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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.
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.
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; Araki, Shohei; Suyama, Kenya; Izawa, Kazuhiko
Proceedings of International Conference on Physics of Reactors 2022 (PHYSOR 2022) (Internet), 10 Pages, 2022/05
The fuel debris is expected to have not only heterogeneous but also non-uniform compositions. Therefore, the calculation method used in their criticality management is required to be validated experimentally. In this study, several core configurations of a new critical assembly "STACY" of JAEA with non-uniform arrangements of uranium oxide fuel rods, concrete rods and stainless steel rods, which are components of the fuel debris, were studied. In each case, the median value of 100 sample patterns was larger than the mean effective multiplication factor. It was also confirmed that there are differences in the effective multiplication factor of more than one dollar by the pattern changing, and that the neutron spectra can change significantly by changing the local neutron moderation conditions. In particular, the effective multiplication factor became smaller when over-moderated regions with large water-to-fuel ratios were formed in the core configurations due to increases in thermal neutron absorption. Such criticality experiments with non-uniform arrangements of multiple compositions will be useful to evaluate the validity of the calculation code.
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.
Tonoike, Kotaro; Watanabe, Tomoaki; Gunji, Satoshi; Yamane, Yuichi; Nagaya, Yasunobu; Umeda, Miki; Izawa, Kazuhiko; Ogawa, Kazuhiko
Proceedings of 11th International Conference on Nuclear Criticality Safety (ICNC 2019) (Internet), 9 Pages, 2019/09
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 administrated a research and development program to tackle this challenge since 2014. The Nuclear Safety Research Center of Japan Atomic Energy Agency, commissioned by the authority, is conducting activities such as computations of criticality characteristics of the fuel debris, development of a criticality analysis code, preparation of criticality experiments, and development of a criticality risk analysis method.
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.
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.
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.
Sakai, Yutaka; Gunji, Kazuhiko; Haga, Takahisa*; Fukaya, Hiroyuki; Sonoda, Takashi; Sakazume, Yoshinori; Akutsu, Hideyuki; Niitsuma, Yasushi; Shirahashi, Koichi; Sato, Takeshi
JAERI-Tech 2004-006, 25 Pages, 2004/02
JAERI-Tech-2004-006.pdf:1.72MB
Analyses 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 2002, analyses of the uranyl nitrate solution fuel from STACY/TRACY on its pre- and post-operations, analyses of the uranyl nitrate solution under preparation stage for the fuel and analyses for nuclear material accountancy purpose, have been conducted. In addition, analyses on the preliminary tests to confirm adjustment condition of plutonium solution fuel for its further use at STACY, and analyses on the americium extraction/separation tests to provide americium for the research on high temperature chemistry of TRU, were conducted. A total number of analytical samples in the FY 2002 were 275. This report summarizes works related to the analyses and management of the analytical laboratory in the FY 2002.
Haga, Takahisa*; Gunji, Kazuhiko; Fukaya, Hiroyuki; Sonoda, Takashi; Sakazume, Yoshinori; Sakai, Yutaka; Niitsuma, Yasushi; Togashi, Yoshihiro; Miyauchi, Masakatsu; Sato, Takeshi; et al.
JAERI-Tech 2004-005, 54 Pages, 2004/02
JAERI-Tech-2004-005.pdf:2.06MB
Criticality experiments using uranyl nitrate solution fuel are being conducted at STACY (the Static Experiment Critical Facility) and TRACY (the Transient Experiment Critical Facility) in NUCEF (the Nuclear Fuel Cycle Safety Engineering Research Facility). Chemical analyses of the solution have been carried out to take necessary data for criticality experiments, for treatment and control of the fuel, and for safeguards purpose at the analytical laboratory placed in NUCEF. About 300 samples are analyzed annually that provide various kinds of data, such as uranium concentration, isolation acid concentration, uranium isotopic composition, concentration of fission product (FP) nuclides, tri-butyl phosphoric acid (TBP) concentration, impurities in the solution fuel and so on. This report summarizes the analytical methods and quality management of the analysis for uranyl nitrate solution relating to the criticality experiments.
