2019 ⦽ǎႊᔍᖒ⠱ʑྜྷ⦺⫭ ⇹ĥ⦺ᚁݡ⫭ םྙ᧞Ḳ
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Characterization of Ceramicrete Waste Form for Disposal of Concrete Waste of
Decommissioned Nuclear Power Plant
Jae-Young Pyo* and Jong HeoPohang University of Science and Technology, 77, Cheongam-ro, Nam-gu, Pohang-si, Gyeongsangbuk-do, Republic of Korea
*
vywodud@postech.ac.kr
1. Introduction
According to the IAEA report, 900 tons of radioactive concrete wastes are expected to be generated in the decommissioning of one PWR unit [1]. In previous researches, most of the radionuclides are present in porous cement paste and thus the volume of radioactive concrete wastes can be reduced by separating aggregates through heating and grinding process [2]. During the heating and grinding process, 80% of the total waste volume is reduced and powder wastes composed of cement paste and sand of less than 1 mm size are generated. In this study, these powder wastes are solidified with ceramicrete. The principle of ceramicrete formation is that calcined MgO and KH2PO4 are dissolved in
the aqueous solution to synthesize MgKPO4Â6H2O.
The objectives of this study are to solidify radioactive concrete wastes with ceramicrete and evaluate the waste acceptance criteria.
2. Experimental Procedures
The simulated concrete was formulated according to the mix proportion used in nuclear power plants constructed by the Korea Hydro & Nuclear Power Co., Ltd [3]. The simulated concrete cured for 28 days were heated at 550oC for 1 hour, pulverized into powder and sieved to 1 mm. The formulation of ceramicrete is shown in Table 1. Cesium and cobalt
nitrates were added to evaluate the leaching characteristics of the ceramicrete. The samples of ceramicrete with a diameter of 15 mm and a length of 30 mm were made to evaluate the waste acceptance criteria of the disposal. Compressive strengths were measured using a universal testing machine. The thermal cycling and immersion tests were carried out according to the ASTM B553 and ASN 16.1 method, respectively. Specific details of these evaluation methods are described in the reference literature [4]. Concentrations of Co, Cs, Mg, K and P ions in the leachates were analyzed by inductively coupled plasma atomic emission spectroscopy (ICP-AES). After the thermal cycling and immersion tests, the compressive strengths of ceramicrete specimens were measured.
Table 1. Composition of ceramicrete prepared (wt.%) Waste MgO KH2PO4 H2O H3BO3 CsNO3 Co(NO3)2
50 10.32 23.24 15.38 0.50 0.29 0.27
3. Results and Discussion
The compressive strengths of ceramicrete specimens cured for 28 days were 56.27 MPa, but decreased to 47.90 and 25.15 MPa after immersion and thermal cycling tests, respectively. Despite the decreases in compressive strengths after immersion and thermal cycling tests, ceramicrete specimens were satisfied with the waste acceptance criteria of >
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2019 ⦽ǎႊᔍᖒ⠱ʑྜྷ⦺⫭⇹ĥ⦺ᚁݡ⫭םྙ᧞Ḳ 3.45 MPa. Fig. 1 shows the results of the ceramicrete components leached for 90 days. During immersion, MgKPO4Â6H2O phase was incongruently dissolvedand K ion was relatively leached. Fig. 2 shows the cumulative fraction leached and leaching rates of Cs ions evaluated to ANS 16.1 test. The leachability index of Cs was 11.06 and satisfied with the waste acceptance criterion of > 6. On the other hand, the leaching of Co ions hardly occurred below the detection limit of ICP-AES. Therefore, the ceramicrete that solidify radioactive concrete wastes have satisfactorily met the waste acceptance criteria.
Fig. 1. Cumulative fraction of elements constituting the ceramicrete.
Fig. 2. Cumulative fraction leached and leaching rate of Cs in the ceramicrete.
4. Conclusion
Ceramicretes for the solidification of waste generated in the decontamination process of radioactive concrete were evaluated according to waste acceptance criteria. The Compressive strengths of the ceramicrete before testing were excellent at 56.27 MPa, and were 25.15 and 47.90 MPa after the thermal cycle and immersion test, respectively, which satisfied the regulation of > 3.45 MPa. The leachability index of Cs was 11.06 and satisfied the regulation of > 6.
ACKNOWLEDGMENT
This work was supported by the National Research Foundation (NRF) grant funded by the Ministry of Science & ICT (NRF-2017M2B2B1072405).
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