Doping-dependent Fe phonon dynamics in LaFeAsOH studied by Fe nuclear resonant inelastic scattering

Kawachi, Shiro*; Hiraka, Haruhiro*; Yamaura, Junichi*; Iimura, Soshi*; Nakamura, Hiroki; Tsutsui, Satoshi*; Yoda, Yoshitaka*; Machida, Masahiko; Hosono, Hideo*; Kobayashi, Hisao*

Physical Review B, 113(2), p.024519_1 - 024519_7, 2026/01

https://doi.org/10.1103/7zd9-5t9z

To investigate the phonon dynamics of iron in the heavily hydride-ion-substituted region of LaFeAsOH (), Fe nuclear resonant inelastic scattering measurements were performed over a wide temperature range from 5 to 300 K on two polycrystalline samples with and 0.51, which exhibit superconducting and antiferromagnetic ground states, respectively. The resulting inelastic scattering spectra revealed distinct differences between the two compositions. The Fe phonon density of states (PDOS) exhibits a pronounced peak at 15 meV for , whereas this peak is absent in . Density functional theory calculations support the interpretation that the PDOS peak at 15 meV is associated with the optical vibrational modes of Fe atoms along the nearest-neighbor direction, mediated by As atoms. The calculations further suggest that the suppression of the PDOS peak at 15 meV for originates from in-plane electronic inequivalence. These finndings suggest that signatures of electronic nematicity may persist over a wide temperature range in the composition, which exhibits magnetic and structural order, whereas such signatures are almost absent in the superconducting composition.

Specific heat anomalies and local symmetry breaking in (anti-)fluorite materials; A Machine learning molecular dynamics study

Kobayashi, Keita; Nakamura, Hiroki; Okumura, Masahiko; Itakura, Mitsuhiro; Machida, Masahiko

Journal of Chemical Physics, 162(24), p.244508_1 - 244508_11, 2025/06

https://doi.org/10.1063/5.0262059

The specific heat anomaly in (anti-)fluorite structures was analyzed using machine learning molecular dynamics (MLMD) methods. By employing the Farthest Point Sampling method and the Bootstrap method, first-principles training data were efficiently generated, and machine learning potentials were created for thorium dioxide (fluorite structure) and lithium oxide (anti-fluorite structure). As a result, the MLMD method accurately reproduced the reported thermal properties of thorium dioxide and lithium oxide. These materials exhibit a specific heat anomaly at high temperatures due to sublattice disordering. However, the details are complex and not fully understood. In this study, by applying a local order parameter methodology, which has been used in the analysis of liquid-liquid phase transitions, we revealed that the anomalous specific heat in (anti-)fluorite structures can be interpreted as a consequence of local symmetry breaking.

Uncertainty analysis of the inverse LASSO estimation scheme on radioactive source distributions inside reactor building rooms from air does rate measurements

Shi, W.*; Machida, Masahiko; Yamada, Susumu; Okamoto, Koji

Progress in Nuclear Energy, 184, p.105710_1 - 105710_10, 2025/06

https://doi.org/10.1016/j.pnucene.2025.105710

Very recently, Least Absolute Shrinkage and Selection Operator (LASSO) has been proposed as a scheme capable to inversely estimate radioactive source distributions inside reactor building rooms from air dose rate measurements together with the predicted lower bound of the measurement numbers for successful reconstructions. However, no one has ever analyzed how the uncertainty of input data including the measurement errors influences the accuracy of the inverse estimation results. In this paper, we therefore perform uncertainty analysis of the LASSO scheme and suggest an uncertainty estimation function derived based on the theory of Candes. We actually demonstrate in two types of numerical tests with different input uncertainties obtained by using Monte Carlo code, Particle and Heavy Ion Transport code System (PHITS) that the calculated errors obey the proposed uncertainty estimation function. Thus, the LASSO scheme allows to successfully estimate radioactive distributions within the predicted uncertainty.

Effects of magnetic ordering on the ground-state energy of plutonium dioxide; A Study using adiabatic connection fluctuation-dissipation theory

Nakamura, Hiroki; Machida, Masahiko

Proceedings of 31st International Conference on Nuclear Engineering (ICONE31) (Internet), 5 Pages, 2024/11

https://doi.org/10.1115/ICONE31-134660

Efficiency and safety improvements in nuclear fuel development demand a comprehensive understanding of the thermal properties of PuO. Because of difficulty of experiments at high temperatures, they have been supplemented by numerical simulations, such as density functional theory (DFT). While DFT struggles to replicate the nonmagnetic insulating ground state of PuO, DFT+U can reproduce nonmagnetic insulating state. However, the obtained state remains less stable than magnetic states and is not the ground state. Adiabatic connection fluctuation-dissipation theory (ACFDT) is expected to be a promising solution, addressing higher-order correlations and exact exchange energies. In this study, we evaluate the ground state energy using ACFDT and find that the nonmagnetic state can be the ground state. This result successfully reproduces the observed nonmagnetic ground state of PuO and has the potential to improve predictions of the thermal properties of nuclear fuel materials.

R&D status of digital technology on inverse estimation of radioactive source distributions and related source countermeasures; Fast Digital Twin Tech. in Decommissioning Field: 3D-ADRES-Indoor FrontEnd

Machida, Masahiko; Yamada, Susumu; Kim, M.; Tanaka, Satoshi*; Tobita, Yasuhiro*; Iwata, Ayako*; Aoki, Yuto; Aoki, Kazuhisa; Yanagisawa, Kenichi*; Yamaguchi, Takashi; et al.

