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Suyama, Kenya; Ueki, Taro; Gunji, Satoshi; Watanabe, Tomoaki; Araki, Shohei; Fukuda, Kodai; Yamane, Yuichi; Izawa, Kazuhiko; Nagaya, Yasunobu; Kikuchi, Takeo; et al.
Proceedings of 12th International Conference on Nuclear Criticality Safety (ICNC2023) (Internet), 6 Pages, 2023/10
To remove and store safely the fuel debris generated by the severe accident of the Fukushima Daiichi Nuclear Power Station in 2011 is one of the most important and challenging topics for decommissioning of the damaged reactors in Fukushima. To validate the adopted method for the evaluation of criticality safety control of the fuel debris through comparison with the experimental data obtained by the criticality experiments, the Nuclear Regulation Authority (NRA) of Japan funds a research and development project which was entrusted to the Nuclear Safety Research Center (NSRC) of Japan Atomic Energy Agency (JAEA) from 2014. In this project, JAEA has been conducting such activities as i) comprehensive computation of the criticality characteristics of the fuel debris and making database (criticality map of the fuel debris), ii) development of new continuous energy Monte Carlo code, iii) evaluation of criticality accident and iv) modification of the critical assembly STACY for the experiments for validation of criticality safety control methodology. After the last ICNC2019, the project has the substantial progress in the modification of STACY which will start officially operation from May 2024 and the development of the Monte Carlo Code "Solomon" suitable for the criticality calculation for materials having spatially random distribution complies with the power spectrum. We present the whole picture of this research and development project and status of each technical topics in the session.
Gunji, Satoshi; Yoshikawa, Tomoki; Araki, Shohei; Izawa, Kazuhiko; Suyama, Kenya
Proceedings of 12th International Conference on Nuclear Criticality Safety (ICNC2023) (Internet), 8 Pages, 2023/10
Since the compositions and properties of the fuel debris are uncertain, critical experiments are required to validate calculation codes and nuclear data used for the safety evaluation. For this purpose, JAEA has been modifying a critical assembly called "STACY". The first criticality of the new STACY is scheduled for spring 2024. This paper reports the consideration results of the core configurations of the new STACY at the first criticality. We prepared two sets of gird plates with different neutron moderation conditions (their intervals are 1.50 cm and 1.27 cm). However, there is a limitation on the number of available UO
fuel rods. In addition, we would like to set the critical water heights for the first criticality at around 95 cm. This is to avoid the reactive effect of the aluminum alloy middle grid plates (Approx. 98 cm high). The core configurations for the first criticality satisfying these conditions were constructed by computational analysis. A square core configuration with the 1.50 cm grid plate that is close to the optimum moderation condition needs 261 fuel rods to reach criticality. As to the 1.27 cm grid plate, we considered two core configurations with 1.80 cm intervals by using a checkerboard arrangement. One of them has two regions core configuration with 1.27 and 1.80 cm intervals, and the other has only 1.80 cm intervals. They need 341 and 201 fuel rods for the criticality, respectively. This paper shows these three core configurations and their calculation models.
Gunji, Satoshi; Araki, Shohei; Arakaki, Yu; Izawa, Kazuhiko; Suyama, Kenya
Proceedings of 12th International Conference on Nuclear Criticality Safety (ICNC2023) (Internet), 9 Pages, 2023/10
JAEA has been modifying a critical assembly called STACY from a solution system to a light-water moderated heterogeneous system to validate computation results of criticality characteristics of fuel debris generated in the accident at TEPCO's Fukushima Daiichi Nuclear Power Station. To experimentally simulate the composition and characteristics of fuel debris, we will prepare several grid plates which make particular neutron moderation conditions and a number of rod-shaped concrete and stainless-steel materials. Experiments to evaluate fuel debris's criticality characteristics are scheduled using these devices and materials. This series of STACY experiments are planned to measure the reactivity of fuel debris-simulated samples, measure the critical mass of core configurations containing structural materials such as concrete and stainless steels, and the change in critical mass when their arrangement becomes non-uniform. Furthermore, two divided cores experiments are scheduled that statically simulate fuel debris falling, and also scheduled that subcriticality measurement experiments with partially different neutron moderation conditions. The experimental plans have been considered taking into account some experimental constraints. This paper shows the schedule of these experiments, as well as the computation results of the optimized core configurations and expected results for each experiment.
