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Nguyen, H. H.
Annals of Nuclear Energy, 230, p.112171_1 - 112171_13, 2026/06
Times Cited Count：0 Percentile：0.00(Nuclear Science & Technology)This study examined the effects of the moderator-to-fuel volume ratio, fuel debris shape, and the number of damaged fuel assemblies on the neutronic characteristics of a partially damaged reactor model, where the fuel assemblies at the core center melt to fuel debris while the fuel assemblies at the outer region remain intact. The investigations were conducted using the Serpent code and the JENDL-5 library. The results show that when fuel debris is surrounded by intact fuel assemblies, the k
can be classified into two groups based on the shape of the fuel debris. Conversely, in scenarios where the fuel debris is not fully encircled by intact fuel assemblies, the shape of the fuel debris has a negligible impact on the k. Additionally, the relationship between the number of neutrons entering and leaving the fuel debris determines how the shape of the fuel debris affects the k.
Aoyagi, Kazuhei; Tamura, Tomonori; Ozaki, Yusuke; Ishii, Eiichi; Motoshima, Takayuki*; Sugawara, Kentaro*
Dai-51-Kai Gamban Rikigaku Ni Kansuru Shimpojiumu Koen Rombunshu(Internet), p.119 - 124, 2025/12
In a high-level radioactive waste disposal, it is important to understand the extent of the Excavation Damaged Zone (EDZ) because it can be one of the factors to determine whether disposal galleries or pits can be excavated or not in the design or construction phases. In this study, we performed a hydro-mechanical coupling analysis to simulate the three-dimensional excavation of the twin galleries which were excavated at a depth of 500 m in the Horonobe Underground Research Laboratory. The analysis revealed that the EDZ was developed 1.5-2.0 m from the gallery wall. The stress acting on the shotcrete was within the ultimate limit state. Based on these results, we estimated that the stability of the twin galleries will be maintained, despite the relatively large extent of the EDZ.
Asahina, Daisuke*; Takamura, Hiroaki*; Tsukamoto, Koji*; Honda, Fumito*; ITO, Takashi*; IWATA, Naoki*; Aydan, 
mer*; Sato, Toshinori
Dai-51-Kai Gamban Rikigaku Ni Kansuru Shimpojiumu Koen Rombunshu(Internet), p.43 - 47, 2025/12
Fault zones commonly contain distinctive structures such as fault cores and damage zones, where physical properties vary locally. These zones are important for evaluating underground safety and groundwater flow in waste disposal, yet quantitative measurements near faults remain limited. We applied Schmidt hammer and needle penetration tests to assess the mechanical properties of the rocks near the Tanakura Fault, in a simple, non-destructive manner. The results showed a clear spatial variation in measured values with distance from the fault gouge boundary, suggesting that combining methods with different scales and sensitivities can provide a more comprehensive evaluation of rock mass properties near faults.
Nguyen, H. H.
Annals of Nuclear Energy, 218, p.111361_1 - 111361_9, 2025/08
Times Cited Count：1 Percentile：64.08(Nuclear Science & Technology)This study examined the criticality characteristics of a partially damaged reactor model, in which fuels located at the core center melt into fuel debris of varying shapes, while fuels situated at the core edges remain intact. The investigation was conducted using the Serpent code with the JENDL-5 nuclear data library. The results of the calculations indicate that when the volume of fuel debris is small and maintained at a constant level, the shape of the fuel debris does not result in significant alterations in the variation law of k of the system. In contrast, for the scenario in which the volume of the fuel debris is variable, the k variation law can be divided into two groups for the reference case with a system temperature of 300 K and no boron in the water. The first group comprises fuel debris with shapes that are cuboid and cylindrical, while the second group comprises fuel debris with shapes that are spherical, cone-shaped, and truncated cone-shaped.
Haga, Katsuhiro; Naoe, Takashi; Kogawa, Hiroyuki; Wakui, Takashi; Kinoshita, Hidetaka; Harada, Masahide
Proceedings of 16th International Particle Accelerator Conference (IPAC25) (Internet), p.3245 - 3249, 2025/06
In April 2024, the beam power at MLF attained 950 kW for the first time for long term user operation, and the beam power at the 3 GeV rapid cycle synchrotron (RCS) outlet was raised to 1 MW. This accomplishment means that the goal of the stable operation of the neutron source with 1 MW was almost achieved at last, and it's time to go on to the new stage of the neutron source R&D. There are two major challenges for the mercury target in the next stage. One is to attain the long-term operation of a mercury target. The service life of the target vessel is primarily determined by cavitation damage that occurs on the inner surface due to the injection of high-intensity pulsed proton beams. Until now, the vessel has been replaced annually to inspect the extent of the damage. However, based on the damage data obtained during 1 MW high-power operation, it has been determined that the vessel can withstand long-term operation for more than two years. Therefore, a new target vessel, which was replaced in 2024, is scheduled to be used for an extended period through 2027. Furthermore, since there are plans to increase the pulse intensity of the RCS in the future, it will be necessary to develop more effective pitting damage suppression techniques and new target vessels that can withstand even stronger proton beam pulses. In this presentation, the present status of the neutron source of MLF and future operation plans will be shown.
