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Ishikawa, Norito; Fukuda, Shoma; Nakajima, Toru; Ogawa, Hiroaki; Fujimura, Yuki; Taguchi, Tomitsugu*
Materials, 17(3), p.547_1 - 547_21, 2024/02
Times Cited Count：0 Percentile：0.04(Chemistry, Physical)Natural monoclinic zirconia (baddeleyite) was irradiated with 340-MeV Au ions, and the irradiation-induced nanostructures (i.e., ion tracks and nanohillocks) were observed using transmission electron microscopy. The diameter of the nanohillocks is approximately 10 nm, which is similar to the maximum molten region size calculated using the analytical thermal spike model. Ion tracks are imaged as strained regions that maintain their crystalline structure. The cross-sections of most of the ion tracks are imaged as parallelopiped or rectangular contrasts as large as 10 nm. These results strongly indicate that the molten region is recrystallized anisotropically, reflecting the lattice structure. Furthermore, low-density track cores are formed in the center of the ion tracks. The formation of low-density track cores can be attributed to the ejection of molten matter toward the surface.
Narukawa, Takafumi; Kondo, Keietsu; Fujimura, Yuki; Kakiuchi, Kazuo; Udagawa, Yutaka; Nemoto, Yoshiyuki
Journal of Nuclear Materials, 587, p.154736_1 - 154736_8, 2023/12
Times Cited Count：1 Percentile：0.01(Materials Science, Multidisciplinary)Sakaguchi, Yoshifumi*; Takata, Shinichi; Kawakita, Yukinobu; Fujimura, Yuki; Kondo, Keietsu
Journal of Physics; Condensed Matter, 35(41), p.415403_1 - 415403_11, 2023/10
Times Cited Count：0 Percentile：0(Physics, Condensed Matter)Narukawa, Takafumi; Kondo, Keietsu; Fujimura, Yuki; Kakiuchi, Kazuo; Udagawa, Yutaka; Nemoto, Yoshiyuki
Journal of Nuclear Materials, 582, p.154467_1 - 154467_12, 2023/08
Times Cited Count：3 Percentile：95.99(Materials Science, Multidisciplinary)Nemoto, Yoshiyuki; Ishijima, Yasuhiro; Kondo, Keietsu; Fujimura, Yuki; Kaji, Yoshiyuki
Journal of Nuclear Materials, 575, p.154209_1 - 154209_19, 2023/03
Times Cited Count：1 Percentile：31.61(Materials Science, Multidisciplinary)Previous studies had shown that in certain conditions, the rate of oxidation of zirconium (Zr) based alloy fuel cladding is higher in air-steam mixtures than in dry air. In severe accidents in the spent fuel pool and in other air ingress accidents in nuclear power plants, the cladding is likely to be oxidized in an air-steam mixture, which makes it crucial to have an in-depth understanding of the nature of oxidation and its kinetics in that environment. Oxidation tests were conducted at 800
C on Zircaloy-4 specimens in a mix of (air+steam) with various component ratios. Oxidation kinetics, details of the oxide layer, and hydrogen pick-up in the specimen were studied to investigate the mechanism of oxidation in each of these sets of conditions. Zirconium nitride precipitation in the oxide layer during the initial stages of the pre-breakaway oxidation stage and the widespread porous oxide growth on the cladding surface in the latter post-BA oxidation stage are related to the oxidation mechanism in the air-steam mixture. The differences in the mechanism of oxidation of the cladding in dry air and air-steam mixtures are discussed based on the experimental results.
Hirade, Tetsuya; Furuta, Hikaru*; Torikai, Yuji*; Fujimura, Yuki; Michishio, Koji*
JJAP Conference Proceedings (Internet), 9, p.011106_1 - 011106_7, 2023/00
Positron annihilation lifetime (PAL) measurements by use of a positron source of 
Na were performed for polycrystalline ITER-grade tungsten samples exposed to low-energy deuterium plasma. The energy of deuterium plasma was low and then it was expected that it would affect just near-surface region. However, we obtained the longer mean positron annihilation lifetime in the tungsten samples exposed to the low-energy deuterium plasma than the virgin tungsten samples. Moreover, almost same longer values were obtained even on the other (no exposed) side of the samples, although the thickness of the samples were about 2 mm. Although, there has been no report of observation of defect formation by existence of hydrogen or deuterium in tungsten, the results indicated that deuterium existence in tungsten can be one of reasons of defects formation.
