capture cobalt/CRUD from contaminated water [4e8]. For example, Granados et al. reported on the adsorption behavior of cobalt as a function of pH, contact time and initial metal ion concentrations for Fe-Mn oxide synthetic material [9]. Oliva et al. used commercial apatite to remove trace levels of Cd, Cu, Ni, Co and Hg, which are hard to remove by calcite or other organic adsorbents [8]. Yuonjin et al. used ammonium molybdophosphateepolyacrylonitrile to remove Co, Sr and Cs which come from radioactive laundry waste water [10]. However, the solidification of Co/CRUD captured adsorbents (secondary waste produced) was not addressed in the aforementioned studies. Radioactive waste management procedures can be simplified and more cost-effective if the material being used as an adsorbent also acts as a matrix for the consolidation of captured nuclear waste. In addition, a significant reduction of secondary waste could be achieved with an adsorbent that can capture metal ions from waste and simultaneously solidify them thus eliminating the need for additional chemical treatments (addition of binders, excessive acid/base treatments or conditioning), loading to other matrices like Portland cement, glass or unsaturated polyesters. Calcium hydroxyapatite (Ca5(PO4)3OH) is a phosphate-based hard ceramic belonging to the apatite family and has the general chemical formula A5(XO4)3Z. The apatite structure has two cationic positions (A and X) and one anionic position (Z) [11e14]. On ionic sites, different ions can be accommodated without affecting the apatite structure due to its ion exchange properties [11]. Calcium hydroxyapatite (HA) has excellent biocompatibility and is being widely used as a bio ceramic material for making artificial dental roots, teeth implants and artificial bones similar to those living bodies [15e17]. The HA is also abundantly present in nature in the form of waste bones and phosphate-based minerals. Both natural and synthetic HA have been extensively investigated for their adsorption and substitutional properties to remove pollutants and toxic heavy metals [8,18,19]. In terms of environmental pollution, it is always desirable to use waste management methods that are the least harmful to the environment and nature. The use of organics, lime and cementitious binders for the stabilization and solidification of contaminants has been considered the best available technology [20]. The additional binders such as biochar, starch and carbodiimide have been used during green shaping of ceramic body to make it easier to obtain a desired density with less defects while pressing. During sintering at higher temperatures, the pyrolysis of these binders results in the emission of carbon dioxide or other hydrocarbon gases [21,22]. Nowadays, geo-polymers, slag, cement kiln and hydroxyapatite have also been tested for the consolidation process [23]. In this study, we applied the synthesized HA to remove single metal ions and a mixture of CRUD metals from an aqueous system. This study further investigated the possibility of binder-free immobilization of a CRUD captured adsorbent in a form of the highly dense and durable sintered matrix. This technique involves the adsorption of CRUD onto the surface of ceramic (calcium hydroxyapatite) nanoparticles. The adsorbent was separated from liquid by centrifugation and ultrafiltration. Then the compaction and consolidation of separated CRUD captured adsorbent was achieved by using conventional pressure-less sintering method.
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2. Experimental procedures 2.1. Synthesis of the adsorbent The wet precipitation method has been used to synthesize crystalline apatite as described elsewhere [13]. Anionic and cationic solutions were prepared by dissolving each of the di-ammonium hydrogen phosphate (NH4)2HPO4 and calcium nitrate Ca(NO3)2$4H2O in 200 ml of ultrapure deionized water, respectively. The Ca/P molar ratio of both the reactants was theoretical set as 1.67. The anionic solution was dropwise mixed with the cationic solution at a speed of 0.06 ml/s under continuous stirring at 200 rpm. During mixing, the temperature and the pH of the solution were continuously monitored and maintained at 30 C and 10.5, respectively. To maintain the pH of the solution, a concentrated ammonia solution was used. The aging of white gel type precipitates that formed during the synthesis was done inside the parent solution for 3 h. During the first hour, the stirring continued at 100 rpm and the temperature was maintained at 30 C, whereas at the end of the first hour, the reaction vessel was removed from the hot plate and kept without stirring at room temperature for 2 h. After that the suspension was filtered, thoroughly washed and dried overnight at a temperature of 100 C in a vacuum oven. The dried precipitates were ground to fine powder with a mortar and pestle. 2.2. Adsorption procedures A detailed adsorption study of HA can be found elsewhere [24]. First, the maximum removal of Co(II) was optimized by varying the solid to liquid ratio (S: L ratio) from 2 to 5 g/L with 1 mM of Co(II) at pH 6. The adsorbent-liquid contact time was set as 24 h with constant stirring at 150 rpm at 25 C. More than 90% removal of the Co(II) was observed for S:L ratio of 4 g/L. By using the optimized S:L ratio, a single element Co(II) solution and a series of corrosion products (Co(II), Cr(III), Cu(II), Ni(II), Mn(II), Fe(II) and Zn(II)) were removed from the contaminated aqueous solution. In the mixture of corrosion products, the concentration of each metal ion was kept constant at 1 mM in 100 ml of deionized water. After 24 h, the solid and liquid phases were separated with a 0.45 mm syringe filter followed by a 5 min centrifugation at 3200 rpm. The concentration of the metal ions in the separated dried adsorbent and the supernatant were measured by ICP-OES. The percentage removal (% R) of the metal ions was calculated by equation (1),
%R ¼ ðCo Ce Þ=Co 100
(1)
where Co and Ce are the initial and final metal ion concentrations (mM), respectively. 2.3. Sintering of pure HA and corrosion product adsorbed HA After the separation and drying of the adsorbent in a vacuum oven at 100 C overnight, the powder was shaped into cylindrical pellets with a stainless steel mold 13 mm in diameter. The pressing at room temperature was conducted under 100 MPa for 30 s by a Carver(R) uniaxial press. For each pellet, 1.20 g of pure HA and the metal adsorbed HA (Co-HA and M-HA) powder was used. The pressure-less conventional sintering of the as-pressed pellets of pure HA Co-HA and M-HA was carried out in the box furnace (Model: S-1700, HANTECH) under an air environment. In the first step, the pure HA was studied for a range of temperatures (500 C, 800 C, 900 C, 1000 C, 1150 C and 1300 C) for 120 min at a heating ramp of 4 C/min to optimize the sintering temperature. After the sintering, the samples were left inside the furnace to cool
