Durability of U-Zr oxide solid solution in aqueous HO solution

Kumagai, Yuta; Kusaka, Ryoji; Takano, Masahide; Watanabe, Masayuki

Journal of Nuclear Materials, 625, p.156553_1 - 156553_7, 2026/04

https://doi.org/10.1016/j.jnucmat.2026.156553

Uranium-zirconium oxide solid solution, (U, Zr)O, is a representative matrix phase found in fuel debris formed during severe nuclear reactor accidents. Understanding its chemical behavior in oxidative aqueous environments is important for evaluating the potential release of radionuclides during water contact. In this study, we investigated the reactivity of (U, Zr)O with hydrogen peroxide (HO) in pure water to assess its resistance to oxidative dissolution, because HO is the dominant oxidant produced by water radiolysis. The dissolution behavior of uranium and zirconium was monitored through repeated HO exposure experiments, and the solid phases were characterized using Raman micro-spectroscopy and X-ray diffraction. Kinetic modeling was performed to interpret experimental data. The results showed that uranium dissolution occurred initially but decreased significantly upon repeated HO exposure, while zirconium dissolution proceeded more slowly. Raman analysis revealed only minor surface changes, with limited formation of uranyl peroxide phases. The kinetic simulation reproduced the experimental trends by assuming a small fraction of redox-active surface sites. These findings suggest that the observed durability of (U, Zr)O against HO-induced oxidative dissolution is not due to the formation of a protective surface layer, but rather reflects the limited redox reactivity of most of the surface. This study provides a quantitative basis for understanding the HO-induced oxidation of (U, Zr)O in water, relevant to the long-term behavior of fuel debris.

Challenge for screening of nuclear fuel debris by innovative spectral imaging and its verification by LIBS mapping (Contract research); FY2023 Nuclear Energy Science & Technology and Human Resource Development Project

Collaborative Laboratories for Advanced Decommissioning Science; The University of Osaka*

JAEA-Review 2025-040, 111 Pages, 2025/12

JAEA-Review-2025-040.pdf:22.28MB

The Collaborative Laboratories for Advanced Decommissioning Science (CLADS), Japan Atomic Energy Agency (JAEA), had been conducting the Nuclear Energy Science & Technology and Human Resource Development Project (hereafter referred to "the Project") in FY2023. The Project aims to contribute to solving problems in the nuclear energy field represented by the decommissioning of the Fukushima Daiichi Nuclear Power Station, Tokyo Electric Power Company Holdings, Inc. (TEPCO). For this purpose, intelligence was collected from all over the world, and basic research and human resource development were promoted by closely integrating/collaborating knowledge and experiences in various fields beyond the barrier of conventional organizations and research fields. The sponsor of the Project was moved from the Ministry of Education, Culture, Sports, Science and Technology to JAEA since the newly adopted proposals in FY2018. On this occasion, JAEA constructed a new research system where JAEA-academia collaboration is reinforced and medium-to-long term research/development and human resource development contributing to the decommissioning are stably and consecutively implemented. Among the adopted proposals in FY2023, this report summarizes the research results of the "Challenge for screening of nuclear fuel debris by innovative spectral imaging and its verification by LIBS mapping" conducted in FY2023. The present study aims to develop a remote-sensing technique to identify the in-reactor materials by a combination of Hyper Spectral Imaging (HSI) and Laser Induced Breakdown Spectroscopy (LIBS). HSI analyzes spectral information of more than 100 colors, and is being applied to classify various materials. On the other hand, material composition cannot be directly evaluated by HSI. Therefore, we thought that the combination of HSI and LIBS could be an accurate and wide-ranging visualization technique. In order to demonstrate the HSI and LIBS, it is necessary to prepare standard materials that simulate in-reactor materials, and to acquire and accumulate training data on them. In this study, the University of Osaka is in charge of the preparation of standard materials and HSI data analysis, Nuclear Fuel Development (NFD) is in charge of the preparation of uranium bearing materials and HSI/LIBS measurements, and JAEA is in charge of LIBS development. On the UK side, the Univ. of Strathclyde, National Nuclear Laboratory (NNL), and Lancaster University participate in the joint research project. The compositions of the standard samples were determined from past experiments and thermodynamic calculation results. Several samples such as UO based composites and concrete were prepared. The HSI data were obtained using a hyperspectral camera installed in the NFD. For LIBS, we worked on the automatic optimization of focal distance as part of the development of remote operation technology.