irradiated with swift heavy ions at grazing incidenceIshikawa, Norito; Fujimura, Yuki; Kondo, Keietsu; Szabo, G. L.*; Wilhelm, R. A.*; Ogawa, Hiroaki; Taguchi, Tomitsugu*
Nanotechnology, 33(23), p.235303_1 - 235303_10, 2022/06
Times Cited Count：4 Percentile：48.5(Nanoscience & Nanotechnology)A single crystal of SrTiO doped with niobium (Nb-STO) was irradiated with 200 MeV Au ions at grazing incidence. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) are used to study the relation between irradiation-induced change of surface topography and corresponding material property changes. As expected, multiple hillocks as high as 5-6 nm are imaged by AFM observation. It is also found that the region in between the adjacent hillocks is slightly elevated rather than depressed. Line-like contrasts along the ion paths are found in both AFM phase images and SEM images, indicating the formation of continuous ion tracks in addition to multiple hillocks. Cross-sectional transmission electron microscopy (TEM) observation shows that the ion tracks in the near-surface region are found to be relatively large, whereas buried ion tracks in the deeper region are relatively small. The results suggest that recrystallization plays an important role in the formation of small ion tracks in the deep region, whereas formation of large ion tracks in the near-surface region is likely due to the absence of recrystallization.
Okubo, Nariaki; Fujimura, Yuki; Tomobe, Masakatsu*
Quantum Beam Science (Internet), 5(3), p.27_1 - 27_9, 2021/09
In an accelerator driven system (ADS), the beam window material of the spallation neutron target is heavily irradiated under severe conditions. Displacement damage and corrosion occur simultaneously because of high-energy neutron and/or proton irradiation in the lead-bismuth flow. The materials used in ADSs need to be compatible with the liquid metal, which is lead-bismuth eutectic (LBE), to prevent issues such as liquid metal embrittlement (LME) and liquid metal corrosion (LMC). In this study, the LMC behavior after ion irradiation of 316L austenitic steel is considered for self-ion irradiations followed by the corrosion tests. The 316L specimens were irradiated by 10.5 MeV-Fe
ions at temperature of 450C up to 50 displacement per atom (dpa). After the corrosion test at 450C in LBE with low oxygen concentration, the surface of the non-irradiated area was not oxidized but corrosive morphology with pits, whereas the irradiated area was covered by an iron/chromium oxide layer. The surface of the irradiated area was covered by the duplex layers of iron and iron/chromium oxides in the case of higher oxygen concentration in LBE. It is suggested that irradiation can advance oxide layer formation because of enhanced Fe diffusion caused by the residual vacancies in 316L steel.
Shamoto, Shinichi; Lee, M. K.*; Fujimura, Yuki; Kondo, Keietsu; Ito, Takashi; Ikeuchi, Kazuhiko*; Yasuda, Satoshi; Chang, L.-J.*
Materials Research Express (Internet), 8(7), p.076303_1 - 076303_6, 2021/07
Times Cited Count：0 Percentile：0(Materials Science, Multidisciplinary)Pb, Ga, and Ga doped lead free Sn-Ag-Cu solders are used to study the gallium effect for the low joint resistivity with silver sheathed DI BISCCO type H tapes. The results are reported.
irradiated with swift heavy ionsKitamura, Akane; Ishikawa, Norito; Kondo, Keietsu; Fujimura, Yuki; Yamamoto, Shunya*; Yamaki, Tetsuya*
Transactions of the Materials Research Society of Japan, 44(3), p.85 - 88, 2019/06
Swift heavy ions can create nanosized hillocks on the surfaces of various ceramics. When these materials are irradiated with swift heavy ions at normal incidence, each ion impact results in the formation of a single hillock on the surfaces. In contrast, irradiation at grazing incidence forms chains of multiple hillocks on the surface, for example, for strontium titanate (SrTiO). So far, chains of multiple hillocks have been investigated using atomic force microscopy (AFM). It should be noted that AFM measurements involve systematic errors of several nanometers due to the finite size of the probe tip. Consequently, it is possible that the image of one hillock may merge with that of a neighboring hillock even if the two hillocks are well separated. In contrast to AFM, field-emission scanning electron microscopy (FE-SEM) is a useful technique for obtaining higher-resolution images. In this study, we observed multiple nanohillocks on the surfaces of SrTiO using FE-SEM. Crystals of SrTiO(100) and 0.05 wt% Nb-doped SrTiO(100) were irradiated with 350 MeV Xe ions, respectively, at grazing incidence, where the angle between the sample surface and the beam was less than 2. On the SrTiO surface, a chain of periodic nanohillocks is created along the ion path. In contrast, black lines accompanied by hillocks are observed on the Nb-doped SrTiO surface.