Toyota, Kodai; Onizawa, Takashi; Wakai, Eiichi*
Research & Development in Material Science (Internet), 21(5), p.2632 - 2637, 2025/06
Suetake, Koya*; Ogata, Sho*; Yasuhara, Hideaki*; Aoyagi, Kazuhei; Inui, Toru*; Kishida, Kiyoshi*
Dai-16-Kai Iwa No Rikigaku Kokunai Shimpojiumu Koen Rombunshu (Internet), p.304 - 309, 2025/01
It is very important to predict the extent of EDZ (excavation disturbed zone) and the behavior of hydraulic conductivity after excavation of shafts and tunnels for geological disposal of radioactive waste. In this study, we attempted to reproduce the in-situ tunnel excavation test and the subsequent permeability test at the Horonobe Underground Research Laboratory (URL) using a three-dimensional tunnel excavation simulator. As a result, similar results to the in-situ tests were obtained for the extent of EDZ propagation by excavation and the results of permeability tests. The simulator is effective in predicting the mechanical effects of excavation, coupled phenomena such as deformation and seepage, and hydraulic conductivity in deep mudstones in Japan.
Kogawa, Hiroyuki; Wakui, Takashi; Futakawa, Masatoshi
Fluids (Internet), 10(1), p.3_1 - 3_15, 2025/01
Microbubbles have been applied in various fields. In mercury targets of spallation neutron sources, where cavitation damage is a crucial issue for the life estimation, microbubbles are injected into the mercury to absorb the thermal expansion of the mercury caused by the pulsed proton beam injection and reduce the macroscopic pressure waves, and result in reducing the damage. Recently, when the proton beam power was increased and the amount of injected gas bubbles was increased, unique damage morphologies were observed on solid-liquid interface. Detailed observation and numerical analyses revealed that the microscopic pressure emitted from the gas bubbles contracting is sufficient to form pit damage; i.e. the directions of streak-like defects which are formed by connecting pit damage coincides with the direction of gas bubble trajectories, and the distances between pits was understandable taking the natural period of gas bubble vibration into account. This indicates that the gas microbubbles to reduce the macroscopic pressure waves have potential to be inceptions of the cavitation damage due to the microscopic pressure emitted from gas bubbles. To completely mitigate the damage, we have to consider the two effects of injecting gas bubbles; reducing macroscopic pressure waves and microscopic pressure due to bubble dynamics.
Wakui, Takashi; Saito, Shigeru; Futakawa, Masatoshi
Jikken Rikigaku, 24(4), p.212 - 218, 2024/12
Irradiation damage is one of the main factors determining the lifetime of the mercury target vessel for spallation neutron source in J-PARC. To understand material degradation of the used vessels, it is planned to conduct an evaluation using inverse analyses with indentation tests on the structural materials of the used vessels and numerical experiments. This evaluation technique was applied to two kinds of ion-irradiated materials with different displacement damage doses, in which the irradiation condition was simulated. It could be confirmed that the ultimate strength increased, and the total elongation decreased with increasing irradiation. These trends are like the material degradation behaviors which have been reported by using small specimen's tensile tests.
Hata, Koji*; Niunoya, Sumio*; Aoyagi, Kazuhei
International Journal of the JSRM (Internet), 20(1), p.240104_1 - 240104_4, 2024/11
Japan Atomic Energy Agency has been conducting R&D activities to enhance the reliability of high-level radioactive waste deep geological disposal technology. In this project, one of the challenges is the development of methods for long-term monitoring of rock mass behavior. Therefore, in January 2014, the long-term measurements of the hydro-mechanical behavior of the rock mass were launched using the three types of optical sensors in the vicinity of the shaft excavated deeper than 350 m at the Horonobe Underground Research Center. The measurement results show that Acoustic emission occur frequently up to 1.5 m from the wall during excavation. In addition, hydraulic conductivity increased by 2-4 orders of magnitude. Elastoplastic analysis revealed that the hydraulic behavior of the rock mass affected the pore pressure fluctuations and caused micro-fractures. Based on this, a conceptual model is developed to represent the excavation damaged zone, which contributes to the safe geological disposal of radioactive waste.
