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Nghiên cứu về cơ chế lão hóa do bức xạ ở bê tông kết cấu trong các cơ sở hạt nhân bằng cách sử dụng bức xạ ion trên các vật liệu mô hình

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Bài viết Nghiên cứu về cơ chế lão hóa do bức xạ ở bê tông kết cấu trong các cơ sở hạt nhân bằng cách sử dụng bức xạ ion trên các vật liệu mô hình nghiên cứu về cơ chế suy thoái do bức xạ trong kết cấu bê tông, đặc biệt là hiện tượng giãn nở thể tích bởi bức xạ (RIVE) do sự chuyển pha từ tinh thể sang vô định hình ở các tinh thể trong cốt liệu đá.

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Nội dung Text: Nghiên cứu về cơ chế lão hóa do bức xạ ở bê tông kết cấu trong các cơ sở hạt nhân bằng cách sử dụng bức xạ ion trên các vật liệu mô hình

Tiểu ban D2: Ứng dụng kỹ thuật hạt nhân trong công nghiệp và các lĩnh vực khác Section D2: Application of nuclear techniques in industries and others NGHIÊN CỨU VỀ CƠ CHẾ LÃO HÓA DO BỨC XẠ Ở BÊ TÔNG KẾT CẤU TRONG CÁC CƠ SỞ HẠT NHÂN BẰNG CÁCH SỬ DỤNG BỨC XẠ ION TRÊN CÁC VẬT LIỆU MÔ HÌNH A STUDY ON THE IRRADIATION DEGRADATION MECHANISM IN CONCRETE STRUCTURES USED IN NUCLEAR FACILITIES USING ION IRRADIATION ON MODEL MATERIALS* NHUT LUUVU (1, 2A*), KENTA MURAKAMI (2), HAMZA SAMOUH (3) , IPPEI MARUYAMA (3), KIYOTERU SUZUKI(4) (1) Center for Non-destructive Evaluation, Vietnam Atomic Energy Institute, Hanoi, Vietnam (2) Nagaoka University of Technology, Nagaoka, Niigata, Japan (3) Nagoya University, Nagoya, Aichi, Japan (4) Mitsubishi Research Institute, Chiyoda, Tokyo, Japan (2a) Former student at Nagaoka University of Technology, Nagaoka, Niigata, Japan * nhut.czech@gmail.com Tóm tắt: Báo cáo này nghiên cứu về cơ chế suy thoái do bức xạ trong kết cấu bê tông, đặc biệt là hiện tượng giãn nở thể tích bởi bức xạ (RIVE) do sự chuyển pha từ tinh thể sang vô định hình ở các tinh thể trong cốt liệu đá. Để làm rõ cơ chế gây ra RIVE, chiếu xạ ion bằng Si được tiến hành trên các mẫu tinh thể thạch anh, albite và microcline ở các thông lượng khác nhau đến 2 x 1016 ions/cm2. Những thay đổi tính chất cơ học và cấu trúc được phân tích bằng kính hiển vi laser, kính hiển vi điện tử truyền qua và độ cứng nano. Kết quả cho thấy các đặc tính cơ học của tinh thể bị chiếu xạ giảm đáng kể với mật độ ion, tuy nhiên sự thay đổi thể tích (RIVE) tăng dần theo mật độ ion. Điều thú vị là quá trình RIVE vẫn tiếp diễn sau khi tinh thể đã vô định hình hoàn toàn. RIVE thay đổi tỷ lệ với ‘số lượng nguyên tử bị đánh bật DPA’ (thông số này thường được sử dụng để định lượng hư hỏng do bức xạ trong vật liệu thùng lò) và RIVE xảy ra khi DPA lớn hơn 0.04 ở tinh thể thạch anh. Từ khóa: lão hóa bê tông; RIVE; tinh thể silicate; chiếu xạ ion; hư hỏng do bức xạ; sự vô định hình Abstract: This study aimed at evaluating and elucidating the irradiation degradation mechanisms in concrete structures used in nuclear facilities, particularly radiation-induced volume expansion (RIVE) due to crystal-to-amorphous transition (amorphization) in rock-forming aggregate minerals. To elucidate RIVE mechanism, the Si ion irradiation was conducted on quartz, albite and microcline mineral at different ion fluences up to 2 x 10 16 ions/cm2. The changes in properties were characterized by Laser Microscopy, Transmission Electron Microscopy and Nano-indentation. The results showed that mechanical properties of irradiated minerals decreased significantly along ion fluence; in contrast, RIVE increased with ion fluence. Importantly, the RIVE in irradiated quartz continued even after the amorphization was completed. Furthermore, RIVE was found to be proportional to the number of knock-on atoms (DPA) and RIVE started to occur above 0.04DPA in irradiated quartz, this implies that the RIVE of aggregate can be indicated by DPA. Keywords: concrete degradation; RIVE; silicate minerals; ion irradiation; radiation damage; amorphization I. INTRODUCTION Concrete biological shield is designed for radiation shielding and reactor support in Light Water Reactors (LWRs). In general, the performance of concrete in LWRs is very good because of well controlling of material selection and safety measures to prevent exposure to chemical