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down to room temperature. In the second step, Co-HA and M-HA were sintered at 500 C and at the optimized sintering temperature (1150 C). The dimensions of each pellet were measured before and after the sintering. The percentage volume contraction was calculated with equation (2):
Vc ð%Þ ¼
ðVb Va Þ 100 Vb
(2)
where Vc (%) is the percentage volume contraction, Vb and Va are the volumes of the pellet before and after sintering. All experiments were performed in triplicate to check the reproducibility of the results. 2.4. Product consistency test (PCT) The Product Consistency Test (PCT) is a standard test of the American Society of Testing Materials (ASTM) to check the chemical stability and structural durability of ceramic waste forms [25e30]. The PCT is performed by immersing the crushed sintered samples (specific surface area ¼ 0.27 m2/g) into an ASTM-type I water at 90 ± 2 C for 7 days. All the procedures were performed in triplicate under static condition in an air-sealed stainless steel autoclave. After 7 days the leachate was analyzed for elemental degradation and pH change of the waste matrix. The elemental composition of the waste matrix was measured before the PCT, and the concentrations of the relevant constituents were measured in the leachate after 7 days PCT test with ICP-OES. The normalized leaching rate of the ith element was calculated by equation (3) as follows:
NLRi ¼
mi s,t,wi
(3)
where NLRi is the normalized leaching rate, mi is the concentration of component i leached into the water after a reaction time t, S is the surface area of the crushed powder, and wi is the weight fraction of an ith component in the solid matrix. 3. Characterization of the sintered matrices High-resolution X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to study the crystal structure, size and morphology of the grains in the sintered matrix. The XRD patterns were measured with XRD (SmartLab, RIGAKU) in a scan range of 10 to 90 with a step size of 0.02 at a tube voltage of 45 kV and a current of 200 mA. The Cu Ka incident radiation source with a wavelength (l ¼ 1.54056 Å) in the reflection geometry was used. For the microscopic analysis of the sintered matrix, SEM (Hitachi S4800) was used. The elemental mapping with the distribution of different waste constituents was recorded by energy dispersive spectrometer (EDS) coupled with SEM. The fractured surface morphology of the sintered samples was observed by mounting the samples on a cross-section holder. The sputter coating was performed for 15 s with an HPC-1SW osmium coater before microscopic analysis. The sintered bulk densities were measured with the Archimedes' method [31]. The sintered samples were boiled at 100 C in ultrapure water (MQ) for 30 min to remove the trapped air and to avoid chances of error. The densities and the relative percentage densities were calculated by using the following relationships [31,32]:
rb ¼ MD rL =ðWs WSM Þ
(4)
%RD ¼ rb =rR 100
(5)
where rb is the bulk density of the sintered sample, and mD, rL, Ws, and WSM are the dry weight, the density of deionized water, the saturated weight, and the suspended weight of pellet inside the deionized water, respectively. %RD is the relative standard density, and rR is the measured bulk density of the starting powder which is 2.94 g/cm3. The bulk density of the starting HA powder was measured with a helium pycnometer. The rb of the CRUD captured HA was assumed to be equal to the rb of the pure HA because the mass fraction of adsorbed CRUD is very small in comparison to mass fraction of pure HA. The ASTM E384 testing procedure was followed to measure the hardness values. The Vickers microindentor (Model: 402 MVD, Wolpert Wilson Instruments) installed with the software Expert Hardness 2007 V2.0 was used to quantify the micro-hardness of the sintered pellets under a constant load of 200 gf for a dwell time of 10 s [33]. All of the samples were fine polished by standard SiC papers with different grits (800e2000) and then by the diamond suspension paste before the hardness measurements [34]. The indents were made in a line crossing the center of the specimen so that the near surface effect could be avoided. The indentation was done at room temperature and a representative hardness value of 10 measurements was taken for each sample. The compressive strength of the sintered samples was measured with the universal testing machine (UTS INSTRON 5583, U.S.A) with a cross-head speed of 0.4 mm/min and a maximum load capacity of 150 kN at room temperature. Three cylindrical specimens with a length to diameter ratio (L/D) of 1.24 were prepared for each set of experiments, and the test was carried out according to the ASTM C39 standard compressive strength test [35].
4. Results and discussions This study was divided into three main steps: the synthesis of HA, CRUD adsorption by HA, and consolidation of the as-spent adsorbent (CRUD captured HA). Detailed adsorption studies on HA investigating the function of pH and contact time as well as the effect of the initial metal ion concentrations have already been done elsewhere [24,36,37]. We investigated the effect of the adsorbent amount on the maximum removal of a single metal ion and a mixture of metal ions from aqueous system. The quantitative measurements of the metal ions were done by inductively coupled plasma-optical emission spectrometry (ICP-OES). It is clear from Fig. 2 that the behavior of the cobalt removal is almost linear with the increasing adsorbent dose up to 4 g/L. This may be due to the excess surface binding sites available to the metal ions due to increase of adsorbent dose. After 4 g/L, the breakthrough value has been achieved with an insignificant effect on the removal of metal ion at S: L ratio of 5 g/L. Then an equimolar (1 mM each) mixture of corrosion products (Co(II), Cr(III), Cu(II), Fe(II), Mn(II), Ni(II) and Zn(II)) has been prepared in 100 ml of MQ water and optimum removal of metal ions (>95%) was obtained by increasing S:L ratio to 40 g/L. It is clear from Fig. 3 that more than 95% of corrosion products can be removed from aqueous media by providing 40 g/L of HA powder at 7 mM of total metal ion concentrations in a mixture. During the adsorption process a decrease in pH was observed from 6 to 4.9, which is due to protons coming from two types of protonated sites at apatite surface (≡POH and ≡CaOHþ 2 ) by the exchange of M(II)/M(III) cations as shown in equations (6) and (7) [38].
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Fig. 1. Flow diagram showing the synthesis of the adsorbent, and then the adsorption and consolidation of the captured corrosion products by hydroxyapatite.
