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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.
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; 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.
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.
Gunji, Satoshi; Watanabe, Tomoaki; Tonoike, Kotaro; Araki, Shohei
no journal, ,
The criticality safety research group had been conducted research using deterministic methods to ensure the criticality safety. However, the retrieval work of fuel debris in the Fukushima Dai-ichi Nuclear Power Station cannot be evaluated by the conventional criticality management methods, therefore it is necessary to develop a risk-informed control method. To solve these research issues, we are making a critical risk basic database that covers the possible composition and properties of fuel debris. Its validity will be confirmed by a critical experiment by modified STACY critical assembly. We are also building a database that can manage the risk of the impact of exposure to critical events. We are developing a new calculation model to evaluate fuel debris. And post irradiation examinations have been conducted to accurately measure the burnup.
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.
Gunji, Satoshi; Watanabe, Tomoaki; Suyama, Kenya; Araki, Shohei; Fukuda, Kodai
no journal, ,
The criticality safety research group had been conducted research using deterministic methods to ensure the criticality safety. However, the retrieval work of fuel debris in the Fukushima Dai-ichi Nuclear Power Station cannot be evaluated by the conventional criticality management methods, therefore it is necessary to develop a risk-informed control method. In this presentation, among the research activities undertaken to address these research issues, the following research topics are presented. (1) Criticality Map Database, (2) Support for the new STACY installation and certification, (3) Study of the new STACY experiments, (4) Post-Irradiation Examination (PIE) / Combustion Analysis
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.
Araki, Shohei; Gunji, Satoshi; Izawa, Kazuhiko; Suyama, Kenya
no journal, ,
In order to add elasticity to the experimental design of STACY, a two-region core with only 400 existing fuel rods was considered. The two-region core was composed of a test region with 1.27-cm lattice pitch and a driver region with 1.8-cm pitch. It was confirmed that if the test region was larger than 11x11, the neutron flux in the center of the test region could approximately simulate that of a one-region core with a 1.27-cm lattice plate.
Gunji, Satoshi; Araki, Shohei; Suyama, Kenya; Izawa, Kazuhiko
no journal, ,
For the start of critical experiments in the modified STACY, it is necessary to identify core tank components that have significant reactivity effects in order to evaluate its critical water height. Therefore, reactivity effects of a few main in-core tank components which are expected having neutron absorption effects were evaluated by Monte Carlo computation. As the results, it was found that a few components have significant reactivities. Computation models for experimental designs need to take these components into account.
Gunji, Satoshi; Araki, Shohei; Yoshikawa, Tomoki; Izawa, Kazuhiko; Suyama, Kenya
no journal, ,
The Static Experiment Critical Facility (STACY) is being modified to achieve the first criticality to clarify criticality characteristics of fuel debris and is scheduled to start operation in May 2024 at the time of the registration. For detailed planning of critical experiments after the operation, the core configurations for the first criticality for each lattice plate with different neutron moderation conditions using 400 UO fuel rods are being studied. In addition, we have confirmed the feasibility of the critical experiments to predict the changes in neutron multiplication factor and the associated critical water height for the two-part core experiment, which is one of the experiments being considered for the modified STACY and which statically simulates the fall and coalescence of fuel debris. The computation was done using MCNP code and JENDL-4.0 or JENDL-5 libraries. In this presentation, in addition to the status of these studies, we will also report on the status of the modification of the STACY.
Araki, Shohei; Gunji, Satoshi; Yoshikawa, Tomoki; Izawa, Kazuhiko; Suyama, Kenya
no journal, ,
In order to study the criticality characteristics of fuel debris, experiments with concrete rods are designed in the modified STACY. We produced some concrete rods, which consist of an Al alloy cladding tube and concrete pellets, by way of trial, and obtained properties such as compositions and density of them. In this study, reactivity worth was estimated based on the concrete rod properties.
Araki, Shohei; Gunji, Satoshi; Hasegawa, Kenta; Tada, Yuta; Yoshikawa, Tomoki; Izawa, Kazuhiko; Suyama, Kenya
no journal, ,
The activation method was investigated to measure the thermal power in the new STACY. The neutron flux and spectra in the core are calculated with the MVP code. The reaction rate of the activation detector in the neutron field was estimated by using the PHITS code. From these results, the relationship between the thermal power and the reaction rate was estimated.
Gunji, Satoshi; Araki, Shohei; Suyama, Kenya
no journal, ,
Experimental core configurations with nonuniform arrangements of concrete- and steel-based structural materials in the new STACY were studied. In this study, the composition of the mortar sample with aluminum cladding based on the prototype results were used to evaluate the effects on the keff values of the nonuniform core arrangements in the STACY core configurations with some of the experimental constraints such as the number of fuel elements.
Araki, Shohei; Gunji, Satoshi; Yoshikawa, Tomoki; Arakaki, Yu; Izawa, Kazuhiko; Suyama, Kenya
no journal, ,
The first criticality of the statistical criticality assembly called "STACY", which JAEA has been modifying, is scheduled for 2024. After the first criticality, experiments using debris structure material rods are being prepared. At the spring 2023 annual meeting, we presented the results of a study on the core characteristics of the experimental core under the condition of using 900 fuel rods. However, it is requested to consider the core configuration with 400 fuel rods due to the fuel procurement. In this study, we reanalyzed the core with 400 fuel rods and confirmed that the core characteristics are included in that of the previous presentation.
Gunji, Satoshi; Araki, Shohei; Yoshikawa, Tomoki; Izawa, Kazuhiko; Suyama, Kenya
no journal, ,
The first criticality of the statistical criticality assembly called "STACY", which JAEA has been modifying, is scheduled for 2024. At the spring 2023 annual meeting, we presented the results of a study on the basic core configuration for the first criticality, which is intended to reduce experimental uncertainties. However, in subsequent regulatory reviews, the Nuclear Regulation Authority instructed to create of core configurations with the strictest safety factors (reactor shutdown margin, etc.) among the permission, therefore the first critical core configurations were changed to cylindrical core configurations with grid intervals of 1.50 and 2.54 cm. This study presents the results of estimations such as critical water levels using multiple nuclear data and calculation codes.