Non-condensable gas accumulation and distribution due to condensation in the CIGMA Facility; Implications for Fukushima Daiichi Unit 3 (1F3)

Hamdani, A.; 相馬 秀; 安部 諭; 柴本 泰照

Progress in Nuclear Energy, 185, p.105771_1 - 105771_13, 2025/07

https://doi.org/10.1016/j.pnucene.2025.105771

This study, motivated by previous TEPSYS analysis, examined how different temperatures on the 4th and 5th floors of the Fukushima Daiichi Unit 3 reactor building (R/B) influenced non-condensable gas distribution during the 2011 severe accident. Understanding this is vital for assessing risks related to gas accumulation, especially since the hydrogen explosion may have involved multiple stages. An experimental study was conducted using the CIGMA facility, designed to mimic the R/B structure, where steam and helium (as a substitute for hydrogen) were injected for 10,000 seconds to simulate leakage. Two cooling conditions were tested: 50C (Case 1) and 90C (Case 2). Results showed that the highest concentration of non-condensable gases was often found downstream rather than near the injection point. In Case 1, after 10,000 seconds, helium concentration reached 65% in the middle region (4th floor) and 45% in the top region (5th floor). Analysis indicated that the gas mixture in the middle region posed a potential detonation risk. This study offers crucial insights for enhancing safety measures and risk mitigation strategies in nuclear reactor designs.

Detailed computational models for nuclear criticality analyses on the first startup cores of NSRR: A TRIGA annular core pulse reactor

柳澤 宏司; 求 惟子

JAEA-Research 2025-001, 99 Pages, 2025/06

JAEA-Research-2025-001.pdf:1.98MB

中性子吸収棒の反応度価値に関する安全検査データのより深い理解と反応度価値の測定技術の向上のために、TRIGA-ACPR(環状炉心パルス炉)に分類されるNSRR(原子炉安全性研究炉)の初回起動炉心の臨界解析用詳細計算モデルを作成した。本モデルの形状、材料、運転データの誤差から伝播する中性子実効増倍率(keff)の不確かさを、最新の核データライブラリJENDL-5及び旧版のJENDLライブラリとMVP第3版コードを用いて詳細に評価した。その結果、本モデルにおけるkの全体的な不確かさは、0.0027から0.0029kの範囲と評価した。本モデルは、TRIGA-ACPRのkのベンチマークとして利用されることが期待される。さらに、全体的な不確かさは、NSRRで測定された吸収棒価値よりも十分小さいことを確認した。よって、本モデルはNSRRにおける吸収棒反応度価値に関する今後の解析にも適用できる。

Numerical analysis of natural convective heat transfer with porous medium using JUPITER

上澤 伸一郎; 山下 晋; 佐野 吉彦*; 吉田 啓之

Journal of Nuclear Science and Technology, 62(6), p.523 - 541, 2025/06

https://doi.org/10.1080/00223131.2025.2451023

東京電力福島第一原子力発電所(1F)の廃炉における汚染水対策として、日本原子力研究開発機構(JAEA)では、燃料デブリの位置や発熱、空隙率の影響を含む、空冷時の熱挙動を計算するため、ポーラスモデルを用いたJUPITERコードによる数値解析手法の開発を進めている。本研究では、ポーラスモデルを用いたJUPITERの妥当性確認を行うため、多孔質体を用いた自然対流熱伝達実験とその数値シミュレーションを実施した。実験とシミュレーションの温度と速度の分布を比較すると、多孔質体の上面付近の温度を除き、シミュレーションの温度分布は実験の温度分布と良く一致した。また、速度分布も実験結果と定性的に一致した。妥当性確認に加えて、本研究では、多孔質体の内部構造に基づく有効熱伝導率が自然対流熱伝達に及ぼす影響について検討するために、様々な有効熱伝導率モデルを用いた数値シミュレーションも実施した。その結果、多孔質媒体内の温度分布や自然対流の速度分布はモデルごとに大きく異なることがわかり、燃料デブリの有効熱伝導率は1Fの熱挙動解析における重要なパラメータの一つであることがわかった。

Numerical analysis of a potential Reactor Pressure Vessel (RPV) boundary failure mechanism in Fukushima Daiichi Nuclear Power Station Unit-2