Okubo, Nariaki; Fujimura, Yuki; Kitamura, Akane; Okuno, Yasuki; Kondo, Keietsu
no journal, ,
An accelerator-driven subcritical reactor is a nuclear reactor design formed by coupling a substantially subcritical nuclear reactor core with a high-energy proton accelerator. In this system, lead-bismuth eutectic (LBE) is supposed to be used as coolant material of reactor core as well as neutron spallation sources. In this study, effect of irradiation on corrosion behavior was evaluated for 316L stainless steel, which is one of candidate ADS target window materials, through the immersion test under LBE with low oxygen concentration followed by ion irradiation experiment.
Okubo, Nariaki; Fujimura, Yuki; Kitamura, Akane; Okuno, Yasuki
no journal, ,
Japan atomic energy agency has been conducting research activities for developing the accelerator driven system (ADS), which is expected to reduce and transmute radioactive wastes involved in spent fuels by nuclear power plants. The ADS adopts a liquid metal of lead-bismuth eutectic (LBE) as a coolant and also spallation target to produce high energy neutrons. In this study, effect of irradiation on corrosion behavior was evaluated for 316L stainless steel and T91 ferritic martensitic steel, which are candidate materials of ADS target window, through the immersion test under LBE with saturated and low oxygen concentration followed by ion irradiation experiment. In the case of soaking in LBE with saturated oxygen concentration for SS316L steels at 450C, 330 hrs, non-irradiated region did not show clear oxide layer, however, irradiated region showed bi-layers of magnetite and spinel type oxides. The formation rate of oxide layer for irradiated region, 26 
m/year, was about twice faster than that of non-irradiated region, 12 m/year. This result suggests that diffusion behavior after irradiation and mass transfer in the interface between LBE and steel surface is important for understanding of irradiation effect on liquid metal corrosion.
Kitamura, Akane; Ishikawa, Norito; Kondo, Keietsu; Fujimura, Yuki; Yamamoto, Shunya*
no journal, ,
Swift heavy ions can create nanosized hillocks on the surfaces of various ceramics. When these materials are irradiated with swift heavy ions at normal incidence, each ion impact results in the formation of a single hillock on the surfaces. In contrast, irradiation at grazing incidence forms chains of multiple hillocks on the surface such as strontium titanate (SrTiO). So far, chains of multiple hillocks have been investigated using atomic force microscopy (AFM). It should be noted that AFM measurements involve systematic errors of several nanometers due to the finite size of the probe tip. Consequently, it is possible that the image of one hillock may merge with that of a neighboring hillock even if the two hillocks are well separated. In contrast to AFM, field-emission scanning electron microscopy (FE-SEM) is a useful technique for obtaining higher-resolution images. In this study, we observed multiple nanohillocks on the surfaces of SrTiO using FE-SEM. Crystals of SrTiO(100) and 0.5 wt% Nb-doped SrTiO(100) were irradiated with 200 MeV Xe ions, respectively. The irradiated angle between the sample surface and the beam was less than 2. On the SrTiO surface, a chain of periodic nanohillocks is created along the ion path. In contrast, black lines accompanied by hillocks are observed on the Nb-doped SrTiO surface. As a result, we proposed a new model of formation process for the hillock chains in the framework of Rayleigh instability.
Shamoto, Shinichi; Ito, Takashi; Yasuda, Satoshi; Fujimura, Yuki; Kondo, Keietsu
no journal, ,
no abstracts in English
Okubo, Nariaki; Fujimura, Yuki
no journal, ,
The accelerator driven system (ADS) adopts a liquid metal of lead-bismuth eutectic (LBE) as a coolant and also spallation target to produce high energy neutrons. In this study, irradiation effect on the corrosion behavior was evaluated for the compativility of 316L stainless steel, which is candidate material of ADS target window, through the immersion test under LBE with saturated and low oxygen concentration followed by ion irradiation experiment. In the case of soaking in LBE with saturated oxygen concentration for SS316L steels at 450C, 330 hrs, non-irradiated region did not show clear oxide layer, however, irradiated region showed bi-layers of magnetite and spinel type oxides. The formation rate of oxide layer for irradiated region was about twice faster than that of non-irradiated region. This result suggests that diffusion behavior after irradiation and mass transfer in the interface between LBE and steel surface is important for understanding of irradiation effect on liquid metal corrosion.