Fig. 2. Effect of the solid to liquid ratio on the removal of Co(II) by hydroxyapatite (Co ¼ 1 mM, pH ¼ 6, contact time ¼ 24 h, ionic strength ¼ 0.1 M NaCl, shaking speed ¼ 150 rpm at RT).
≡POH þ M 2þ ⇔ ≡POM þ þ Hþ
(6)
≡CaOH2þ þ M 2þ ⇔ ≡CaOHM2þ þ Hþ
(7)
The main focus of this study was to reduce the secondary waste volume and to provide a facile route for waste management by direct consolidation of an as-spent adsorbent after separation and drying of the adsorbent. Therefore, pure HA, cobalt adsorbed HA (Co-HA) and corrosion product adsorbed HA (M-HA) powders dried in a vacuum oven at 100 C for 12 h were shaped into cylindrical pellets with a stainless steel mold, as shown in Fig. 1. The pellets
Fig. 3. Effect of the S: L ratio on the removal of CRUD mixture by hydroxyapatite (Co of each metal ion ¼ 1 mM, pH ¼ 6, contact time ¼ 24 h, shaking speed ¼ 150 rpm at RT).
were sintered at 500, 800, 900, 1000, 1150 and 1300 C in a box furnace in an air environment, as already discussed in Section 2.3. The pellets were crushed into powder and the XRD analysis was carried out to investigate the effect of sintering temperatures on the crystallinity and decomposition of the HA and M-HA. The phases were identified with the standard JCPDS card No. 00-064-0738 available in the system software [39,40]. No peak shifting phenomenon was observed in all samples except the appearance of bTCP-Ca3(PO4)2 on XRD patterns for HA-1150 C or M-HA-1150 C as shown in Fig. 4 which corresponds to the decomposition of HPO2 4 group present in a wet precipitated HA at this processing temperature [11,13]. There is an insignificant increase in crystallinity of samples heat
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Relative sintered density and the micro-hardness are the key parameters to confirm the sintering phenomena [42]. Fig. 5 (a & b) shows the effect of the sintering temperature on the relative sintered density and the micro-hardness, respectively. It is worth noting that the relative sintered density (Fig. 5(a)) increased from 60% to 99.8% with the increase of sintering temperature from 500 to 1300 C. The maximum hardness of 4.17 ± 0.27 GPa was measured for the sintered pure HA sample at 1300 C. Based on the densification and the volume contraction results, the optimized sintering temperature has been taken as 1150 C. The micro-hardness values for pure HA, Co-HA and M-HA at the optimized sintering temperature (1150 C) were 3.70 ± 0.69, 3.63 ± 0.61 and 3.41 ± 0.46 GPa, respectively. The volumetric contraction of pure HA sintered in the temperature range starting from 500 to 1300 C was also measured. About 60% of volume was decreased at 1150 C, as compared to 4e5% at 500 C. The full densification at 1150 C is due to the removal of the porosity and the grain boundary diffusion to form larger grains which is the temperature-driven phenomenon. The high temperature offers sufficient energy for the thermal activation of the grain boundary migration and diffusion [43]. It can be seen from Fig. 5 (a & b) that the adsorbed metal ions did not play a
Fig. 4. High-resolution XRD patterns showing the increasing crystallinity of the sintered samples as the temperature was increased (A shows the most intense peak of bTCP-Ca3(PO4)2).
treated at 500 C (HA and M-HA) compared with the non-heat treated CRUD adsorbed HA (M-HA-As adsorbed) while the samples sintered at 1150 C showed the highest crystallinity which is evident from the sharpness of XRD peaks (Fig. 4). The sample sintered at 1300 C also showed good stability and only the formation of small fraction of b-TCP-Ca3(PO4)2 can be seen in the XRD patterns (Fig. 4). Generally, 1200e1450 C is the temperature range reported for the decomposition of HA depending on the characteristics of the HA [41].
Fig. 6. Compressive strength of the pure HA, Co-HA and M-HA: M is a mixture of corrosion products containing Co(II), Cu(II), Ni(II), Fe(II), Zn(II), Mn(II), and Cr(III).
Fig. 5. Effect of the sintering temperature on (a) the relative sintered density and (b) the micro-hardness of the pure HA.
S. Iqbal et al. / Journal of Nuclear Materials 507 (2018) 218e225
significant role in lowering the sintering temperature of the immobilized matrix. The compressive strength is an important physical parameter that ensures the integrity of the waste form during its interim storage, transportation and final disposal under a geological repository environment. The waste form should have an acceptable value for the compressive strength which ensures its stable performance during storage and transportation. Fig. 6 shows the behavior of the compressive strength as the strain was increased. It is clear from Fig. 6 that the samples sintered at 500 C do not bear a significant load and have a very low compressive strength. On the other hand, the samples sintered at 1150 C showed an excellent
Table 1 Comparison of our data with standard compressive strength data [44,45]. Solidified matrix
Minimum Requirement (MPa)
Portland Cement U.S standard Portland Cement Russian standard Unsaturated Polyester Polyethylene Vitrified Glass Calcium Hydroxyapatite (this study)
3.45 MPa 4.93 MPa 5.17 MPa 6.89 MPa 34.50 MPa 207.34 ± 9.46 MPa
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compressive strength for the pure HA as well as the Co-HA and MHA. The high compressive strength was achieved because of the high densification and low porosity of the adsorbent. The compressive strength of 207.3 ± 9.5 MPa achieved in this study is several times higher than the compressive strength of the Portland cement (2.9e9.8 MPa) and other consolidation matrices being used for the immobilization of radioactive waste, as reported in Table 1. The reported compressive strength for HA is considerably higher than the US established regulatory requirements for Portland cement (3.45 MPa) [44]. SEM images in Fig. 7 show the effect of the sintering temperature on the grain size of the pure HA, Co-HA, and M-HA at 500 C and 1150 C. It is obvious from the SEM images that the particles are loosely packed and less diffused in both the HA and Co-HA at 500 C. On the other hand, more compact and densely packed structures can be seen in the images for the HA, Co-HA, and M-HA at 1150 C which show a significant grain growth and enlarged particle size. The increase in micro-hardness (Fig. 5(b)) and compressive strength (Fig. 6) are in agreement with studies by Muralitharan et al. and Karimzadeh et al. [34,46]. They investigated the effect of sintering temperature on the strength of HA and showed that the micro-hardness and compressive strength increases with increase in temperature unless dehydroxylation starts
Fig. 7. SEM micrographs of the fractured surfaces of the pure HA, the enlarged view in the inset of each at top left corner, the mapping and distribution of metal ions in the sintered pellets with different colored dots were observed by EDS analysis.