Li, X.; 山路 哲史*; 佐藤 一憲*; 山下 拓哉

Annals of Nuclear Energy, 214, p.111217_1 - 111217_13, 2025/05

https://doi.org/10.1016/j.anucene.2025.111217

The decommissioning of Fukushima Daiichi NPP Unit-2 requires understanding of reactor damage and fuel debris distribution for effective debris retrieval. This study numerically analyzes potential Reactor Pressure Vessel (RPV) boundary failure due to eutectic melting of Control Rod Drive (CRD) housings during reheating after debris bed dryout. The Moving Particle Semi-implicit (MPS) method, with an enthalpy-based temperature algorithm and Boussinesq approximation, is applied to simulate melt/solid interactions in a 2-D model of the lower plenum. The CRD housing melting temperature is set at 1523 K based on a quasi-binary phase diagram of 304 Stainless Steel (SS) and Zirconium (Zr) and ELSA experiments. Results suggest local RPV failure at CRD housings, leading to melt release and refreezing. The estimated failure occurs 8-12 hours post-dryout (ca. 12:00-16:00 on 3/15/2011), providing insights into melt progression and boundary breach scenarios in Unit-2.

Ingestion doses from radionuclides in seafood before and after the Fukushima Daiichi Nuclear Power Plant accident

森 愛理; Johansen, M. P.*; McGinnity, P.*; 高原 省五

Communications Earth & Environment (Internet), 6, p.356_1 - 356_11, 2025/05

https://doi.org/10.1038/s43247-025-02338-6

The presence of radionuclides in seafood following the Fukushima Daiichi Nuclear Power Plant accident in March 2011 have led to widespread and persistent concerns over seafood safety. We assess seafood ingestion doses before and after the accident for adults in the Tohoku Region of Northeast Japan. Using a Monte Carlo approach, we evaluate 23 anthropogenic and natural radionuclides alongside realistic seafood consumption rates. In the first year after the accident, the ingestion dose from accident-derived radionuclides was 19 Sv for consumers exposed to the 95th percentile dose, contributing only 2% of the total seafood ingestion dose, which includes natural radionuclides such as Po and Pb. After the third year, the dose from accident-derived radionuclides was indistinguishable to that from pre-accident background levels. These findings suggest that, with seafood restrictions in place, the impact of accident-related releases on seafood ingestion doses was minor and relatively short-lived compared with that of natural radionuclides.

Development of a method for the determination of spontaneous fission nuclides in irradiated fuel and applicability to Pu quantification in fuel debris by dual time neutron measurements

長谷 竹晃; 小菅 義広*; 相楽 洋*; 中岫 翔; 能見 貴佳; 奥村 啓介

Progress in Nuclear Science and Technology (Internet), 7, p.41 - 46, 2025/05

https://doi.org/10.15669/pnst.7.41

This paper provides an overview of plutonium quantification in irradiated fuel including fuel debris at the Fukushima Daiichi Nuclear Power Plants, named Dual Time Measurement (DTM) method. Spontaneous fission nuclides in irradiated fuel decrease exponentially with the passage of time according to the mainly half-life of Cm-244 (half-life of about 18.11 years). By measuring neutrons two times with long time intervals, Pu-240 effective mass (half-life of about 6,500 years) and Cm-244 mass can be quantified. Pu mass can be quantified by utilizing the correlation between ratio of Cm-244/ Pu-240 effective mass and Pu/ Pu-240 effective mass. The applicability of DTM method was evaluated numerically. The results show that long time interval was required to reduce the random errors. In the case that the interval between the first and second measurements is 32 years, Pu-240 effective mass and Pu can be quantified with uncertainties of 10-50% depending on the presence of water in storage canister and the burnup condition of irradiated fuel including the mixture of several burnup compositions in fuel debris.

Three-dimensional localization and radioactivity quantification of radiation sources through inverse estimation based on Compton camera measurements

佐藤 優樹

Radiation Protection Dosimetry, p.ncaf046_1 - ncaf046_11, 2025/05

https://doi.org/10.1093/rpd/ncaf046

Radioactive substances released during the Fukushima Daiichi Nuclear Power Station accident were deposited on various equipment and building structures within the plant site. To minimize worker exposure and plan effective remediation strategies, accurately identifying the locations of these deposits is crucial. In response to this need, the current study presents a method to accurately determine the locations of multiple radiation sources in three dimensions and quantify their radioactivity levels. In particular, the method involves the application of an inverse estimation technique to data captured using a Compton camera. Here, the target region is first divided into multiple regions of interest. Image data are then acquired by placing a radiation source of known radioactivity in each region of interest. The resulting images are subsequently multiplied by each coefficient and summed to reproduce the image data of multiple unknown radiation sources. The radioactivity of each unknown radiation source is then derived based on the coefficients determined through inverse estimation.