Investigation of effects of nano interfacial phenomena on dissolution aggregation of alpha nanoparticles by using micro nano technologies (Contract research); FY2023 Nuclear Energy Science & Technology and Human Resource Development Project

Collaborative Laboratories for Advanced Decommissioning Science; Institute of Science Tokyo*

JAEA-Review 2025-026, 72 Pages, 2025/11

JAEA-Review-2025-026.pdf:7.97MB

The Collaborative Laboratories for Advanced Decommissioning Science (CLADS), Japan Atomic Energy Agency (JAEA), had been conducting the Nuclear Energy Science & Technology and Human Resource Development Project (hereafter referred to "the Project") in FY2023. The Project aims to contribute to solving problems in the nuclear energy field represented by the decommissioning of the Fukushima Daiichi Nuclear Power Station, Tokyo Electric Power Company Holdings, Inc. (TEPCO). For this purpose, intelligence was collected from all over the world, and basic research and human resource development were promoted by closely integrating/collaborating knowledge and experiences in various fields beyond the barrier of conventional organizations and research fields. The sponsor of the Project was moved from the Ministry of Education, Culture, Sports, Science and Technology to JAEA since the newly adopted proposals in FY2018. On this occasion, JAEA constructed a new research system where JAEA-academia collaboration is reinforced and medium-to-long term research/development and human resource development contributing to the decommissioning are stably and consecutively implemented. Among the adopted proposals in FY2022, this report summarizes the research results of the "Investigation of effects of nano interfacial phenomena on dissolution aggregation of alpha nanoparticles by using micro nano technologies" conducted in FY2023. To ensure the safety of retrieval and storage management of nuclear fuel debris generated by the Fukushima-Daiichi Nuclear Power Station accident, understanding of dissolution-denaturation behavior of the fuel debris alpha particles is one of the most crucial issues. This research aims to create novel microfluidic real-time measurement device for elucidating dissolution, aggregation, and denaturation processes of metal oxide nanoparticles under various solution environments, and clarify their nano-size and interfacial effects. In this year, we conducted bulk and micro dissolution tests of simulated fuel debris particles (UO mechanical-treated nanoparticles, UO chemical-treated nanoparticles, and (U,Zr)O nanoparticles), and successfully clarified that the effects of particle sizes, reaction times, and HO concentrations on the dissolution behavior of each nanoparticle. In particular, it was found that (U,Zr)O nanoparticles have different degrees of Zr catalytic reactions depending on HO concentrations, resulting in the generation of different amounts of gas and U. Moreover, we developed a new microfluidic device which enables to instantly react the nanoparticles with HO solutions, and determined dynamic aggregation and dissolution rates of the nanoparticles. The research was carried out in close collaboration with UK researchers, and achieved the expected goal of this year.

Study on water stopping, repair and stabilization of lower PCV by geopolymer, etc. (Contract research); FY2023 Nuclear Energy Science & Technology and Human Resource Development Project

Collaborative Laboratories for Advanced Decommissioning Science; The University of Tokyo*

JAEA-Review 2025-008, 134 Pages, 2025/09

JAEA-Review-2025-008.pdf:10.0MB

The Collaborative Laboratories for Advanced Decommissioning Science (CLADS), Japan Atomic Energy Agency (JAEA), had been conducting the Nuclear Energy Science & Technology and Human Resource Development Project (hereafter referred to "the Project") in FY2023. The Project aims to contribute to solving problems in the nuclear energy field represented by the decommissioning of the Fukushima Daiichi Nuclear Power Station, Tokyo Electric Power Company Holdings, Inc. (TEPCO). For this purpose, intelligence was collected from all over the world, and basic research and human resource development were promoted by closely integrating/collaborating knowledge and experiences in various fields beyond the barrier of conventional organizations and research fields. The sponsor of the Project was moved from the Ministry of Education, Culture, Sports, Science and Technology to JAEA since the newly adopted proposals in FY2018. On this occasion, JAEA constructed a new research system where JAEA-academia collaboration is reinforced and medium-to-long term research/development and human resource development contributing to the decommissioning are stably and consecutively implemented. Among the adopted proposals in FY2021, this report summarizes the research results of the "Study on water stopping, repair and stabilization of lower PCV by geopolymer, etc." conducted from FY2021 to FY2023. Since the final year of this proposal was FY2023, the results for 3 fiscal years were summarized. In order to retrieve fuel debris, it is necessary to shut off the water at the bottom of the dry well and repair it in order to control the PCV water level. Therefore, in this study, we evaluated a construction method of stopping the water of the jet deflector with an improved geopolymer and repairing the lower part of the dry well by experiments and simulations. In addition, after understanding the properties of the fuel debris coated with the geopolymer, the long-term life of the waste body was evaluated. As a result, it was predicted that by utilizing geopolymers, it is possible to stop water and repair the lower part of the PCV in consideration of steps from construction to waste management.