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Table 2 The normalized leaching rate (NLRi) of each element from the HA matrix. Metal
Co(II)
Cr(III)
Cu(II)
Fe(II)
Mn(II)
Ni(II)
Zn(II)
NLRi (g/m2/day)
4.4 106
3.4 102
3.1 106
5.3 106
3.8 106
4.4 106
3.3 106
and HA structure collapses. Different microstructures in the sintered matrix can be obtained at a given temperature depending on the characteristics of the starting powder which lead to the different mechanical properties. However, the microstructure obtained in our study had shown no adverse effects on mechanical properties of the sintered matrix. The presence of cobalt/CRUD in the sintered matrices was observed from the EDS and in the area mapping results of the sintered samples. The EDS results showed the homogenized distribution of the CRUD elements in the sintered matrix. PCT was performed as described in Section 2.4 to check the chemical stability of the consolidated matrix and the leaching rate of the immobilized corrosion products. The calculated NLRi values for each metal are listed in Table 2. It is obvious from Table 2 that the NLRi values for all the metals are in the range of 3.1#106 to 5.3#106 g/m2/day except for Cr(III) (3.4 102 g/m2/day). Usually, the NLRi values in the range of 104 to 106 g/m2/day, calculated based on PCT, are considered to be reasonable for nuclear waste management [47,48]. In the case of Cr(III), the significantly higher NLRi value might be due to the lower Cr(III)-HA complex stability at a higher temperature [49]. Overall, the good mechanical properties and chemical durability make the sintered matrix a suitable candidate for nuclear waste immobilization. 5. Conclusions In this study, a simplified route to capture and directly solidify CRUD radionuclides was introduced. We found that the direct solidification of CRUD captured hydroxyapatite is possible without compromising the micro-hardness, compressive strength and chemical stability. The captured CRUD remained stable in the sintered matrices at high temperatures (1150 C). It was observed that the waste volume was reduced up to 60% during the sintering process achieving the relative sintered density of 99.8 ± 0.8%. The solidified matrix provides an excellent compressive strength (207.3 ± 9.5 MPa) which fulfills the criteria for the waste confinement in a solid body. Because no additional chemical treatments, additives and sophisticated equipment involved during this consolidation process, this adsorption and solidification process is simple and highly efficient to immobilize the primary waste as well as to reduce secondary waste. Acknowledgement This work was supported by the Nuclear Safety Research Program through the Korea Foundation of Nuclear Safety (KOFONS) with the granted financial resource from the Nuclear Safety and Security Commission (No. 1305032) and by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT ((No. NRF2015R1A5A1037627) and (No. NRF-2016R1A5A1013919)), Republic of Korea. References [1] J.W. Yeon, Y. Jung, S.I. Pyun, Deposition behaviour of corrosion products on the Zircaloy heat transfer surface, J. Nucl. Mater. 354 (2006) 163e170. [2] G. Hirschberg, P. Baradlai, K. Varga, G. Myburg, J. Schunk, P. Tilky, P. Stoddart, Accumulation of radioactive corrosion products on steel surfaces of VVER type
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Duran, Novel cermaic forming methods with are active organic binder, in: Environmental Issues Waste Manag. Technol. Mater. Nucl. Ind. XII, 2009, pp. 235e242. [23] W.Y. Xia, Y.S. Feng, F. Jin, L.M. Zhang, Y.J. Du, Stabilization and solidification of a heavy metal contaminated site soil using a hydroxyapatite based binder, Construct. Build. Mater. 156 (2017) 199e207. [24] I. Smi ciklas, S. Dimovi c, I. Ple cas, M. Mitri c, Removal of Co2þ from aqueous solutions by hydroxyapatite, Water Res. 40 (2006) 2267e2274. [25] ASTM C1285-02, Standard test methods for determining chemical durability of nuclear, hazardous, and mixed waste glasses and multiphase glass Ceramics : the product consistency test ( PCT ) 1, ASTM, Conshohocken, USA 15 (2002) (2002) 1e26. [26] A. Wagh, Chapter 18 e applications of CBPCs to radioactive and hazardous waste immobilization, in: Chemical Bonded Phosphate Ceramics, second ed., Elsevier, 2016, pp. 283e346, 2016. [27] K. Gray, F. Poc, Project Title: radionuclide Incorporation and Long Term Performance of Apatite Waste Forms 11-3105, U.S. Deptt. Enegy, 2015. [28] S.E. Vinokurov, Y.M. Kulyako, O.M. Slyuntchev, S.I. Rovny, B.F. Myasoedov, Low-temperature immobilization of actinides and other components of highlevel waste in magnesium potassium phosphate matrices, J. Nucl. Mater. 385 (2009) 189e192. [29] G.L. Smith, D.R. Bates, R.W. Goles, L.R. Greenwood, R.C. Lettau, G.F. Piepel,
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Contents lists available at ScienceDirect
Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Environmentally benign and novel management route for radioactive corrosion products by hydroxyapatite Sajid Iqbal, Muhmood ul Hassan, Ho Jin Ryu, Jong-Il Yun* Department of Nuclear and Quantum Engineering, KAIST, Daejeon 34141, Republic of Korea
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
The direct solidification of the captured corrosion products by HA has been investigated. No organic binders were used for solidification of M-HA matrix. The sintered matrix showed good densification and mechanical properties. ASTM PCT test approved the chemical durability of sintered matrix.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 January 2018 Received in revised form 4 May 2018 Accepted 6 May 2018 Available online 7 May 2018
A novel and environmentally benign route for the direct solidification of captured corrosion products (Co, Cr, Fe, Ni, Cu, Zn, and Mn) generated from the primary coolant system of nuclear power plants is introduced in this study. Synthesized calcium hydroxyapatite was used to remove individual and mixtures of corrosion products from aqueous solutions. The results show more than a 95% removal of these corrosion products for both cases. The direct solidification of adsorbed corrosion products was done by pressure-less conventional sintering. The sintered matrix revealed a good hardness (3.70 ± 0.60 GPa) and a relative sintered density > 98% after heat treatment at 1150 C. The measured compressive strength (207.3 ± 9.5 MPa) was significantly higher than the established waste immobilization criteria of the US (3.5 MPa) and Russia (4.9 MPa). The corrosion products consolidated matrix had a normalized leaching rate ranging from 3.4#102 to 3.1#106 g/m2/day. Moreover, no additional chemical treatments, additives (gypsum, slaked lime, sodium silicate, etc.), and sophisticated equipment were needed for the adsorption and solidification process. Therefore, the proposed waste management route has no adverse effects on the ecosystem and can be highly efficient to immobilize adsorbed waste and to reduce the secondary waste volume. © 2018 Elsevier B.V. All rights reserved.