Comparison of analysis results based on flight methods using a CZT detector system on an unmanned aerial vehicle near the Fukushima nuclear power plant

Joung, S.*; Ji, Y.-Y.*; Choi, Y.*; Lee, E.*; Ji, W.*; 佐々木 美雪; 越智 康太郎; 眞田 幸尚

Journal of Instrumentation (Internet), 20(4), p.P04027_1 - P04027_10, 2025/04

https://doi.org/10.1088/1748-0221/20/04/P04027

An airborne survey system named the MARK-A1 was previously developed to be mounted on an unmanned aerial vehicle for the purpose of application in contaminated areas with high dose rate levels. The MARK-A1 system consists of a CZT detector, signal processing unit, and positioning and interface units linked to a PC on the ground. The weight of the system is below 1 kg for loading on a commercial drone. In the current work, for experimental verification, field testing was conducted in a high dose rate environment near the Fukushima Daiichi Nuclear Power Plant. With the cooperation of the Japan Atomic Energy Agency, the MARK-A1 was installed on an unmanned aerial vehicle to conduct airborne surveys using two flight methods, namely a flat flight and a step flight, at various speeds. The airborne survey data were converted to gamma dose rates at a height of 1 m above the ground using a flat source model to create contamination maps. For a comparative evaluation of the airborne survey results, an in situ survey was also conducted in the survey area, and it was confirmed that the step flight method better matched the surface survey results.

Crystal structures of ReO under hydrostatic pressure; A Combined neutron, X-ray, Raman, and first-principles calculation study

Efthimiopoulos, I.*; Klotz, S.*; Kunc, K.*; Baptiste, B.*; Chauvigne, P.*; 服部 高典

Physical Review B, 111(13), p.134103_1 - 134103_13, 2025/04

https://doi.org/10.1103/PhysRevB.111.134103

X線回折、中性子回折、ラマン散乱、第一原理計算を用いて、ReOの高圧力下での挙動を15GPaまで包括的に調べた。常圧=3構造は0.7GPaで空間群=3の立方晶相に連続的に相転移し、その後少なくとも15GPaまで安定であることがわかった。過去この圧力領域で報告されていた単斜晶構造や菱面体晶=3構造への転移は、試料に高輝度放射光X線を照射したことによる試料の劣化によるものであり、人工的なものであることが分かった。また今回、=3相の構造の圧力依存性と正確な状態方程式および天然試料と同位体濃縮O試料のラマン散乱データを示した。このデータから、リジッドなReO八面体が圧力とともに回転することによって、相転移および高密度化が起こることが分かった。

クリープ破断時間および高温引張強度予測モデルの連合学習

櫻井 惇也*; 鳥形 啓輔*; 松永 学*; 高梨 直人*; 日比野 真也*; 木津 健一*; 森田 聡*; 井元 雅弘*; 下畠 伸朗*; 豊田 晃大; et al.

鉄と鋼, 111(5), p.246 - 262, 2025/04

https://doi.org/10.2355/tetsutohagane.TETSU-2024-124

Creep testing is time-consuming and costly, leading institutions to limit the number of tests conducted to the minimum necessary for their specific objectives. By pooling data from each institution, it is anticipated that predictive models can be developed for a wide range of materials, including welded joints and degraded materials exposed to service conditions. However, the data obtained by each institution is often highly confidential, making it challenging to share with others. Federated learning, a type of privacy-preserving computation technology, allows for learning while keeping data confidential. Utilizing this approach, it is possible to develop creep life prediction models by leveraging data from various institutions. In this paper, we constructed global deep neural network models for predicting creep rupture life of heat-resistant ferritic steels in collaboration with eight institutions using the federated learning system we developed for this purpose. Each institution built a local model using only its own data for comparison. While these local models demonstrated good predictive accuracy for their respective datasets, their predictive performance declined when applied to data from other institutions. In contrast, the global model constructed using federated learning showed reasonably good predictive performance across all institutions. The distance between each institution's data was defined in the space of explanatory variables, with the NIMS data, which had the largest dataset, serving as the reference point. The global model maintained high predictive accuracy regardless of the distance from the NIMS data, whereas the predictive accuracy of the NIMS local model significantly decreased as the distance increased.