Gunji, Satoshi; Araki, Shohei; Watanabe, Tomoaki; Fernex, F.*; Leclaire, N.*; Bardelay, A.*; Suyama, Kenya
Proceedings of 12th International Conference on Nuclear Criticality Safety (ICNC2023) (Internet), 9 Pages, 2023/10
Institut de radioprotection et de s
ret
nuclaire (IRSN) and Japan Atomic Energy Agency (JAEA) have a long-standing partnership in the field of criticality safety. In this collaboration, IRSN and JAEA are planning a joint experiment using the new STACY critical assembly, modified by JAEA. In order to compare the codes (MVP3, MORET6, etc.) and nuclear data (JENDL and JEFF) used by both institutes in the planning of the STACY experiment, benchmark calculations of the Apparatus B and TCA, which are critical assemblies once owned by both institutes, benchmarks from the ICSBEP handbook and the computational model of the new STACY were performed. Including the new STACY calculation model, the calculations include several different neutron moderation conditions and critical water heights. There were slight systematic differences in the calculation results, which may have originated from the processing and/or format of the nuclear data libraries. However, it was found that the calculated results, including the new codes and the new nuclear data, are in good agreement with the experimental values. Therefore, there are no issues to use them for the design of experiments for the new STACY. Furthermore, the impact of the new TSL data included in JENDL-5 on the effective multiplication factor was investigated. Experimental validation for them will be completed by critical experiments of the new STACY by both institutes.
Araki, Shohei; Gunji, Satoshi; Arakaki, Yu; Yoshikawa, Tomoki; Murakami, Takahiko; Kobayashi, Fuyumi; Izawa, Kazuhiko; Suyama, Kenya
Proceedings of 12th International Conference on Nuclear Criticality Safety (ICNC2023) (Internet), 8 Pages, 2023/10
New experiments simulating fuel debris in the new criticality assembly, STACY, are designed to contribute to the validation of criticality calculations for criticality control of the fuel debris in the Fukushima Daiichi Nuclear Power Plant accident. In the new STACY experiment, a two-region core consisting of a driver region and a test region was investigated in order to configure a debris-simulated core with under-moderation condition (lattice pitch 1.27-cm) having the constraint of available fuel rod number. The test region with a 1.27-cm lattice pitch is surrounded by the driver region, in which fuel rods are arranged in a checkerboard pattern on a 1.27-cm lattice plate, with a 1.80-cm lattice pitch. Neutron spectra and sensitivity were calculated by using MCNP6 and ENDF/B-VII. The core which has a 17
17 test region with 373 fuel rods is the largest two-region core under the constraint. It was found that the core which has a 1717 test region can simulate the neutron spectra of under-moderation condition in a 1313 region inside the test region with the root-mean square percentage error of less than 5%. It was also confirmed that the sensitivity of 
Si and 
Ca (n,
) reactions when the concrete simulant, was loaded could be simulated.