Keywords: Waste immobilization Nano-ceramic Hydroxyapatite Radioactive corrosion products
1. Introduction Corrosion products, known by Chalk River Unidentified Deposits (CRUD), such as 58Co, 60Co, 51Cr, 64Cu, 54Mn, and 59Fe are the main
* Corresponding author. E-mail address: [email protected] (J.-I. Yun). https://doi.org/10.1016/j.jnucmat.2018.05.016 0022-3115/© 2018 Elsevier B.V. All rights reserved.
sources of radiation build-up inside nuclear reactor vessel and pose serious problems in routine overhaul maintenance work. CRUD are released into the primary coolant due to the degradation of mechanical parts of the primary coolant system in nuclear power plants. The CRUD are transported to the reactor core and get activated under the high neutron flux. These are called radioactive CRUD which consequently increase the radiation level. CRUD can
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deposit on the surface of fuel rods and cause a reduction in the thermal conductivity. In certain cases, they may also damage the fuel rods due to stress corrosion cracking phenomena [1e3]. Therefore, it is necessary to use efficient adsorbents to maintain the coolant chemistry and to remove any metal ions that form due to the corrosion of structural materials of the nuclear power plant. Many natural wastes, organic matter, and synthesized adsorbents have been successfully used to capture cobalt/CRUD from contaminated water [4e8]. For example, Granados et al. reported on the adsorption behavior of cobalt as a function of pH, contact time and initial metal ion concentrations for Fe-Mn oxide synthetic material [9]. Oliva et al. used commercial apatite to remove trace levels of Cd, Cu, Ni, Co and Hg, which are hard to remove by calcite or other organic adsorbents [8]. Yuonjin et al. used ammonium molybdophosphateepolyacrylonitrile to remove Co, Sr and Cs which come from radioactive laundry waste water [10]. However, the solidification of Co/CRUD captured adsorbents (secondary waste produced) was not addressed in the aforementioned studies. Radioactive waste management procedures can be simplified and more cost-effective if the material being used as an adsorbent also acts as a matrix for the consolidation of captured nuclear waste. In addition, a significant reduction of secondary waste could be achieved with an adsorbent that can capture metal ions from waste and simultaneously solidify them thus eliminating the need for additional chemical treatments (addition of binders, excessive acid/base treatments or conditioning), loading to other matrices like Portland cement, glass or unsaturated polyesters. Calcium hydroxyapatite (Ca5(PO4)3OH) is a phosphate-based hard ceramic belonging to the apatite family and has the general chemical formula A5(XO4)3Z. The apatite structure has two cationic positions (A and X) and one anionic position (Z) [11e14]. On ionic sites, different ions can be accommodated without affecting the apatite structure due to its ion exchange properties [11]. Calcium hydroxyapatite (HA) has excellent biocompatibility and is being widely used as a bio ceramic material for making artificial dental roots, teeth implants and artificial bones similar to those living bodies [15e17]. The HA is also abundantly present in nature in the form of waste bones and phosphate-based minerals. Both natural and synthetic HA have been extensively investigated for their adsorption and substitutional properties to remove pollutants and toxic heavy metals [8,18,19]. In terms of environmental pollution, it is always desirable to use waste management methods that are the least harmful to the environment and nature. The use of organics, lime and cementitious binders for the stabilization and solidification of contaminants has been considered the best available technology [20]. The additional binders such as biochar, starch and carbodiimide have been used during green shaping of ceramic body to make it easier to obtain a desired density with less defects while pressing. During sintering at higher temperatures, the pyrolysis of these binders results in the emission of carbon dioxide or other hydrocarbon gases [21,22]. Nowadays, geo-polymers, slag, cement kiln and hydroxyapatite have also been tested for the consolidation process [23]. In this study, we applied the synthesized HA to remove single metal ions and a mixture of CRUD metals from an aqueous system. This study further investigated the possibility of binder-free immobilization of a CRUD captured adsorbent in a form of the highly dense and durable sintered matrix. This technique involves the adsorption of CRUD onto the surface of ceramic (calcium hydroxyapatite) nanoparticles. The adsorbent was separated from liquid by centrifugation and ultrafiltration. Then the compaction and consolidation of separated CRUD captured adsorbent was achieved by using conventional pressure-less sintering method.
