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Sono, Hiroki; Izawa, Kazuhiko; Yoritsune, Tsutomu; Suyama, Kenya; Tonoike, Kotaro
Proceedings of 12th International Conference on Nuclear Criticality Safety (ICNC2023) (Internet), 7 Pages, 2023/10
Japan Atomic Energy Agency (JAEA) has constructed and operated nine critical assemblies. Of these nine facilities as of 2023, four have already been dismantled, four are under decommissioning, and only STACY is active but under temporally shutdown. STACY is scheduled to restart in 2024 after core modification from a "critical assembly using uranium nitrate solution fuel" to a "general-purpose critical assembly using uranium fuel rods and light-water moderator." The immediate objective of new STACY is to acquire criticality data for fuel debris removal from the damaged reactors in Fukushima-Daiichi Nuclear Power Plant. After the critical experiment program regarding fuel debris, the new STACY is expected to be used for various R&D on next-generation power reactors and others. In addition, the new STACY will serve as an educational and training reactor. These activities are useful not only for Japan but also for international collaborative research and joint use.
Kobayashi, Fuyumi; Fukaya, Hiroyuki; Izawa, Kazuhiko; Kida, Takashi; Sono, Hiroki; Suyama, Kenya
Proceedings of 12th International Conference on Nuclear Criticality Safety (ICNC2023) (Internet), 7 Pages, 2023/10
In the criticality experiment in the new STACY, pseudo fuel debris samples are used to acquire data for validation of the system used for 1F debris criticality safety assessment. The pseudo fuel debris is a pellet with a diameter of 8 mm and a height of 10 mm containing uranium oxide and structural materials (iron, silicon, zirconium, etc.). The pellets are made by mixing, pressing and sintering uranium dioxide powder and structural materials powder. The UO
powder uses the same composition of uranium as the STACY driver fuel rods, in order to reduce the errors in fuel composition. The pseudo fuel debris fabrication devices and analysis equipment are installed at the BECKY in order to evaluate the critical properties of fuel debris with high accuracy in dimension and analysis. This equipment is located in the same laboratory and can quickly respond to experimental needs such as preparation of the pseudo fuel debris and analysis before and after irradiation.
Yonomoto, Taisuke; Nakashima, Hiroshi*; Sono, Hiroki; Kishimoto, Katsumi; Izawa, Kazuhiko; Kinase, Masami; Osa, Akihiko; Ogawa, Kazuhiko; Horiguchi, Hironori; Inoi, Hiroyuki; et al.
JAEA-Review 2020-056, 51 Pages, 2021/03
JAEA-Review-2020-056.pdf:3.26MB
A group named as "The group for investigation of reasonable safety assurance based on graded approach", which consists of about 10 staffs from Sector of Nuclear Science Research, Safety and Nuclear Security Administration Department, departments for management of nuclear facility, Sector of Nuclear Safety Research and Emergency Preparedness, aims to realize effective graded approach (GA) about management of facilities and regulatory compliance of JAEA. The group started its activities in September, 2019 and has had discussions through 10 meetings and email communications. In the meetings, basic ideas of GA, status of compliance with new regulatory standards at each facility, new inspection system, etc were discussed, while individual investigation at each facility were shared among the members. This report is compiled with expectation that it will help promote rational and effective safety management based on GA by sharing contents of the activity widely inside and outside JAEA.
