Analysis of fracture conditions of Cr-coated Zr alloy claddings under LOCA conditions calculated using FEMAXI fuel performance code

Luu, V. N.; 谷口 良徳; 宇田川 豊; 田崎 雄大; 勝山 仁哉

Annals of Nuclear Energy, 230, p.112114_1 - 112114_14, 2026/06

https://doi.org/10.1016/j.anucene.2026.112114

Fracture behavior of chromium (Cr) coated cladding under loss of coolant accident (LOCA) conditions was investigated utilizing the FEMAXI fuel performance code. Cr coating degradation models were introduced to FEMAXI to calculate oxygen diffusion behavior within the cladding tube. The FEMAXI code reasonably simulated the observed evolution of cladding metallic and oxide layers under the simulated LOCA conditions, accounting for factors such as wall thinning due to cladding high temperature creep, Cr layer thinning by CrO formation and Cr/Zr interdiffusion, weight increase by oxygen absorption, associated oxide growth, and increased oxygen concentration in -Zr phase. According to sensitivity analyses of the cladding oxygen concentration, where the effects of wall thickness change and eutectic reactions were taken into account, the fracture condition of the Cr-coated cladding samples can be reasonably modelled by the fracture criteria based on the remaining -Zr thickness with an oxygen concentration of 0.9 wt%.

A Novel kinetic model for dissolution and precipitation of oxide on stainless-steel surface in stagnant liquid sodium

河口 宗道*; 池田 明日香; 斉藤 淳一

Annals of Nuclear Energy, 226, p.111880_1 - 111880_9, 2026/02

https://doi.org/10.1016/j.anucene.2025.111880

This study performed sodium experiments and developed a new kinetic model to investigate the oxide dissolution and precipitation behavior on the stainless-steel (SS) surface in stagnant liquid sodium. The experiment revealed that the oxygen of NaFeO on the SS surface was dissolved into the liquid sodium with v 9.3 10 wt.ratio/h in less than 20 h, and the oxide precipitation occurred on the SS surface with v 1.4 10 wt.ratio/h after the dissolution. Furthermore, the phase-field (PF) calculation code was developed to investigate the dependence of six parameters (T, c, , D , k, and t) of the oxide precipitation velocity in the liquid sodium. As a result, the precipitation velocity increased linearly as the oxygen concentration (c) and the oxygen diffusion coefficient (D) in liquid sodium increased. In contrast, its velocity decreased exponentially as the sodium temperature (T) and the interfacial energy of oxide () increased. The quasi-partial coefficient (k) and the time step (t) did not affect the calculation results at all. In these sensitivity analyses, the oxide precipitation velocity obtained by the PF calculation shows consistency with the laboratory-scale experimental findings of Latge et al.

Ambient dose rate variation in the Fukushima region visualized using explainable AI techniques

吉田 竜*; 操上 広志; 長尾 郁弥; 高橋 成雄*; 眞田 幸尚

Journal of Environmental Radioactivity, 293, p.107900_1 - 10790_13, 2026/02

https://doi.org/10.1016/j.jenvrad.2026.107900

Following the Fukushima Daiichi Nuclear Power Station accident in 2011, ambient dose rates in the surrounding region have gradually declined due to radioactive decay and decontamination efforts. However, spatial variations in dose rate reduction remain insufficiently understood, particularly in forested areas where contamination persists. This study investigates long-term trends in ambient dose rate changes using explainable AI techniques. A 12-year integrated dose rate map, combining fixed-point, walk, carborne, and airborne survey data, was used to analyze temporal and spatial patterns. We developed a predictive model using Light Gradient Boosting Machine (LightGBM) to estimate dose rate reduction ratios based on geographic and environmental features. SHapley Additive Explanations (SHAP) were applied to quantify the contribution of each variable and enhance model interpretability. Our findings revealed that land use significantly influences dose rate reduction, with urban and agricultural areas showing faster declines due to infrastructure and human activity including decontamination works, while forests exhibit slower reductions. Notably, topographical features such as elevation and slope affect dose rate trends in undisturbed forests, with valleys and depressions showing stagnation. This study provided the first visual validation of area-wide decontamination effects and demonstrates the utility of explainable AI in environmental radiation analysis. The proposed approach offers a robust framework for geospatial interpretation and supports future policymaking for regional recovery and forest utilization.