219
2. Experimental procedures 2.1. Synthesis of the adsorbent The wet precipitation method has been used to synthesize crystalline apatite as described elsewhere [13]. Anionic and cationic solutions were prepared by dissolving each of the di-ammonium hydrogen phosphate (NH4)2HPO4 and calcium nitrate Ca(NO3)2$4H2O in 200 ml of ultrapure deionized water, respectively. The Ca/P molar ratio of both the reactants was theoretical set as 1.67. The anionic solution was dropwise mixed with the cationic solution at a speed of 0.06 ml/s under continuous stirring at 200 rpm. During mixing, the temperature and the pH of the solution were continuously monitored and maintained at 30 C and 10.5, respectively. To maintain the pH of the solution, a concentrated ammonia solution was used. The aging of white gel type precipitates that formed during the synthesis was done inside the parent solution for 3 h. During the first hour, the stirring continued at 100 rpm and the temperature was maintained at 30 C, whereas at the end of the first hour, the reaction vessel was removed from the hot plate and kept without stirring at room temperature for 2 h. After that the suspension was filtered, thoroughly washed and dried overnight at a temperature of 100 C in a vacuum oven. The dried precipitates were ground to fine powder with a mortar and pestle. 2.2. Adsorption procedures A detailed adsorption study of HA can be found elsewhere [24]. First, the maximum removal of Co(II) was optimized by varying the solid to liquid ratio (S: L ratio) from 2 to 5 g/L with 1 mM of Co(II) at pH 6. The adsorbent-liquid contact time was set as 24 h with constant stirring at 150 rpm at 25 C. More than 90% removal of the Co(II) was observed for S:L ratio of 4 g/L. By using the optimized S:L ratio, a single element Co(II) solution and a series of corrosion products (Co(II), Cr(III), Cu(II), Ni(II), Mn(II), Fe(II) and Zn(II)) were removed from the contaminated aqueous solution. In the mixture of corrosion products, the concentration of each metal ion was kept constant at 1 mM in 100 ml of deionized water. After 24 h, the solid and liquid phases were separated with a 0.45 mm syringe filter followed by a 5 min centrifugation at 3200 rpm. The concentration of the metal ions in the separated dried adsorbent and the supernatant were measured by ICP-OES. The percentage removal (% R) of the metal ions was calculated by equation (1),
%R ¼ ðCo Ce Þ=Co 100
(1)
where Co and Ce are the initial and final metal ion concentrations (mM), respectively. 2.3. Sintering of pure HA and corrosion product adsorbed HA After the separation and drying of the adsorbent in a vacuum oven at 100 C overnight, the powder was shaped into cylindrical pellets with a stainless steel mold 13 mm in diameter. The pressing at room temperature was conducted under 100 MPa for 30 s by a Carver(R) uniaxial press. For each pellet, 1.20 g of pure HA and the metal adsorbed HA (Co-HA and M-HA) powder was used. The pressure-less conventional sintering of the as-pressed pellets of pure HA Co-HA and M-HA was carried out in the box furnace (Model: S-1700, HANTECH) under an air environment. In the first step, the pure HA was studied for a range of temperatures (500 C, 800 C, 900 C, 1000 C, 1150 C and 1300 C) for 120 min at a heating ramp of 4 C/min to optimize the sintering temperature. After the sintering, the samples were left inside the furnace to cool
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down to room temperature. In the second step, Co-HA and M-HA were sintered at 500 C and at the optimized sintering temperature (1150 C). The dimensions of each pellet were measured before and after the sintering. The percentage volume contraction was calculated with equation (2):
Vc ð%Þ ¼
ðVb Va Þ 100 Vb
(2)
where Vc (%) is the percentage volume contraction, Vb and Va are the volumes of the pellet before and after sintering. All experiments were performed in triplicate to check the reproducibility of the results. 2.4. Product consistency test (PCT) The Product Consistency Test (PCT) is a standard test of the American Society of Testing Materials (ASTM) to check the chemical stability and structural durability of ceramic waste forms [25e30]. The PCT is performed by immersing the crushed sintered samples (specific surface area ¼ 0.27 m2/g) into an ASTM-type I water at 90 ± 2 C for 7 days. All the procedures were performed in triplicate under static condition in an air-sealed stainless steel autoclave. After 7 days the leachate was analyzed for elemental degradation and pH change of the waste matrix. The elemental composition of the waste matrix was measured before the PCT, and the concentrations of the relevant constituents were measured in the leachate after 7 days PCT test with ICP-OES. The normalized leaching rate of the ith element was calculated by equation (3) as follows:
NLRi ¼
mi s,t,wi
(3)
where NLRi is the normalized leaching rate, mi is the concentration of component i leached into the water after a reaction time t, S is the surface area of the crushed powder, and wi is the weight fraction of an ith component in the solid matrix. 3. Characterization of the sintered matrices High-resolution X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to study the crystal structure, size and morphology of the grains in the sintered matrix. The XRD patterns were measured with XRD (SmartLab, RIGAKU) in a scan range of 10 to 90 with a step size of 0.02 at a tube voltage of 45 kV and a current of 200 mA. The Cu Ka incident radiation source with a wavelength (l ¼ 1.54056 Å) in the reflection geometry was used. For the microscopic analysis of the sintered matrix, SEM (Hitachi S4800) was used. The elemental mapping with the distribution of different waste constituents was recorded by energy dispersive spectrometer (EDS) coupled with SEM. The fractured surface morphology of the sintered samples was observed by mounting the samples on a cross-section holder. The sputter coating was performed for 15 s with an HPC-1SW osmium coater before microscopic analysis. The sintered bulk densities were measured with the Archimedes' method [31]. The sintered samples were boiled at 100 C in ultrapure water (MQ) for 30 min to remove the trapped air and to avoid chances of error. The densities and the relative percentage densities were calculated by using the following relationships [31,32]:
rb ¼ MD rL =ðWs WSM Þ
(4)
%RD ¼ rb =rR 100
(5)
where rb is the bulk density of the sintered sample, and mD, rL, Ws, and WSM are the dry weight, the density of deionized water, the saturated weight, and the suspended weight of pellet inside the deionized water, respectively. %RD is the relative standard density, and rR is the measured bulk density of the starting powder which is 2.94 g/cm3. The bulk density of the starting HA powder was measured with a helium pycnometer. The rb of the CRUD captured HA was assumed to be equal to the rb of the pure HA because the mass fraction of adsorbed CRUD is very small in comparison to mass fraction of pure HA. The ASTM E384 testing procedure was followed to measure the hardness values. The Vickers microindentor (Model: 402 MVD, Wolpert Wilson Instruments) installed with the software Expert Hardness 2007 V2.0 was used to quantify the micro-hardness of the sintered pellets under a constant load of 200 gf for a dwell time of 10 s [33]. All of the samples were fine polished by standard SiC papers with different grits (800e2000) and then by the diamond suspension paste before the hardness measurements [34]. The indents were made in a line crossing the center of the specimen so that the near surface effect could be avoided. The indentation was done at room temperature and a representative hardness value of 10 measurements was taken for each sample. The compressive strength of the sintered samples was measured with the universal testing machine (UTS INSTRON 5583, U.S.A) with a cross-head speed of 0.4 mm/min and a maximum load capacity of 150 kN at room temperature. Three cylindrical specimens with a length to diameter ratio (L/D) of 1.24 were prepared for each set of experiments, and the test was carried out according to the ASTM C39 standard compressive strength test [35].