Kawaguchi, Maho*; Shiba, Shigeki*; Iwahashi, Daiki*; Okawa, Tsuyoshi*; Gunji, Satoshi; Izawa, Kazuhiko; Suyama, Kenya
Proceedings of 12th International Conference on Nuclear Criticality Safety (ICNC2023) (Internet), 8 Pages, 2023/10
The Nuclear Regulation Authority (NRA) has been working on an experimental approach for evaluating the criticality of fuel debris produced by the Fukushima Daiichi Nuclear Power Plant (FDNP) accident since 2014, collaborating with the Japan Atomic Energy Agency (JAEA). As part of the approach, JAEA has modified the STAtic experiment Critical facilitY (STACY) for critical experiments to evaluate characteriscs of pseudo-fuel debris. As the preliminary analyses, we verified critical characteristics with major nuclear data libraries for the proposed core configuration patterns. The three-dimensional continuous-energy Monte Carlo neutron and photon transport code, SERPENT-V2.2.0 was used with the latest JENDL, JENDL-5. As a result, larger multiplication factors of JENDL-5 across the modified STACY core configuration patterns were evaluated in comparison to the other libraries. And, 
H scattering and 
U fission sensitivity coefficients of JENDL-5 were different from those of the other libraries. Comparing among analyses with those libraries, the updated S(
, 
) of JENDL-5 might affect the result of critical characteristics in the critical analyses for the modified STACY core configuration.
Araki, Shohei; Gunji, Satoshi; Arakaki, Yu; Murakami, Takahiko; Yoshikawa, Tomoki; Hasegawa, Kenta; Tada, Yuta; Izawa, Kazuhiko; Suyama, Kenya
Proceedings of 4th Reactor Physics Asia Conference (RPHA2023) (Internet), 4 Pages, 2023/10
To conduct integrated thermal power measurements for the performance test of the modified STACY, we re-analyzed the experimental data measured in the solution fuel STACY using the activation method. We validated its feasibility under the limited number of activation detectors. The re-analyzed results of the activation method by using MVP and PHITS with JENDL-4.0 indicated that the effect of the difference of the position between activation detectors was small enough, and the results agreed with that of the fission product analysis within almost 10%. It is conceivable that the activation method could be adopted instead of the fission product analysis.
variations by nonuniform core configurationsGunji, Satoshi; Araki, Shohei; Suyama, Kenya
Nuclear Science and Engineering, 197(8), p.2017 - 2029, 2023/08
Times Cited Count：0 Percentile：0.01(Nuclear Science & Technology)The fuel debris generated by the accident at the Tokyo Electric Power Company's Fukushima Daiichi Nuclear Power Plant is expected to have not only heterogeneous but also nonuniform compositions. Similarly, damaged fuel assemblies remaining in the reactor vessels also have nonuniform configurations due to some missing fuel rods. This non-uniformity may cause changing neutron multiplication factors. The effect of non-uniformity on the neutron multiplication factor is clarified by computations, and the possibility of experimentally validating the computations used for criticality management is being investigated. For this purpose, in this study the criticality effects of several core configurations of a new critical assembly, STACY, of the Japan Atomic Energy Agency with nonuniform arrangements of uranium oxide fuel rods, concrete rods, and stainless-steel rods were studied to confirm benchmarking potential. The difference in these arrangements changed the neutron multiplication factor by more than 1 $. We confirmed that changes in local neutron moderation conditions and the clustering of specific components caused this effect. In addition, the feasibility of benchmark experimental cores with nonuniform arrangements is evaluated. If benchmarking of such experiments could be realized, it would help to validate calculation codes and to develop criticality management methods by machine learning.
Shiba, Shigeki*; Iwahashi, Daiki*; Okawa, Tsuyoshi*; Gunji, Satoshi; Izawa, Kazuhiko; Suyama, Kenya
Proceedings of 30th International Conference on Nuclear Engineering (ICONE30) (Internet), 9 Pages, 2023/05
The Nuclear Regulation Authority (NRA) has tackled the experimental approach for determining the criticality of pseudo-fuel debris plausibly simulating actual fuel debris since 2014, collaborating with the Japan Atomic Energy Agency. To elucidate the characteristics of the pseudo-fuel debris, the Japan Atomic Energy Agency modified the STACY (STAtic experiment Critical facilitY) to conduct critical experiments simulating fuel debris. Thus, we proposed three types of modified STACY core configurations. In critical experiments in the modified STACY core, it is important to judge whether the proposed modified STACY core configurations are representative of molten core-concrete interaction debris or not. In this study, we built pseudo-fuel debris models considering a volume ratio of pseudo-fuel debris to moderation (V
/V
) and calculated uncertainty-based similarity values (C
) between the modified STACY core configurations and pseudo-fuel debris models using Tools for Sensitivity and Uncertainty Analysis Methodology Implementation-Indices and Parameters (TSUNAMI-IP) in SCALE 6.2. Consequently, the modified STACY core configuration loading structure rods we proposed completely resulted in high similarity to the pseudo-fuel debris models through V
/V
values. The main contributions to C values were 
U 
, U 
, and 
Fe (n,), except for the pseudo-fuel debris model, including extremely high concrete components.