RIST News, (70), p.3 - 22, 2024/09

Inside the Fukushima Daiichi Nuclear Power Plant (1F), there are many locations with high radiation levels due to contamination by radioactive materials that leaked from the reactor. These pose a significant obstacle to the smooth progress of decommissioning work. To help solve this issue, the Japan Atomic Energy Agency (JAEA), under a subsidy from the Ministry of Economy, Trade, and Industry's decommissioning and contaminated water management project, is conducting research and development on digital technologies to improve the radiation environment inside the decommissioning site. This project, titled "Development of Technology to Improve the Environment Inside Reactor Buildings (Enhancing Digital Technology for Environment and Source Distribution to Reduce Radiation Exposure)," began in April of FY 2023. In this project, the aim is to develop three interconnected systems: FrontEnd, Pro, and BackEnd. The FrontEnd system, based on the previously developed 3D-ADRES-Indoor (prototype) from FY 2021-2022, will be upgraded to a high-speed digital twin technology usable on-site. The Pro system will carry out detailed analysis in rooms such as the new office building at 1F, while the BackEnd system will serve as a database to centrally manage the collected and analyzed data. This report focuses on the FrontEnd system, which will be used on-site. After point cloud measurement, the system will quickly create a 3D mesh model, estimate the radiation source from dose rate measurements, and refine the position and intensity of the estimated source using recalculation techniques (re-observation instructions and re-estimation). The results of verification tests conducted on Unit 5 are also presented. Furthermore, the report briefly discusses the future research and development plans for this project.

A Science-based mixed oxide property model for developing advanced oxide nuclear fuels

Kato, Masato; Oki, Takumi; Watanabe, Masashi; Hirooka, Shun; Vauchy, R.; Ozawa, Takayuki; Uwaba, Tomoyuki; Ikusawa, Yoshihisa; Nakamura, Hiroki; Machida, Masahiko

Journal of the American Ceramic Society, 107(5), p.2998 - 3011, 2024/05

https://doi.org/10.1111/jace.19609

Effectiveness of fused LASSO for prediction of distribution of radioactive materials in reactor buildings

Yamada, Susumu; Yoshida, Toru*; Hasegawa, Yukihiro*; Machida, Masahiko

Proceedings of Waste Management Symposia 2024 (WM2024) (Internet), 15 Pages, 2024/03

In order to safely carry out the decommission of reactor buildings, it is extremely important to identify the radiation source distribution. It has been reported that when the structural model of the building is constructed by uniform cells, the source distribution can be estimated from the measured air dose rates by minimizing an evaluation function using the Least Absolute Shrinkage and Selection Operator (LASSO). Moreover, if cells are non-uniform, we can estimate the distribution using the fused LASSO which minimizes the evaluation function that takes account of the connectivity between the adjacent cells. However, when a group of some cells is considered disconnected from the surrounding ones due to the precision of the measured structural data, the concentration of the group can be singularly high. Therefore, in order to avoid the problem, we propose a new evaluation function that can prevent the singularity. We estimated the distribution for the test model using the proposed evaluation function and confirmed the validity of the function. Moreover, we succeeded in estimating the source distribution in the pool canal circulation system room in JMTR in the Japan Atomic Energy Agency by the fused LASSO for the new function more accurately than previous analysis.

Application of Bayesian estimation to X-ray fluorescent image

Matsuyama, Tsugufumi*; Igarashi, Momo*; Yasuda, Sora*; Murakami, Masashi; Yoshida, Yukihiko; Hayashi, Kazunori*; Machida, Masahiko; Tsuji, Koichi*

Analytical Chemistry, 9 Pages, 2024/01

http://dx.doi.org/10.2139/ssrn.4697562

Application of radiocaesium microscale observation methodology to parmelioid lichen and ultrastructural analyses using STEM-EDS

Dohi, Terumi; Iijima, Kazuki; Machida, Masahiko; Suno, Hiroya*; Omura, Yoshihito*; Fujiwara, Kenso; Kimura, Shigeru*; Kanno, Futoshi*

Environmental Radiochemical Analysis VII, p.50 - 57, 2023/12

https://doi.org/10.1039/9781837670758

Estimation of temporal variation of discharged inventory of radioactive strontium Sr (Sr) from port of Fukushima Daiichi Nuclear Power Plant; Analysis of the temporal variation from the accident to March 2022 and evaluation of its impact on Fukushima coast and offshore areas

Machida, Masahiko; Iwata, Ayako; Yamada, Susumu; Otosaka, Shigeyoshi*; Kobayashi, Takuya; Funasaka, Hideyuki*; Morita, Takami*

Nihon Genshiryoku Gakkai Wabun Rombunshi (Internet), 22(4), p.119 - 139, 2023/11

https://doi.org/10.3327/taesj.J22.016

We estimate monthly discharged inventory of Sr from port of Fukushima Daiichi Nuclear Power Plant (1F) from Jun. 2013 to Mar. 2022 by using the Voronoi tessellation method inside the port, following the monitoring of Sr sea water radioactivity concentration inside the port. The results suggest that the closure of sea side impermeable wall is the most effective for the reduction of discharged one. In addition, the results roughly reveal the monthly discharged inventory required to observe visible enhancement of the sea radioactivity concentration from the background level in each area. Such outcome is significant for considering environmental impacts on the planned future releasing of the treated water accumulated in 1F site.