attack; however, extending lifetime leads to high accumulated irradiation dose which would change concrete properties i.e., reduction in compressive strength and elastic modulus due to the radiation induced volume expansion (RIVE) in concrete aggregates and/or radiation heating in cement paste, which has received large attention recently. Concrete is made by mixing aggregates, cement powder, and water with an appropriate water-to- cement ratio. Because aggregates make up around 70-80% of total concrete volume, the properties of aggregates could strongly affect the mechanical properties of concrete. Feldspars are most abundant among silicate minerals existing in aggregate, such as albite-NaAlSi3O8 , microcline (KAlSi3O8). In silicate minerals, the basic structure [SiO4]4- (Si tetrahedron) is the building block, thus mineral properties are commonly associated with the topology of the network that is built by basic structures. Therefore, quartz (SiO2), albite and microcline can be used as surrogate materials to evaluate the irradiation effect on concrete aggregate. 394
Tuyển tập báo cáo Hội nghị Khoa học và Công nghệ hạt nhân toàn quốc lần thứ 14 Proceedings of Vietnam conference on nuclear science and technology VINANST-14 Irradiation can cause the disordering of lattice structure and leading amorphization with volume expansion due to the reduction in density [1]. Because aggregate is well-crystalline, it would transform to amorphous phase under irradiation, leading to high internal stress that can cause cracking in irradiated concrete. For this reason, RIVE is considered the main causes of reduction in mechanical properties caused by neutron. Thus, it is important to find the critical irradiation level for neutron irradiation in concrete, beyond which concrete starts to degrade the properties, such as compressive strength and Young’s modulus. The expected conditions at 80 years of operation in the US reactors have been evaluated [2]. The maximum irradiation of the inner CBS wall is expected to be 6.1x10 19 (n/cm2) for fast neutron. For Japanese PWR reactors, it would reach to 4.7 x 1019 (n/cm2) after 60 years operation [3]. These expected neutron fluences are much higher than a reference value of 1.0 x 10 19 (n/cm2) for concrete degradation, which was suggested by some researchers [4,5]. However, it is difficult to utilize the data on neutron irradiated concrete from test reactor due to several reasons, for instance, the heterogeneity of concrete and the diversity of neutron spectrum in each reactor that may cause a high uncertainty of reference value. Compared with neutron, irradiation using high energy ions has several advantages such as low cost, high dose rate, negligible sample activation, as well as easy to control irradiation conditions, thus it is utilized in this study for extracting the RIVE mechanism in representative silicate minerals: quartz, albite and microcline. II. METHODOLOGY Three natural minerals, namely quartz, albite, and microcline, with size approximately 5 mm × 10 mm × 0.5 mm were prepared. The minerals were polished, then irradiated with 3 MeV Si 2+ ion at the High Fluence Irradiation Facility (HIT) at the University of Tokyo. The detail of the irradiation condition is shown in Table 1. In the 3 MeV Si2+ ion irradiation condition, the ion range and displacement damage were estimated using SRIM-2013 [6], as shown in Fig.1, and the range of 3 MeV Si ions in quartz, albite, and microcline was estimated to be 2.24, 2.25, and 2.38 micrometers, respectively. Table 1. Irradiation conditions in Tandem Accelerators Nuclear energy Energy Range R Flux DPA per Ave. damage rate Temp. Ions loss dE/dx (keV) (nm) (ions/cm2.s) ion (dpa/s) (oC) (keV/nm) Si 3000 1.3 2236 1.5 x 1012 4137 3.5 x 10-4 44 Figure 1. Damage profile in felspar minerals indued by 3MeV Si2+ , simulated by SRIM For evaluating the RIVE, about half of the polished surfaces were covered by an aluminum foil during irradiation to create an irradiated and unirradiated region. After irradiation, the step changes at the 395