Fuel debris criticality analysis technology using non-contact measurement method (Contract research); FY2023 Nuclear Energy Science & Technology and Human Resource Development Project

Collaborative Laboratories for Advanced Decommissioning Science; Institute of Science Tokyo*

JAEA-Review 2025-010, 62 Pages, 2025/08

JAEA-Review-2025-010.pdf:3.63MB

The Collaborative Laboratories for Advanced Decommissioning Science (CLADS), Japan Atomic Energy Agency (JAEA), had been conducting the Nuclear Energy Science & Technology and Human Resource Development Project (hereafter referred to "the Project") in FY2023. The Project aims to contribute to solving problems in the nuclear energy field represented by the decommissioning of the Fukushima Daiichi Nuclear Power Station, Tokyo Electric Power Company Holdings, Inc. (TEPCO). For this purpose, intelligence was collected from all over the world, and basic research and human resource development were promoted by closely integrating/collaborating knowledge and experiences in various fields beyond the barrier of conventional organizations and research fields. The sponsor of the Project was moved from the Ministry of Education, Culture, Sports, Science and Technology to JAEA since the newly adopted proposals in FY2018. On this occasion, JAEA constructed a new research system where JAEA-academia collaboration is reinforced and medium-to-long term research/development and human resource development contributing to the decommissioning are stably and consecutively implemented. Among the adopted proposals in FY2021, this report summarizes the research results of the "Fuel debris criticality analysis technology using non-contact measurement method" conducted from FY2021 to FY2023. The purpose of research was to improve the fuel debris criticality analysis technology using non-contact measurement method by the development of the fuel debris criticality characteristics measurement system and the multi-region integral kinetic analysis code. It was performed by Institute of Science Tokyo, Tokyo City University, National Institute of Advanced Industrial Science and Technology, and Nagaoka University of Technology. We developed the fuel debris criticality characteristics measurement system which has a two layer structure surrounding a canister containing fuel debris fragments with He proportional counters. The operational validation and performance evaluation were performed on the developed detector system. We have demonstrated the feasibility and accuracy of measuring the amount of fissile material and water content. MIK2.0-MVP code, which can calculate fission reaction rate attributed to both prompt and delayed neutrons and also can take the movement of fuel debris into calculation, was developed. After parallelizing the tally process of C() function, MIK2.0-MVP code will be applicable to weakly coupled reactors which include moving fuel debris particles if a supercomputer will be used for the tally process of C() function and if the coupling of MIK2.0-MVP code with MPS will be weak.

Development of a theoretical scaling factor method for the inventory estimation of difficult-to-measure nuclide Cs-135 in fuel debris and radioactive wastes

Sakamoto, Masahiro; Okumura, Keisuke; Kanno, Ikuo; Matsumura, Taichi; Terashima, Kenichi; Riyana, E. S.; Kaneko, Junichi*; Mizokami, Masato*; Mizokami, Shinya*

Journal of Nuclear Science and Technology, 62(8), p.756 - 765, 2025/08

https://doi.org/10.1080/00223131.2025.2478926

Development of extremely small amount analysis technology for fuel debris analysis (Contract Research); FY2023 Nuclear Energy Science & Technology and Human Resource Development Project

Collaborative Laboratories for Advanced Decommissioning Science; Tohoku University*