4. Results and discussions This study was divided into three main steps: the synthesis of HA, CRUD adsorption by HA, and consolidation of the as-spent adsorbent (CRUD captured HA). Detailed adsorption studies on HA investigating the function of pH and contact time as well as the effect of the initial metal ion concentrations have already been done elsewhere [24,36,37]. We investigated the effect of the adsorbent amount on the maximum removal of a single metal ion and a mixture of metal ions from aqueous system. The quantitative measurements of the metal ions were done by inductively coupled plasma-optical emission spectrometry (ICP-OES). It is clear from Fig. 2 that the behavior of the cobalt removal is almost linear with the increasing adsorbent dose up to 4 g/L. This may be due to the excess surface binding sites available to the metal ions due to increase of adsorbent dose. After 4 g/L, the breakthrough value has been achieved with an insignificant effect on the removal of metal ion at S: L ratio of 5 g/L. Then an equimolar (1 mM each) mixture of corrosion products (Co(II), Cr(III), Cu(II), Fe(II), Mn(II), Ni(II) and Zn(II)) has been prepared in 100 ml of MQ water and optimum removal of metal ions (>95%) was obtained by increasing S:L ratio to 40 g/L. It is clear from Fig. 3 that more than 95% of corrosion products can be removed from aqueous media by providing 40 g/L of HA powder at 7 mM of total metal ion concentrations in a mixture. During the adsorption process a decrease in pH was observed from 6 to 4.9, which is due to protons coming from two types of protonated sites at apatite surface (≡POH and ≡CaOHþ 2 ) by the exchange of M(II)/M(III) cations as shown in equations (6) and (7) [38].
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221
Fig. 1. Flow diagram showing the synthesis of the adsorbent, and then the adsorption and consolidation of the captured corrosion products by hydroxyapatite.
Fig. 2. Effect of the solid to liquid ratio on the removal of Co(II) by hydroxyapatite (Co ¼ 1 mM, pH ¼ 6, contact time ¼ 24 h, ionic strength ¼ 0.1 M NaCl, shaking speed ¼ 150 rpm at RT).
≡POH þ M 2þ ⇔ ≡POM þ þ Hþ
(6)
≡CaOH2þ þ M 2þ ⇔ ≡CaOHM2þ þ Hþ
(7)
The main focus of this study was to reduce the secondary waste volume and to provide a facile route for waste management by direct consolidation of an as-spent adsorbent after separation and drying of the adsorbent. Therefore, pure HA, cobalt adsorbed HA (Co-HA) and corrosion product adsorbed HA (M-HA) powders dried in a vacuum oven at 100 C for 12 h were shaped into cylindrical pellets with a stainless steel mold, as shown in Fig. 1. The pellets
Fig. 3. Effect of the S: L ratio on the removal of CRUD mixture by hydroxyapatite (Co of each metal ion ¼ 1 mM, pH ¼ 6, contact time ¼ 24 h, shaking speed ¼ 150 rpm at RT).
were sintered at 500, 800, 900, 1000, 1150 and 1300 C in a box furnace in an air environment, as already discussed in Section 2.3. The pellets were crushed into powder and the XRD analysis was carried out to investigate the effect of sintering temperatures on the crystallinity and decomposition of the HA and M-HA. The phases were identified with the standard JCPDS card No. 00-064-0738 available in the system software [39,40]. No peak shifting phenomenon was observed in all samples except the appearance of bTCP-Ca3(PO4)2 on XRD patterns for HA-1150 C or M-HA-1150 C as shown in Fig. 4 which corresponds to the decomposition of HPO2 4 group present in a wet precipitated HA at this processing temperature [11,13]. There is an insignificant increase in crystallinity of samples heat
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Relative sintered density and the micro-hardness are the key parameters to confirm the sintering phenomena [42]. Fig. 5 (a & b) shows the effect of the sintering temperature on the relative sintered density and the micro-hardness, respectively. It is worth noting that the relative sintered density (Fig. 5(a)) increased from 60% to 99.8% with the increase of sintering temperature from 500 to 1300 C. The maximum hardness of 4.17 ± 0.27 GPa was measured for the sintered pure HA sample at 1300 C. Based on the densification and the volume contraction results, the optimized sintering temperature has been taken as 1150 C. The micro-hardness values for pure HA, Co-HA and M-HA at the optimized sintering temperature (1150 C) were 3.70 ± 0.69, 3.63 ± 0.61 and 3.41 ± 0.46 GPa, respectively. The volumetric contraction of pure HA sintered in the temperature range starting from 500 to 1300 C was also measured. About 60% of volume was decreased at 1150 C, as compared to 4e5% at 500 C. The full densification at 1150 C is due to the removal of the porosity and the grain boundary diffusion to form larger grains which is the temperature-driven phenomenon. The high temperature offers sufficient energy for the thermal activation of the grain boundary migration and diffusion [43]. It can be seen from Fig. 5 (a & b) that the adsorbed metal ions did not play a
Fig. 4. High-resolution XRD patterns showing the increasing crystallinity of the sintered samples as the temperature was increased (A shows the most intense peak of bTCP-Ca3(PO4)2).
treated at 500 C (HA and M-HA) compared with the non-heat treated CRUD adsorbed HA (M-HA-As adsorbed) while the samples sintered at 1150 C showed the highest crystallinity which is evident from the sharpness of XRD peaks (Fig. 4). The sample sintered at 1300 C also showed good stability and only the formation of small fraction of b-TCP-Ca3(PO4)2 can be seen in the XRD patterns (Fig. 4). Generally, 1200e1450 C is the temperature range reported for the decomposition of HA depending on the characteristics of the HA [41].