Tiểu ban D2: Ứng dụng kỹ thuật hạt nhân trong công nghiệp và các lĩnh vực khác Section D2: Application of nuclear techniques in industries and others boundaries were measured by using laser microscopy (Fig.2). The change in mechanical properties of irradiated region was measured by nanoindentation machine (Shimadzu, DUH-211) using a Berkovich tip, which can control the minimum load and displacement increment to 0.196 µN and 0.1 nm, respectively. In addition, the irradiated cross section was picked up and polished down to approximately 100 nanometers in thickness using a micro-scale fabricator, namely focused ion beam, following microstructure observation by 200 kV transmission electron microscope (TEM) in JEOL JEM-2000FX. Figure 2. The representative of step height measurements using Laser Microscopy III. RESULT & DISCUSSION Changes in hardness and Young’s modulus Fig.3 shows a typical measurement of indentation test at unirradiated and irradiated regions. It clearly shows that the change in this region is uniform and both hardness and Young’s modulus are significantly decreased due to ion irradiation. Figure 3. Representative nanoindentation test at the boundary between irradiated and unirradiated quartz Fig.4 presents the changes of hardness of minerals with ion fluence at 20mN load, corresponds indentation depth between 15% to 20% of irradiated layer thickness for all specimens. For the hardness, it decreased with ion fluence and almost remained unchanged after 2 x 1014 cm2. It is interesting that the relative change at saturation is about the same for all minerals by reduction about 40%. The hardness value of pristine state is highest for quartz 14.80 GPa and about 10GPa for albite and microcline. The reduction 396
Tuyển tập báo cáo Hội nghị Khoa học và Công nghệ hạt nhân toàn quốc lần thứ 14 Proceedings of Vietnam conference on nuclear science and technology VINANST-14 of hardness is caused by the softening of amorphized phase. For Young’s modulus, the value of modulus gradually decreased with increasing ion fluences in case of quartz, but it was almost unchanged after ion fluence 2 x 1014 cm2 for albite and microcline. This suggests the different mechanism of RIVE in quartz and that in feldspar minerals (albite and microcline) because the change in modulus is proportional to the change in density (or volume). The obtained Young’s modulus of unirradiated quartz at 20mN was 112.40±0.74GPa, this is quite close to reported value for c-axis Young’s modulus 105GPa, though it is 7% different. Slightly higher modulus value of quartz was also reported by Oliver and Pharr [7], who suggested the reason is due to the influence of anisotropy of the measurement of modulus when using indentation technique. Figure 4. (a) the indentation hardness and (b) elastic modulus of ion-irradiated quartz, albite and microcline at different ion fluences Changes in steps at the boundaries of unirradiated and irradiated region Fig.5 shows the change of step height with ion fluence for three minerals. The step heights exhibited similar trends in all minerals, which increased as increasing ion fluences and approached saturation at 6 x 1015 cm2. The expansion level was highest for quartz (~405nm), followed by albite (~206nm) and lowest for microcline (~ 138nm). The shape of irradiated and unirradiated parts is flat and appears clear steps, this implies that the expansion almost occurs in one dimension. Given the step heights at saturation, total volume change can be estimated by step heights normalized by ion ranges as 18.1%, 9.1% and 5.8% for respective quartz, albite and microcline. Figure 5. The step height of minerals following silicon irradiation Observing the cross-section of some irradiated samples using TEM have found that in quartz, the irradiated layer is almost transformed to amorphous state at the ion dose of 2x1014 cm2, but the swelling continued at higher dose. In albite and microcline, the amorphization completed at 1x1015cm2 and there is almost negligible swelling for additional irradiation. The discrepancy may be related to the alkali ion (K 397