Sono, Hiroki; Sukegawa, Kazuhiro; Nomura, Norio; Okuda, Eiichi; Study Team on Safety and Maintenance; Study Team on Quality Management; Task Force on New Nuclear Regulatory Inspection Systems
JAEA-Technology 2020-013, 460 Pages, 2020/11
JAEA-Technology-2020-013.pdf:13.46MB
Japan Atomic Energy Agency (JAEA) has completed the introduction of a new frame work of safety, maintenance and quality management activities under the new acts on the Regulation of nuclear source material, nuclear fuel material and reactors since April 2020, in consideration of variety, specialty and similarity of nuclear facilities of JAEA (Power reactor in the research and development stage, Reprocessing facility, Fabrication facility, Waste treatment facility, Waste burial facility, Research reactor and Nuclear fuel material usage facility). The JAEA task forces on new nuclear regulatory inspection systems prepared new guidelines on (1) Safety and maintenance, (2) Independent inspection, (3) Welding inspection, (4) Free-access response, (5) Performance indicators and (6) Corrective action program for the JAEA's nuclear facilities. New Quality management systems and new Safety regulations were also prepared as a typical pattern of these facilities. JAEA will steadily improve these guidelines, quality management systems and safety regulations, reviewing the official activities under the new regulatory inspection system together with the Nuclear Regulation Authority and other nuclear operators.
Nagao, Fumiya; Niizato, Tadafumi; Sasaki, Yoshito; Ito, Satomi; Watanabe, Takayoshi; Dohi, Terumi; Nakanishi, Takahiro; Sakuma, Kazuyuki; Hagiwara, Hiroki; Funaki, Hironori; et al.
JAEA-Research 2020-007, 249 Pages, 2020/10
JAEA-Research-2020-007.pdf:15.83MB
The accident of the Fukushima Daiichi Nuclear Power Station, Tokyo Electric Power Company Holdings, Inc. occurred due to the Great East Japan Earthquake, Sanriku offshore earthquake, of 9.0 magnitude and the accompanying tsunami. As a result, large amount of radioactive materials was released into the environment. Under these circumstances, Japan Atomic Energy Agency (JAEA) has been conducting "Long-term Assessment of Transport of Radioactive Contaminants in the Environment of Fukushima" concerning radioactive materials released in environment, especially migration behavior of radioactive cesium since November 2012. This report is a summary of the research results that have been obtained in environmental dynamics research conducted by JAEA in Fukushima Prefecture.
Hagiwara, Hiroki; Nakanishi, Takahiro; Konishi, Hiromi*; Tsuruta, Tadahiko; Misono, Toshiharu; Fujiwara, Kenso; Kitamura, Akihiro
Journal of Environmental Radioactivity, 220-221, p.106294_1 - 106294_9, 2020/09
Times Cited Count：0 Percentile：0.00(Environmental Sciences)Nagao, Fumiya; Niizato, Tadafumi; Sasaki, Yoshito; Ito, Satomi; Watanabe, Takayoshi; Dohi, Terumi; Nakanishi, Takahiro; Sakuma, Kazuyuki; Hagiwara, Hiroki; Funaki, Hironori; et al.
JAEA-Research 2019-002, 235 Pages, 2019/08
JAEA-Research-2019-002.pdf:21.04MB
The accident of the Fukushima Daiichi Nuclear Power Station (hereinafter referred to 1F), Tokyo Electric Power Company Holdings, Inc. occurred due to the Great East Japan Earthquake, Sanriku offshore earthquake, of 9.0 magnitude and the accompanying tsunami. As a result, large amount of radioactive materials was released into the environment. Under these circumstances, JAEA has been conducting Long-term Environmental Dynamics Research concerning radioactive materials released in environment, especially migration behavior of radioactive cesium since November 2012. This report is a summary of the research results that have been obtained in environmental dynamics research conducted by JAEA in Fukushima Prefecture.
Myagmarjav, O.; Iwatsuki, Jin; Tanaka, Nobuyuki; Noguchi, Hiroki; Kamiji, Yu; Ioka, Ikuo; Kubo, Shinji; Nomura, Mikihiro*; Yamaki, Tetsuya*; Sawada, Shinichi*; et al.