Fig. 6. Compressive strength of the pure HA, Co-HA and M-HA: M is a mixture of corrosion products containing Co(II), Cu(II), Ni(II), Fe(II), Zn(II), Mn(II), and Cr(III).
Fig. 5. Effect of the sintering temperature on (a) the relative sintered density and (b) the micro-hardness of the pure HA.
S. Iqbal et al. / Journal of Nuclear Materials 507 (2018) 218e225
significant role in lowering the sintering temperature of the immobilized matrix. The compressive strength is an important physical parameter that ensures the integrity of the waste form during its interim storage, transportation and final disposal under a geological repository environment. The waste form should have an acceptable value for the compressive strength which ensures its stable performance during storage and transportation. Fig. 6 shows the behavior of the compressive strength as the strain was increased. It is clear from Fig. 6 that the samples sintered at 500 C do not bear a significant load and have a very low compressive strength. On the other hand, the samples sintered at 1150 C showed an excellent
Table 1 Comparison of our data with standard compressive strength data [44,45]. Solidified matrix
Minimum Requirement (MPa)
Portland Cement U.S standard Portland Cement Russian standard Unsaturated Polyester Polyethylene Vitrified Glass Calcium Hydroxyapatite (this study)
3.45 MPa 4.93 MPa 5.17 MPa 6.89 MPa 34.50 MPa 207.34 ± 9.46 MPa
223
compressive strength for the pure HA as well as the Co-HA and MHA. The high compressive strength was achieved because of the high densification and low porosity of the adsorbent. The compressive strength of 207.3 ± 9.5 MPa achieved in this study is several times higher than the compressive strength of the Portland cement (2.9e9.8 MPa) and other consolidation matrices being used for the immobilization of radioactive waste, as reported in Table 1. The reported compressive strength for HA is considerably higher than the US established regulatory requirements for Portland cement (3.45 MPa) [44]. SEM images in Fig. 7 show the effect of the sintering temperature on the grain size of the pure HA, Co-HA, and M-HA at 500 C and 1150 C. It is obvious from the SEM images that the particles are loosely packed and less diffused in both the HA and Co-HA at 500 C. On the other hand, more compact and densely packed structures can be seen in the images for the HA, Co-HA, and M-HA at 1150 C which show a significant grain growth and enlarged particle size. The increase in micro-hardness (Fig. 5(b)) and compressive strength (Fig. 6) are in agreement with studies by Muralitharan et al. and Karimzadeh et al. [34,46]. They investigated the effect of sintering temperature on the strength of HA and showed that the micro-hardness and compressive strength increases with increase in temperature unless dehydroxylation starts
Fig. 7. SEM micrographs of the fractured surfaces of the pure HA, the enlarged view in the inset of each at top left corner, the mapping and distribution of metal ions in the sintered pellets with different colored dots were observed by EDS analysis.
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S. Iqbal et al. / Journal of Nuclear Materials 507 (2018) 218e225
Table 2 The normalized leaching rate (NLRi) of each element from the HA matrix. Metal
Co(II)
Cr(III)
Cu(II)
Fe(II)
Mn(II)
Ni(II)
Zn(II)
NLRi (g/m2/day)
4.4 106
3.4 102
3.1 106
5.3 106
3.8 106
4.4 106
3.3 106
and HA structure collapses. Different microstructures in the sintered matrix can be obtained at a given temperature depending on the characteristics of the starting powder which lead to the different mechanical properties. However, the microstructure obtained in our study had shown no adverse effects on mechanical properties of the sintered matrix. The presence of cobalt/CRUD in the sintered matrices was observed from the EDS and in the area mapping results of the sintered samples. The EDS results showed the homogenized distribution of the CRUD elements in the sintered matrix. PCT was performed as described in Section 2.4 to check the chemical stability of the consolidated matrix and the leaching rate of the immobilized corrosion products. The calculated NLRi values for each metal are listed in Table 2. It is obvious from Table 2 that the NLRi values for all the metals are in the range of 3.1#106 to 5.3#106 g/m2/day except for Cr(III) (3.4 102 g/m2/day). Usually, the NLRi values in the range of 104 to 106 g/m2/day, calculated based on PCT, are considered to be reasonable for nuclear waste management [47,48]. In the case of Cr(III), the significantly higher NLRi value might be due to the lower Cr(III)-HA complex stability at a higher temperature [49]. Overall, the good mechanical properties and chemical durability make the sintered matrix a suitable candidate for nuclear waste immobilization. 5. Conclusions In this study, a simplified route to capture and directly solidify CRUD radionuclides was introduced. We found that the direct solidification of CRUD captured hydroxyapatite is possible without compromising the micro-hardness, compressive strength and chemical stability. The captured CRUD remained stable in the sintered matrices at high temperatures (1150 C). It was observed that the waste volume was reduced up to 60% during the sintering process achieving the relative sintered density of 99.8 ± 0.8%. The solidified matrix provides an excellent compressive strength (207.3 ± 9.5 MPa) which fulfills the criteria for the waste confinement in a solid body. Because no additional chemical treatments, additives and sophisticated equipment involved during this consolidation process, this adsorption and solidification process is simple and highly efficient to immobilize the primary waste as well as to reduce secondary waste. Acknowledgement This work was supported by the Nuclear Safety Research Program through the Korea Foundation of Nuclear Safety (KOFONS) with the granted financial resource from the Nuclear Safety and Security Commission (No. 1305032) and by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT ((No. NRF2015R1A5A1037627) and (No. NRF-2016R1A5A1013919)), Republic of Korea. References [1] J.W. Yeon, Y. Jung, S.I. Pyun, Deposition behaviour of corrosion products on the Zircaloy heat transfer surface, J. Nucl. Mater. 354 (2006) 163e170. [2] G. Hirschberg, P. Baradlai, K. Varga, G. Myburg, J. Schunk, P. Tilky, P. Stoddart, Accumulation of radioactive corrosion products on steel surfaces of VVER type
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