Gunji, Satoshi; Tonoike, Kotaro; Clavel, J.-B.*; Duhamel, I.*
Journal of Nuclear Science and Technology, 58(1), p.51 - 61, 2021/01
Times Cited Count：1 Percentile：12.16(Nuclear Science & Technology)The new critical assembly STACY will be able to contribute to the validation of criticality calculations related to the fuel debris. The experimental core designs are in progress in the frame of JAEA/IRSN collaboration. This paper presents the method applied to optimize the design of the new STACY core to measure the criticality characteristics of pseudo fuel debris that simulated Molten Core Concrete Interaction (MCCI) of the fuel debris. To ensure that a core configuration is relevant for code validation, it is important to evaluate the reactivity worth of the main isotopes of interest and their k
sensitivity to their cross sections. In the case of the fuel debris described in this study, especially for the concrete composition, silicon is the nucleus with the highest k sensitivity to the cross section. For this purpose, some parameters of the core configuration, as for example the lattice pitches or the core dimensions, were adjusted using optimization algorithm to find efficiently the optimal core configurations to obtain high sensitivity of silicon capture cross section. Based on these results, realistic series of experiments for fuel debris in the new STACY could be defined to obtain an interesting feedback for the MCCI. This methodology is useful to design other experimental conditions of the new STACY.
Araki, Shohei; Gunji, Satoshi; Tonoike, Kotaro; Kobayashi, Fuyumi; Izawa, Kazuhiko; Ogawa, Kazuhiko
Proceedings of European Research Reactor Conference 2020 (RRFM 2020) (Internet), 7 Pages, 2020/10
Critical experiments of thermal neutron system are still expected to be playing an important role for wide technical issues. The Japan Atomic Energy Agency (JAEA) is renovating the Static Experimental Critical Facility (STACY) to maintain the experimental capability. The new STACY is designed as a general-purpose criticality facility. Its core mainly consists of low enriched UO fuel rods, grid plates, and light water moderator. The first experiment campaign in the new STACY aims to obtain criticality characteristics of fuel debris, which will be used in validation of criticality analysis methods. The designs of the experimental core configurations are in progress.
Izawa, Kazuhiko; Ishii, Junichi; Okubo, Takuya; Ogawa, Kazuhiko; Tonoike, Kotaro
Proceedings of 11th International Conference on Nuclear Criticality Safety (ICNC 2019) (Internet), 9 Pages, 2019/09
Japan Atomic Energy Agency, JAEA, is conducting the renewal program of the heterogeneous water moderated critical assembly STACY (Static Experiment Critical Facility) in order to verify the criticality calculation considering fuel debris which have been produced in the accident of Fukushima Daiichi Nuclear Power Station. The first criticality of the new STACY is scheduled at the beginning of 2021. After the first criticality, it is necessary to perform a series of critical experiments with a series of basic experimental core in order to gain a proficiency of operators and grasp the uncertainty that accompanies the result of critical experiments in STACY. Prior to the construction of the new STACY, a series of neutronic calculation was carried out for licensing and planning first series of critical experiment. In this paper, possible core configuration of the basic experimental core and their limitations are discussed and presented.