International Journal of Hydrogen Energy, 44(35), p.19141 - 19152, 2019/07
Times Cited Count：18 Percentile：49.82(Chemistry, Physical)Seki, Masakazu; Maekawa, Tomoyuki; Izawa, Kazuhiko; Sono, Hiroki
JAEA-Technology 2017-038, 52 Pages, 2018/03
JAEA-Technology-2017-038.pdf:4.6MB
The Japan Atomic Energy Agency is conducting a reactor modification project of the Static Experiment Critical Facility (STACY). In the modification, STACY is to be converted from a thermal reactor using solution fuel into that using fuel rods and light water moderator. Reactivity of the modified STACY core is controlled by the water level fed in the core tank as well as the present STACY. In order to verify the basic design of the water feed and drain system of the modified STACY, we constructed a mockup test apparatus with almost the same structure and specifications as the modified STACY. In the mockup test, performance checks were pursued regarding limitation of maximum flow of water feeding, adjustment of the flow rate of water feeding, stop of water feeding and others. This report describes the outline and results of the mock-up test of the water feed and drain system of the modified STACY.
Gunji, Satoshi; Tonoike, Kotaro; Izawa, Kazuhiko; Sono, Hiroki
Progress in Nuclear Energy, 101(Part C), p.321 - 328, 2017/11
Times Cited Count：3 Percentile：26.88(Nuclear Science & Technology)Criticality safety of fuel debris, particularly MCCI (Molten-Core-Concrete-Interaction) products, is one of the major safety issues for decommissioning of Fukushima Daiichi Nuclear Power Station. Criticality or subcriticality condition of the fuel debris is still uncertain; its composition, location, neutron moderation, etc. are not yet confirmed. The effectiveness of neutron poison in cooling water is also uncertain for use as a criticality control of fuel debris. A database of computational models is being built by Japan Atomic Energy Agency (JAEA), covering a wide range of possible conditions of such composition, neutron moderation, etc., to facilitate assessing criticality characteristics once fuel debris samples are taken and their conditions are known. The computational models also include uncertainties which are to be clarified by critical experiments. These experiments are planned and will be conducted by JAEA with the modified STACY (STAtic experiment Critical facilitY) and samples to simulate fuel debris compositions. Each of the samples will be cladded by a zircalloy tube whose outer shape is compatible with the fuel rod of STACY and loaded into an array of the fuel rods. This report introduces a study of experimental core configurations to measure the reactivity worth of samples simulating MCCI products. Parameters to be varied in the computation models for the experimental series are:(1) Uranium dioxide with 
U enrichments of 3, 4, and 5 wt.%; (2) Concrete volume fraction in the samples of 0, 20, 40, 60, and 80%; and (3) Porosity of the samples filled from 0 to 80% where the sample void is filled with water. It is concluded that the measurement is feasible in both under- and over-moderated conditions. Additionally, the required amount of samples was estimated.
Kobayashi, Fuyumi; Sumiya, Masato; Kida, Takashi; Kokusen, Junya; Uchida, Shoji; Kaminaga, Jota; Oki, Keiichi; Fukaya, Hiroyuki; Sono, Hiroki
JAEA-Technology 2016-025, 42 Pages, 2016/11
JAEA-Technology-2016-025.pdf:17.88MB
A preliminary test on MOX fuel dissolution for the STACY critical experiments had been conducted in 2000 through 2003 at Nuclear Science Research Institute of JAEA. Accordingly, the uranyl / plutonium nitrate solution should be reconverted into oxide powder to store the fuel for a long period. For this storage, the moisture content in the oxide powder should be controlled from the viewpoint of criticality safety. The stabilization of uranium / plutonium solution was carried out under a precipitation process using ammonia or oxalic acid solution, and a calcination process using a sintering furnace. As a result of the stabilization operation, recovery rate was 95.6% for uranium and 95.0% for plutonium. Further, the recovered oxide powder was calcined again in nitrogen atmosphere and sealed immediately with a plastic bag to keep its moisture content low and to prevent from reabsorbing atmospheric moisture.