Aoyagi, Kazuhei; Ozaki, Yusuke; Tamura, Tomonori; Ishii, Eiichi
Proceedings of 4th International Conference on Coupled Processes in Fractured Geological Media; Observation, Modeling, and Application (CouFrac2024) (Internet), 10 Pages, 2024/11
In high-level radioactive waste disposal, it is crucial to estimate the transmissivity of gallery excavation-induced fractures, i.e., excavation damaged zone (EDZ) fractures, because EDZ fractures can be a radionuclide migration pathway after the backfilling of the facility is completed. From previous research, the transmissivity of the fracture can be estimated through the empirical equation using the parameter ductility index (DI), which corresponds to the effective mean stress normalized to the tensile strength of the rock. In this research, we performed a hydromechanical coupling analysis of a gallery excavation at the Horonobe Underground Research Laboratory to estimate the transmissivity of the EDZ fracture before the excavation. At first, we simulated the gallery excavation at 350 m and showed that the measured transmissivity was within the range of the estimated transmissivity using the DI. After that, we also predicted the excavation of a gallery at 500 m by setting the hydromechanical parameters acquired from the laboratory tests before the excavation. The estimated transmissivity at 500 m was one order of magnitude less than that at 350 m. This result might be related to the closure of the fracture under high-stress conditions and low rock strength.
Kai, Takeshi; Toigawa, Tomohiro; Matsuya, Yusuke*; Hirata, Yuho; Tezuka, Tomoya*; Tsuchida, Hidetsugu*; Yokoya, Akinari*
Scientific Reports (Internet), 14, p.24722_1 - 24722_15, 2024/10
Times Cited Count：5 Percentile：49.14(Multidisciplinary Sciences)Scientific insight of water radiolysis is essential to estimate the direct and indirect effects of radiation DNA damage. Secondary electrons produced by water radiolysis are responsible for both effects. Here, we use a first-principles code to calculate the femtosecond dynamics of secondary electrons produced as a result of 20-30 eV energy deposition to water and analyze the formation mechanism of radiolytic chemical species produced in a nano-size ultra-small space region. From the results, it was clarified that the chemical species produced by water radiolysis begin to densify in the ultra-small region of a few nanometers when the deposition energy exceeds 25 eV. Our results provide important scientific insights into the formation of clustered DNA damage, which is believed to cause biological effects such as cell death.
Wakui, Takashi; Takagishi, Yoichi*; Futakawa, Masatoshi
Zairyo, 73(6), p.520 - 526, 2024/06
Cavitation damage is one of crucial issues to predict the structural endurability of the mercury targets for highly intensive pulsed neutron sources. Based on the comparison with numerical simulation on the pit shape and results of the basic test, the cavitation bubble collapsing was assumed to be resulted in the micro jet with the impact velocity of 160-200 m/s, imposing then impact pressure of 3-4 GPa at the input power simulating the operation condition in the mercury targets. It was statistically understandable that cavitation damage evolution was proportional to 4th power of the input power approximately, as taking the aggressivity of cavitation bubbles, the distribution of the maximum diameter of grown bubbles and the space of distribution of bubbles in the mercury into account.
Hata, Koji*; Niunoya, Sumio*; Aoyagi, Kazuhei; Miyara, Nobukatsu*
Journal of Rock Mechanics and Geotechnical Engineering, 16(2), p.365 - 378, 2024/02
Times Cited Count：13 Percentile：85.79(Engineering, Geological)Excavation of underground caverns, such as mountain tunnels and energy-storage caverns, may cause the damages to the surrounding rock as a result of the stress redistribution. In this influenced zone, new cracks and discontinuities are created or propagate in the rock mass. Therefore, it is effective to measure and evaluate the acoustic emission (AE) events generated by the rocks, which is a small elastic vibration, and permeability change. The authors have developed a long-term measurement device that incorporates an optical AE (O-AE) sensor, an optical pore pressure sensor, and an optical temperature sensor in a single multi-optical measurement probe (MOP). Japan Atomic Energy Agency has been conducting R&D activities to enhance the reliability of high-level radioactive waste (HLW) deep geological disposal technology. In a high-level radioactive disposal project, one of the challenges is the development of methods for long-term monitoring of rock mass behavior. Therefore, in January 2014, the long-term measurements of the hydro-mechanical behavior of the rock mass were launched using the developed MOP in the vicinity of 350 m below the surface at the Horonobe Underground Research Center. The measurement results show that AEs occur frequently up to 1.5 m from the wall during excavation. In addition, hydraulic conductivity increased by 2 to 4 orders of magnitude. Elastoplastic analysis revealed that the hydraulic behavior of the rock mass affected the pore pressure fluctuations and caused micro-fractures. Based on this, a conceptual model is developed to represent the excavation damaged zone (EDZ), which contributes to the safe geological disposal of radioactive waste.