Gunji, Satoshi; Clavel, J.-B.*; Tonoike, Kotaro; Duhamel, I.*
Proceedings of 11th International Conference on Nuclear Criticality Safety (ICNC 2019) (Internet), 11 Pages, 2019/09
The new criticality experiments facility STACY will be able to contribute to the validation of criticality calculations related to the fuel debris. The experimental core design is in progress in the frame of JAEA/IRSN collaboration. This paper presents the method applied to optimize the design of core configurations of the new STACY to measure the criticality characteristics of pseudo fuel debris focused on Molten Core Concrete Interaction (MCCI) debris. To ensure that a core configuration is relevant for code validation, it is important to evaluate the reactivity worth of the main isotopes and the keff sensitivity to their cross sections. To obtain maximum sensitivity of Si capture reaction, some parameters of the core configuration, as for example the lattice pitch or the core dimensions, were adjusted using optimization algorithm to research efficiently the optimal core configurations.
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：28.82(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.
Izawa, Kazuhiko; Tonoike, Kotaro; Leclaire, N.*; Duhamel, I.*
Proceedings of International Conference on Nuclear Criticality Safety (ICNC 2015) (DVD-ROM), p.965 - 976, 2015/09
In the post-Fukushima prospect, Japan Atomic Energy Agency, JAEA, intends to re-open its Static Experiment Critical Facility STACY, in order to contribute to the validation of fuel debris calculations in conditions close to those affected by the Fukushima Daiichi Nuclear Power Station accident. The reactor core of STACY is being modified by JAEA from homogeneous solution fuel core to heterogeneous pin-type fuel core. The first step of critical experiments is to establish reference experiments involving lattices of UO
rods without fuel debris. A design of such experiments is in progress for the widest range of moderation ratios in the frame of JAEA/IRSN collaboration. For that purpose, perturbation calculation technique and stochastic geometry modeling technique were used with the MCNP5 and the JENDL-4.0 evaluated nuclear data library. The proposed paper will discuss all the aforementioned issues.
Kokusen, Junya; Seki, Masakazu; Abe, Masayuki; Nakazaki, Masato; Kida, Takashi; Umeda, Miki; Kihara, Takehiro; Sugikawa, Susumu
JAERI-Tech 2005-004, 53 Pages, 2005/03
JAERI-Tech-2005-004.pdf:5.92MB
This report presents operating records of dissolution of uranium dioxide and concentration of uranyl nitrate solution and acid removal, which have been performed from 1994 through 2003, for the purpose of feeding 10% and 6% enriched uranyl nitrate solution fuel to Static Experimental Critical Facility(STACY) and Transient Experimental Critical Facility(TRACY) in Nuclear Fuel Safety Engineering Facility(NUCEF).
Shimizu, Kaori; Gunji, Kazuhiko*; Haga, Takahisa*; Fukaya, Hiroyuki; Sonoda, Takashi; Sakazume, Yoshinori; Sakai, Yutaka*; Akutsu, Hideyuki; Niitsuma, Yasushi*; Inoue, Takeshi; et al.
JAERI-Tech 2004-078, 27 Pages, 2005/02
JAERI-Tech-2004-078.pdf:1.84MB
Analysis of the uranyl nitrate solution fuel are carried out at the analytical laboratory, NUCEF (the Nuclear Fuel Cycle Engineering Research Facility), which provide essential data for the operations of STACY (the Static Experiment Critical Facility), TRACY (the Transient Experiment Critical Facility) and the fuel treatment system.In the FY 2003, analysis of the uranyl nitrate solution fuel from STACY/TRACY on its pre- and post-operations, analysis of the uranyl nitrate solution under preparation stage for the fuel and analysis for nuclear material accountancy purpose, have been conducted. In addition, analysis on the third U/Pu extraction/separation tests among the preliminary tests to confirm adjustment condition of plutonium solution fuel for its further use at STACY from 2000, and analysis on the experiments to treat extraction waste, were conducted. A total number of analytical samples in the FY 2003 were 156.This report summarizes works related to the analysis and management of the analytical laboratory in the FY 2003.