Tiểu ban D2: Ứng dụng kỹ thuật hạt nhân trong công nghiệp và các lĩnh vực khác Section D2: Application of nuclear techniques in industries and others and Na) in feldspar structure, which may affect the swelling behaviour of amorphous structure. Since alkali ion is mobile under irradiation, it can come to compensate the charge caused by lattice damage to prevent the structure relaxation; however, in case of quartz, the lattice damage due to additional irradiation can induce the structure reorganization, thus swelling continues. Figure 6. RIVE as a function of knock-on displacements Because quartz is the most common mineral in aggregate and has showed largest expansion, the present results suggests that the degradation caused by RIVE can be monitored by the change of this mineral during long term operation. Based on the damage profile calculated by SRIM, the number of knock-on atoms (DPA) caused by a single ion can be estimated, so that the ion fluence can be converted into DPA. Fig. 6 shows the change of step as a function of DPA in case of quartz. Available data from literature are also plotted together [8,9]. Because different ion used in different researchers, the step changes were normalized by divided with the ion range. It can be seen that the data is consistent and it showed that the swelling starts at around 0.04DPA, corresponding to about 3 percent of expansion in one- dimension. Note that, this value is determined based on the dimension change, the amorphization dose might differ, which depends on the number of defects or fraction of small amorphous regions formed by irradiation. IV. CONCLUSION The effect of irradiation on dimension change and mechanical properties in quartz, albite and microcline was studied by using 3MeV Silicon irradiation. It was found that mechanical properties of irradiated minerals decreased significantly along ion fluence; meanwhile, RIVE increased with ion fluence. Importantly, the RIVE in irradiated quartz continued even after the amorphization was completed, in contrast to that in albite and microcline, negligible RIVE after amorphization. This suggest a different mechanism of RIVE in quartz and feldspars, which could be related to alkali ions. Furthermore, RIVE was found to be proportional to the number of knock-on atoms (DPA) and RIVE started to occur above 0.04DPA in irradiated quartz, this implies that the RIVE of aggregate can be indicated by DPA. ACKNOWLEDGEMENT A part of this study is conducted by the JCAMP project funded by METI, Japan. REFERENCE [1] R.K. Eby, R.C. Ewing, R.C. Birtcher, The amorphization of complex silicates by ion-beam irradiation, J. Mater. Res. 7 (1992) 3080–3102. [2] U.S. DOE: Office of Nuclear Energy, Light Water Reactor Sustainability Program: expected condition of concrete exposed to radiation at age 80 years of reactor operation, 2015. http://www.energy.gov/sites/prod/files/FY- 15_LWRS_IPP_Final_0.pdf. [3] T.M. Rosseel, I. Maruyama, Y. Le Pape, O. Kontani, A.B. Giorla, I. Remec, J.J. Wall, M. Sircar, C. Andrade, M. 398
Tuyển tập báo cáo Hội nghị Khoa học và Công nghệ hạt nhân toàn quốc lần thứ 14 Proceedings of Vietnam conference on nuclear science and technology VINANST-14 Ordonez, Review of the Current State of Knowledge on the Effects of Radiation on Concrete, J. Adv. Concr. Technol. 14 (2016) 368–383. [4] K.G. Field, I. Remec, Y. Le Pape, Radiation effects in concrete for nuclear power plants -Part I: Quantification of radiation exposure and radiation effects, Nucl. Eng. Des. 282 (2015) 126–143. [5] I. Maruyama, O. Kontani, M. Takizawa, S. Sawada, S. Ishikawao, J. Yasukouchi, O. Sato, J. Etoh, T. Igari, Development of Soundness Assessment Procedure for Concrete Members Affected by Neutron and Gamma-Ray Irradiation, J. Adv. Concr. Technol. 15 (2017) 440–523. [6] J.F. Ziegler, M.D. Ziegler, J.P. Biersack, SRIM - The stopping and range of ions in matter (2010), Nucl. Instruments Methods Phys. Res. B. 268 (2010) 1818–1823. [7] W. C. Oliver, G.M. Pharr, An improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments, J. Mater. Res. 7 (1992) 1654–1583. https://doi.org/10.1177/003591571901200704. [8] F. Harbsmeier, W. Bolse, Ion beam induced amorphization in α quartz, J. Appl. Phys. 83 (1998) 4049–4054. [9] W. Primark, Radiation-Induced Stress Relaxation in Quartz and Vitreous Silica, J. Appl. Phys. 35 (1963) 1342–1347. 399

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