Konno, Chikara
Journal of Nuclear Science and Technology, 61(1), p.121 - 126, 2024/01
Times Cited Count：3 Percentile：44.12(Nuclear Science & Technology)The JENDL-4.0/HE neutron and proton ACE files were produced in 2017 and those of 22 nuclei for neutron and 25 nuclei for proton were bundled in the PHITS code. Recently it was found that the following five data in the JENDL-4.0/HE neutron and proton ACE files had any problems; ACE files for 
N and 
O, heating numbers, damage energy production cross sections, secondary neutron multiplicities and fission cross sections. Thus new JENDL-4.0/HE neutron and proton ACE files were produced with the problems fixed. This paper describes the problems and how to produce the new neutron and proton ACE files in detail.
Wakui, Takashi; Takagishi, Yoichi*; Futakawa, Masatoshi
Materials, 16(17), p.5830_1 - 5830_16, 2023/09
Times Cited Count：0 Percentile：0.00(Chemistry, Physical)Cavitation damage on the mercury target vessel is induced by proton beam injection in mercury. The prediction method of the cavitation damage using Monte Carlo simulations was proposed taking into account of the uncertainties of the position of cavitation bubbles and impact pressure distributions. The distribution of impact pressure attributed to individual cavitation bubble collapsing was assumed to be the Gaussian distribution, and the probability distribution of the maximum value of impact pressures was assumed to be three kinds of distributions; the delta function, the Gaussian and Weibull distributions. Two parameters were estimated using Bayesian optimization by comparing the distribution of the cavitation damage obtained from experiment with that of accumulated plastic strain obtained from the simulation. It was found that the results obtained using the Weibull distribution reproduced the actual cavitation erosion phenomenon better than the other results.
Wakui, Takashi; Takagishi, Yoichi*; Futakawa, Masatoshi; Tanabe, Makoto*
Jikken Rikigaku, 23(2), p.168 - 174, 2023/06
Cavitation damage on the inner surface of the mercury target for the spallation neutron source occurs by proton bombarding in mercury. The prediction method of the cavitation damage using Monte Carlo simulations was suggested taking variability of the bubble core position and impact pressure distribution into account. The impact pressure distribution was estimated using the inverse analysis with Bayesian optimization was conducted with comparison between cavitation damage distribution obtained from experiment and the cumulative plastic strain distribution obtained from simulation. The average value and spread of maximum impact pressure estimated assuming the Gaussian distribution were 3.1 GPa and 1.2 
m, respectively. Simulation results reproduced experimental results and it can be said that this evaluation method is useful.
CMiyazawa, Takeshi; Kikuchi, Yuta*; Ando, Masami*; Yu, J.-H.*; Yabuuchi, Kiyohiro*; Nozawa, Takashi*; Tanigawa, Hiroyasu*; Nogami, Shuhei*; Hasegawa, Akira*
Journal of Nuclear Materials, 575, p.154239_1 - 154239_11, 2023/03
Times Cited Count：8 Percentile：77.55(Materials Science, Multidisciplinary)Kataoka, Takahiro*; Ishida, Tsuyoshi*; Naoe, Shota*; Kanzaki, Norie; Sakoda, Akihiro; Tanaka, Hiroshi; Mitsunobu, Fumihiro*; Yamaoka, Kiyonori*
Journal of Radiation Research (Internet), 63(5), p.719 - 729, 2022/09
Times Cited Count：4 Percentile：34.93(Biology)Naoe, Takashi; Kinoshita, Hidetaka; Wakui, Takashi; Kogawa, Hiroyuki; Haga, Katsuhiro
JAEA-Technology 2022-018, 43 Pages, 2022/08
JAEA-Technology-2022-018.pdf:7.84MB
In the liquid mercury target system for the pulsed spallation neutron source of Materials and Life science experimental Facility (MLF) at the Japan in the Japan Proton Accelerator Research Complex (J-PARC), cavitation that is generated by the high-energy proton beam-induced pressure waves, resulting severe erosion damage on the interior surface of the mercury target vessel. The erosion damage is increased with increasing the proton beam power, and has the possibility to cause the leakage of mercury by the penetrated damage and/or the fatigue failure originated from erosion pits during operation. To achieve the long term stable operation under high-power proton beam, the mitigation technologies for cavitation erosion consisting of surface modification on the vessel interior surface, helium gas microbubble injection, double-walled beam window structure has been applied. The damage on interior surface of the vessel is never observed during the beam operation. Therefore, after the target operation term ends, we have cut out specimen from the target nose of the target vessel to inspect damaged surface in detail for verification of the cavitation damage mitigation technologies and lifetime estimation. We have developed the techniques of specimen cutting out by remote handling under high-radiation environment. Cutting method was gradually updated based on experience in actual cutting for the used target vessel. In this report, techniques of specimen cutting out for the beam entrance portion of the target vessel in high-radiation environment and overview of the results of specimen cutting from actual target vessels are described.