Sakazume, Yoshinori; Gunji, Kazuhiko; Haga, Takahisa*; Fukaya, Hiroyuki; Sonoda, Takashi; Sakai, Yutaka; Niitsuma, Yasushi; Shirahashi, Koichi; Sato, Takeshi
JAERI-Tech 2002-073, 25 Pages, 2002/09
JAERI-Tech-2002-073.pdf:2.51MB
Analytical results of uranyl nitrate solution are essential data for the operation of the Static Experiment Critical Facility (STACY), the Transient Experiment Critical Facility (TRACY) and the fuel treatment system in the Nuclear Fuel Cycle Safety Engineering Research Facility (NUCEF). Analytical works were carried out for the determination of fuel characteristics before and after criticality experiments, fuel preparation and nuclear material accountancy in FY. 2001. Moreover,as to preparation of critical experiments at STACY, plutonium preliminary tests were carried out to confirm the treatment condition (characteristics of the dissolution of the mixed oxide (MOX) powder and the extraction & separation for uranium / plutonium) of plutonium nitrate solution. Analytical works were carried out on the preliminary tests. A total number of analytical samples in FY. 2001 were 322 samples.This report summarizes the data on analytical works in FY.2001.
Tanoue, Takahiro; Gunji, Kazuhiko; Haga, Takahisa*; Fukaya, Hiroyuki; Sonoda, Takashi; Sakazume, Yoshinori; Niitsuma, Yasushi; Miyauchi, Masakatsu; Shirahashi, Koichi; Sato, Takeshi
JAERI-Tech 2001-071, 30 Pages, 2001/11
JAERI-Tech-2001-071.pdf:4.15MB
Analytical results of uranyl nitrate solution are essential data for the operation of the Static Experiment Critical Facility (STACY), the Transient Experiment Critical Facility (TRACY) and the fuel treatment system in the Nuclear Fuel Cycle Safety Engineering Research Facility (NUCEF). Analytical works were carried out for the determination of fuel characteristics before and after criticality experiments, fuel preparation and nuclear material accountancy in FY. 2000. In accordance with handling and dissolution of mixed oxide(MOX) powder in the fuel treatment system of NUCEF ,the preliminary tests for dissolution of the MOX powder in nitric acid solution have started since the end of 2000 ,after the MOX powder transfer to the NUCEF. The analytical works were carried out for the preliminary tests. A total number of analytical samples in FY. 2000 were 483 samples . This report summarizes the data on analytical works in FY.2000.
Araki, Shohei; Izawa, Kazuhiko; Gunji, Satoshi; Suyama, Kenya; Ishii, Junichi; Seki, Masakazu; Kobayashi, Fuyumi; Fukaya, Hiroyuki
no journal, ,
To measure critical characteristics of fuel debris, the Static Experiment Critical Facility (STACY) is being converted to the heterogeneous thermal system using fuel rods and light water moderator from the homogeneous system using solution fuel. This report presents an overview and progress of the modified STACY.
Gunji, Satoshi; Araki, Shohei; Suyama, Kenya; Izawa, Kazuhiko
no journal, ,
JAEA has been planning to measure of critical characteristics of pseudo fuel debris using the STACY critical assembly. For this purpose, we calculated the reactivities that of several insertion materials for two insertion methods into the experimental core configurations.
Araki, Shohei; Izawa, Kazuhiko; Gunji, Satoshi; Arakaki, Yu; Suyama, Kenya
no journal, ,
JAEA has been modifying the Static Experiment Critical Facility (STACY) to study the criticality characteristics of fuel debris. The concrete rod consists of a clad tube (9.5-mm od, 7.5-mm id, 1495-mm length) and a concrete simulant. It can be installed inthe core for fuel debris experiments. This report presents the status of the test production of a concrete rod.