Kaji, Yoshiyuki; Aoki, So; Fujimura, Yuki; Kondo, Keietsu
no journal, ,
In this study, the influence of the long term thermal aging (LTA) on SCC initiation susceptibility of L-grade SSs was examined. Results of CBB testing on Type 316L CW and CW + LTA under the BWR simulated water condition showed the increase of SCC susceptibility to initiation after the LTA treatment. The material examinations on those materials were conducted to identify the SCC accelerating factors. The initial microstructure in Type 316L CW and CW + LTA was characterized to dislocation tangles and dislocation cell structures, respectively, indicating that the LTA treatment induced the rearrangement of dislocations toward the energetically favorable state. Whereas, no sensitization of the grain boundaries of Type 316L CW + LTA was confirmed by means of TEM/EDX analysis. The examination of surface deformation microstructure by bending deformation was conducted by the consecutive SEM observation before and after CBB testing. The more heterogeneous deformation was enhanced by the LTA treatment, that is, the reduction in dislocation motion and increase of deformation twinning. It was considered from those obtained results that the concentration of local stress around grain boundaries caused by the more heterogeneous deformation might be one of the promoting factors for increasing the SCC susceptibility in non-sensitized Type 316L CW + LTA.
Okubo, Nariaki; Fujimura, Yuki
no journal, ,
An accelerator-driven subcritical system is a nuclear reactor design formed by coupling a substantially subcritical nuclear reactor core with a high-energy proton accelerator. In this system, lead-bismuth eutectic (LBE) is supposed to be used as coolant material of reactor core as well as neutron spallation sources. In this study, effect of irradiation on corrosion behavior was evaluated for 316L stainless steel and T91 ferritic/martensitic steel, which are ADS in-core candidate materials, through the immersion test under LBE with high and low oxygen concentration followed by ion irradiation experiment. Irradiations of 10 MeV-Fe ions were conducted for T91 and SS316L steels at 450C. After irradiation, the specimen was soaked in static LBE pod and started the corrosion test for about 400 hrs at 450C. The oxygen concentration in LBE was maintained at conditions of saturated concentration of around 10
wt% and at low concentration below 10
wt%. The surface morphology and cross sectional corrosion behavior were evaluated by SEM (Scanning Electron Microscope). In the case of soaking in LBE with saturated oxygen concentration for SS316L steels at 450C, 400hrs, non-irradiated region did not show clear oxide layer, however, irradiated region showed bi-layers of magnetite and spinel type oxide. In the case of low oxygen concentration, non-irradiated region showed pitting erosion, however, irradiated region showed bi-layer oxide formation. This result suggests that diffusion behavior after irradiation and mass transfer in the interface between LBE and steel surface is important for understanding of irradiation effect on liquid metal corrosion.
Ishikawa, Norito; Kondo, Keietsu; Fujimura, Yuki; Kitamura, Akane
no journal, ,
Radiation damages are induced by swift heavy ions with energy above 1 MeV/u where the energy deposition is mainly done through ionization and electronic excitation process. The previous studies of X-ray diffraction and transmission electron microscopy show that lattice expansion is induced at low fluence, and at high fluence amorphization was induced in single crystalline Al
O irradiated with swift heavy ions (e.g. amorphization by irradiation with 160 MeV Xe ions at 3.5
10
ions/cm
). Since the origin of the amorphization is still unclear, it is important to examine the irradiation-induced lattice expansion in detail.
Shamoto, Shinichi; Fujimura, Yuki; Kondo, Keietsu; Ito, Takashi; Yasuda, Satoshi; Ikeuchi, Kazuhiko*
no journal, ,
The junction resistivity of Bi2223 superconducting wires has been studied at 77K with Ag added Pb-Sn-Bi low-temperature solders, leading to low temperature connection with low resistivity even at 200C.