JAEA-Review 2024-064, 118 Pages, 2025/06

JAEA-Review-2024-064.pdf:6.73MB

The Collaborative Laboratories for Advanced Decommissioning Science (CLADS), Japan Atomic Energy Agency (JAEA), had been conducting the Nuclear Energy Science & Technology and Human Resource Development Project (hereafter referred to "the Project") in FY2023. The Project aims to contribute to solving problems in the nuclear energy field represented by the decommissioning of the Fukushima Daiichi Nuclear Power Station (1F), Tokyo Electric Power Company Holdings, Inc. (TEPCO). For this purpose, intelligence was collected from all over the world, and basic research and human resource development were promoted by closely integrating/collaborating knowledge and experiences in various fields beyond the barrier of conventional organizations and research fields. The sponsor of the Project was moved from the Ministry of Education, Culture, Sports, Science and Technology to JAEA since the newly adopted proposals in FY2018. On this occasion, JAEA constructed a new research system where JAEA-academia collaboration is reinforced and medium-to-long term research/development and human resource development contributing to the decommissioning are stably and consecutively implemented. Among the adopted proposals in FY2019, this report summarizes the research results of the "Development of extremely small amount analysis technology for fuel debris analysis" conducted from FY2019 to FY2023. Understanding the properties of fuel debris is necessary for handling, criticality control, storage control, etc. A key technique is the chemical analysis of actinide nuclides. We developed sample pretreatment technology and separation / analysis process required for chemical analysis. The purpose of this study is to streamline future planned fuel debris analysis. To promote 1F decommissioning, we will train human resources through on-the-job training. In particular, we applied the extremely small amount analysis (ICP-MS/MS), which has recently been successful in the fields of analytical chemistry and radiochemistry, to the nuclear field. This method allows high-accuracy analysis without pretreatment to isolate the nuclide to be measured. The separation pretreatment can be skipped and a rapid analysis process can be established.

Characterization of neutrons emitted by an expected small amount of fuel debris in a trial retrieval from Fukushima Daiichi Nuclear Power Station

Matsumura, Taichi; Okumura, Keisuke; Sakamoto, Masahiro; Terashima, Kenichi; Riyana, E. S.; Kondo, Kazuhiro*

Nuclear Engineering and Design, 432, p.113791_1 - 113791_9, 2025/02

https://doi.org/10.1016/j.nucengdes.2024.113791

Study on water stopping, repair and stabilization of lower PCV by geopolymer, etc. (Contract research); FY2022 Nuclear Energy Science & Technology and Human Resource Development Project

Collaborative Laboratories for Advanced Decommissioning Science; The University of Tokyo*

JAEA-Review 2024-021, 126 Pages, 2024/11

JAEA-Review-2024-021.pdf:6.51MB

The Collaborative Laboratories for Advanced Decommissioning Science (CLADS), Japan Atomic Energy Agency (JAEA), had been conducting the Nuclear Energy Science & Technology and Human Resource Development Project (hereafter referred to "the Project") in FY2022. The Project aims to contribute to solving problems in the nuclear energy field represented by the decommissioning of the Fukushima Daiichi Nuclear Power Station, Tokyo Electric Power Company Holdings, Inc. (TEPCO). For this purpose, intelligence was collected from all over the world, and basic research and human resource development were promoted by closely integrating/collaborating knowledge and experiences in various fields beyond the barrier of conventional organizations and research fields. The sponsor of the Project was moved from the Ministry of Education, Culture, Sports, Science and Technology to JAEA since the newly adopted proposals in FY2018. On this occasion, JAEA constructed a new research system where JAEA-academia collaboration is reinforced and medium-to-long term research/development and human resource development contributing to the decommissioning are stably and consecutively implemented. Among the adopted proposals in FY2021, this report summarizes the research results of the "Study on water stopping, repair and stabilization of lower PCV by geopolymer, etc" conducted in FY2022. The present study aims to propose a construction method to stop jet deflectors by improved geopolymer and ultra-heavy muddy water, and to repair the lower part of the dry well. In addition, in order to increase the options for on-site construction in unknown situations such as deposition conditions, we will examine a wide range of construction outside the pedestal, and evaluate the feasibility of the construction method by the latest thermal flow simulation method.

Fuel debris simulants for the gripper design of decommissioning robot manipulator in Fukushima Daiichi (1F)

Nakashima, Shinsuke*; Moro, A.*; Komatsu, Ren*; Faragasso, A.*; Matsuhira, Nobuto*; Woo, H.*; Kawabata, Kuniaki; Yamashita, Atsushi*; Asama, Hajime*

Proceedings of 2024 IEEE International Symposium on Safety, Security, and Rescue Robotics (SSRR2024), p.160 - 165, 2024/11

https://doi.otg/10.1109/SSRR62954.2024.10770049