Evaluation of sources and seasonal variations of dissolved Cs in stream water in forested catchments

佐久間 一幸; 吉村 和也; 中西 貴宏; 林 誠二*; 辻 英樹*; 舟木 泰智; 飯島 和毅

Science of the Total Environment, 1014, p.181397_1 - 181397_9, 2026/02

https://doi.org/10.1016/j.scitotenv.2026.181397

2011年福島第一原子力発電所事故により陸域環境へ多くの放射性セシウムが放出された。河川水中の溶存態放射性セシウムの発生源と季節変動を制御する要因を理解することは、環境放射能汚染の評価と低減に極めて重要である。2015年から2021年にかけ、渓流水、湧水、地下水、落葉溶出水、土壌間隙水、土壌浸透水における溶存態Cs濃度と溶存有機炭素(DOC)を調査した。源流域では、溶存態Cs濃度は地下水中の濃度と比較して湧水直後に増加した。約3年間の渓流水モニタリングにおいて、溶存態Cs濃度は水温、DOC濃度、K濃度と相関を示した。三成分混合モデルにより、渓流水の組成は主に地下水、森林落葉層/表土、土壌間隙水に由来することが明らかとなった。特に夏季の渓流水には土壌間隙水が顕著に影響しており、降水に従い地下水位の変動が要因と考えられる。溶出水も冬季の渓流水に比べ夏季の渓流水により影響を与えていた。これらの知見は、溶存態Csの季節変動が、温度依存性のある森林落葉層/表土からの溶出と土壌間隙水からの寄与増加によって駆動されることを示唆している。本研究は森林源流域河川における溶存態Csの発生源と季節的駆動要因を特定した初めての研究である。

再処理特別研究棟の解体廃棄物に対する廃棄体化を考慮した分別の導入

中嶋 瞭太; 酒井 達弥; 谷 陸; 半田 雄一; 砂押 瑞穂*; 井上 秀毅*; 山田 悟志; 清水 修

JAEA-Technology 2025-012, 39 Pages, 2026/01

JAEA-Technology-2025-012.pdf:2.31MB

再処理特別研究棟は1996年から廃止措置に移行し、施設内の設備・機器の解体作業を実施してきた。2022年10月からのグローブボックス等の解体撤去作業では、埋設施設へ処分する際に要求される技術上の基準に適合する廃棄体を作製することを目的として、「解体物分別マニュアル」を作成し、発生した解体廃棄物の分別・仕分け作業を実施した。本報告は、「解体物分別マニュアル」に従い実施したグローブボックス等の解体撤去作業で発生した解体廃棄物の分別・仕分けの結果及び得られた知見についてまとめたものである。

令和6年度大型計算機システム利用による研究成果報告集

システム計算科学センター 高性能計算技術利用推進室

JAEA-Review 2025-044, 140 Pages, 2026/01

JAEA-Review-2025-044.pdf:8.77MB

日本原子力研究開発機構では、原子力の総合的研究開発機関として原子力に係わるさまざまな分野の研究開発を行っており、これらの研究開発の多くにおいて計算科学技術が活用されている。日本原子力研究開発機構における計算科学技術を活用した研究開発の論文発表は、過去十数年にわたり、毎年度、全体の約2割を占めている。大型計算機システムはこの計算科学技術を支える重要なインフラとなっている。大型計算機システムは、第4期中長期計画にて重点化して取り組むとされた「安全性向上等の革新的技術開発によるカーボンニュートラルへの貢献」、「原子力科学技術に係る多様な研究開発の推進によるイノベーションの創出」、「東京電力福島第一原子力発電所事故の対処に係わる研究開発の推進」、「高レベル放射性廃棄物の処理処分に関する技術開発の着実な実施」、「原子力安全規制行政及び原子力防災に対する支援とそのための安全研究の推進」等の研究開発活動に利用された。本報告は、令和6年度における大型計算機システムを利用した研究開発の成果を中心に、それを支える利用支援、利用実績、システムの概要等をまとめたものである。