Gunji, Satoshi; Tonoike, Kotaro; Izawa, Kazuhiko; Sono, Hiroki
Proceedings of International Conference on the Physics of Reactors; Unifying Theory and Experiments in the 21st Century (PHYSOR 2016) (USB Flash Drive), p.3927 - 3936, 2016/05
Criticality safety of fuel debris including MCCI products is one of the major safety is-sues for decommissioning of Fukushima Daiichi Nuclear Power Station. Criticality or subcriticality condition of the fuel debris is still uncertain since its composition, location, neutron moderation, etc. are not confirmed. Also uncertain in criticality control of fuel debris is the effectiveness of neutron poison in cooling water. A database is being built by computation in JAEA, covering a wide range of possible conditions of such composition, neutron moderation, etc., to facilitate assessing criticality characteristics when fuel debris samples are taken and their conditions are known. The computation also has uncertainties to be clarified by critical experiments, which is planned by JAEA to be conducted with the modified STACY and samples simulating fuel debris compositions. This report introduces a study of experimental core configurations for reactivity worth measurements of samples simulating MCCI products. It is concluded that the measurement is feasible in both under- and over-moderated conditions. Additionally, required amount of samples was estimated.
Tonoike, Kotaro; Yamane, Yuichi; Umeda, Miki; Izawa, Kazuhiko; Sono, Hiroki
Proceedings of International Conference on Nuclear Criticality Safety (ICNC 2015) (DVD-ROM), p.20 - 27, 2015/09
From the viewpoint of safety regulation, criticality control of the fuel debris in the Fukushima Daiichi Nuclear Power Station would be a risk-informed control to mitigate consequences of criticality events, instead of a deterministic control to prevent such events. The Nuclear Regulation Authority of Japan has set up a research and development program to tackle this challenge. The Nuclear Safety Research Center of Japan Atomic Energy Agency, commissioned by the authority, has launched activities such as computations of criticality characteristics of the fuel debris, development of criticality risk assessment method, and preparation of criticality experiments to support them.
Seki, Masakazu; Izawa, Kazuhiko; Sono, Hiroki
JAEA-Technology 2014-047, 22 Pages, 2015/03
JAEA-Technology-2014-047.pdf:2.33MB
The Japan Atomic Energy Agency is conducting a reactor modification project of the Static Experiment Critical Facility (STACY). In the modification, STACY is to be converted from a thermal reactor using solution fuel into that using fuel rods and light water moderator. Reactivity of the modified STACY is controlled by the water level fed in the core tank as well as the present STACY. Regarding water level detection, however, a float-type water-feed-stop switch is adopted in the modified STACY because the electro-conductivity-type switch of the present STACY for uranyl nitrate solution cannot detect demineralized water used in the modified STACY. For safety operation of the modified STACY, the float-type switch needs accurate and reliable detection of water level at any temperature. This report describes a mock-up test on accuracy verification of the float-type water-feed-stop switch in whole range of water temperature (room temperature 
70
C) in the modified STACY operation.
Izawa, Kazuhiko; Tonoike, Kotaro; Sono, Hiroki; Miyoshi, Yoshinori
JAEA-Conf 2014-003, Appendix (CD-ROM), 13 Pages, 2015/03
Critical assemblies of thermal neutron system are decreasing in number in spite of their important roles in the reactor physics research. On the other hand, the extension of the utilization term of the LWRs brings new research themes requiring critical experiments of thermal neutron system. JAEA is modifying the Static Critical Experiment Facility (STACY) to revive the critical experiments. The modified STACY will be an infrastructure for the experimental research of reactor physics on thermal neutron system. The primary mission of the modified STACY at present is the critical experiments for fuel debris to contribute to the criticality safety control of the fuel debris generated by the severe accident of the Fukushima Daiichi Nuclear Power Station. This report introduces the plan of criticality safety research in Japan Atomic Energy Agency following the accident, and describes the role of the modified STACY in the retrieval work of fuel debris from the damaged reactor.
Tonoike, Kotaro; Sono, Hiroki; Umeda, Miki; Yamane, Yuichi; Kugo, Teruhiko; Suyama, Kenya
Nuclear Back-end and Transmutation Technology for Waste Disposal, p.251 - 259, 2015/00
In the Three Mile Island Unit 2 reactor accident, a large amount of fuel debris was formed whose criticality condition is unknown except the possible highest U/U enrichment. The fuel debris had to be cooled and shielded by water in which the minimum critical mass is much smaller than the total mass of fuel debris. To overcome this uncertain situation, the coolant water was borated with sufficient concentration to secure the subcritical condition. The situation is more severe in the damaged reactors of Fukushima Daiichi Nuclear Power Station, where the coolant water flow is practically "once through". Boron must be endlessly added to the water to secure the subcritical condition of the fuel debris, which is not feasible. The water is not borated relying on the circumstantial evidence that the xenon gas monitoring in the containment vessels does not show a sign of criticality. The criticality condition of fuel debris may worsen due to the gradual drop of its temperature, or the change of its geometry by aftershocks or the retrieval work, that may lead the criticality. To avoid criticality and its severe consequences, a certain principle of criticality control must be established. There may be options, such as prevention of the criticality by coolant water boration or by neutronic monitoring, prevention of the severe consequences by intervention measures against criticality, etc. Every option has merits and demerits that must be adequately evaluated toward selection of the best principle.
Tonoike, Kotaro; Izawa, Kazuhiko; Sono, Hiroki; Umeda, Miki; Yamane, Yuichi
Transactions of the American Nuclear Society, 110(1), p.282 - 285, 2014/06
no abstracts in English
Tonoike, Kotaro; Sono, Hiroki; Umeda, Miki; Yamane, Yuichi; Kugo, Teruhiko; Suyama, Kenya
Proceedings of International Nuclear Fuel Cycle Conference; Nuclear Energy at a Crossroads (GLOBAL 2013) (CD-ROM), p.729 - 735, 2013/09
JAEA is conducting studies on criticality control of the fuel debris formed in the accident of Fukushima-Daiichi site. A new control principle must be established, referring principles for existing facilities, and based on criticality characteristics of the debris. In accordance with the principle, safe and practical control has to be realized for the debris whose condition is uncertain at present. This report outlines the present condition of debris and Fukushima site, introduces examples of criticality analysis, and discusses control principles. Research subjects are also proposed to realize the control.
-concrete systemIzawa, Kazuhiko; Uchida, Yuriko; Okubo, Kiyoshi; Totsuka, Masayoshi; Sono, Hiroki; Tonoike, Kotaro
Journal of Nuclear Science and Technology, 49(11), p.1043 - 1047, 2012/11
Times Cited Count：12 Percentile：65.44(Nuclear Science & Technology)Possibility of criticality of fuel debris in a form of UO-concrete mixture is evaluated by calculating infinite multiplication factor (
) for a study of criticality control on the fuel debris generated through the molten core concrete interaction (MCCI) in a severe accident of a light water reactor (LWR). The infinite multiplication factor can be greater than unity, which means that handling of the mixture is subject to criticality control. This paper shows that concrete have efficient slowing-down capability of neutron and points out the necessity of further investigations on the criticality of low-enriched UO-concrete system for actual handling of fuel debris.
Izawa, Kazuhiko; Aoyama, Yasuo; Sono, Hiroki; Ogawa, Kazuhiko; Yanagisawa, Hiroshi; Miyoshi, Yoshinori
Proceedings of 9th International Conference on Nuclear Criticality (ICNC 2011) (CD-ROM), 11 Pages, 2012/02
For reactor physics and criticality safety researches, the Static Experiment Critical Facility (STACY) will be modified. In the modification, the present STACY, solution-fuel-type homogeneous cores, will be converted to fuel-pin-type heterogeneous cores moderated by light water. For nuclear safety design of the modified STACY, computational analyses have been carried out by using a Monte Carlo code MVP and a transport code system DANTSYS with cross-section data based on the JENDL-3.3. In the analyses, basic nuclear characteristics have been evaluated, such as criticality, water-level worth and reactor shutdown margin. By the results of these analyses, the feasibility of reactivity control mechanism and the sufficiency of reactor shutdown margin of the modified STACY were confirmed. In addition, temperature and void coefficients of reactivity and kinetic parameters were obtained to comprehend nuclear characteristics of the modified STACY.