Residual stress relief effect in gradient structural steel and remaining life evaluation under stochastic fatigue loads

Qin, T. Y.*; Hu, F. F.*; 徐 平光; Zhang, R.*; Su, Y. H.; Ao, N.*; Li, Z. W.*; 篠原 武尚; 菖蒲 敬久; Wu, S. C.*

International Journal of Fatigue, 202, p.109233_1 - 109233_16, 2026/01

https://doi.org/10.1016/j.ijfatigue.2025.109233

The surface induction-hardened S38C medium carbon steel shows a good balance of strength and toughness, but complicates the evaluation of fatigue resistance, mainly because of gradient residual stress (RS) and grains. An integrated fatigue resistance assessment (AIFA) framework was proposed to consider the residual stress relief under stochastic loads. To this end, quasi-in situ neutron diffraction and Bragg-edge imaging were combined to probe the evolution of residual stress during crack propagation. Firstly, a rigid-flexible coupled vehicle dynamics model was adopted to obtain the time-domain variable amplitude loading spectrum. Then, Fortran subroutines were developed to assign these data into full-scale S38C axle model, and the remaining life was predicted using the damage tolerance approach. The results demonstrate that crack propagation would accelerate when residual stress is considered in the case of the crack depth exceeding 3.0 mm. It is, for the first time, found that 15 mm- and 5 mm-thickness fan-shaped specimens can retain the axial and hoop residual strain in terms of diffraction angle variation, respectively, for full-scale structural S38C steel axles. In the absence of RS, the remaining life of the axle decreases sharply from 624,800 to 51,300 km as the crack depth increases from 3.0 to 16 mm. Compared with the standard method under constant amplitude loading without residual stress relief, the present AIFA method provides the more accurate but conservative fatigue life prediction.

Development of LASSO based optimized scheme for reconstructing radioactive source distributions using monitoring air dose rates

Shi, W.*; 町田 昌彦; 山田 進; 岡本 孝司*

Measurement, 258(Part D), p.119444_1 - 119444_15, 2026/01

https://doi.org/10.1016/j.measurement.2025.119444

Clarifying the distribution of radioactive sources within nuclear facilities is crucial for ensuring worker safety during decommissioning and for responding to accidents. However, air dose rate measurements in restricted areas are often limited due to complex structures and high radiation levels in contaminated rooms. To address this, we have proposed a machine learning-based approach, the Least Absolute Shrinkage and Selection Operator (LASSO), to reconstruct radioactive source distributions in simplified room models. LASSO method indicates the good performance of reconstructing radioactive source with high accuracy inside simple room model. However, in more complex environments, obstacles can degrade reconstruction accuracy. To overcome these limitations, we developed an optimized scheme based on the LASSO method to improve inverse estimation in complex rooms. In this scheme, the impact of shielding structures is mitigated by normalizing the radioactive contributions from sources. A series of numerical simulations demonstrate that the optimized approach outperforms the non-optimized version in accurately reconstructing source distributions. Furthermore, experiments in a room with complex structures validate the effectiveness of the optimized method. The inverse estimations performed on experimental data confirm that the use of a normalized contribution matrix significantly improves accuracy by reducing the influence of shielding. Conclusively, this paper optimizes LASSO scheme for reconstructing radioactive source distributions in complex building room using air dose rate measurements. It shows significant improvements over existing scheme and is verified to be successfully applied in complicated situations with high accuracy. We confirm that optimized LASSO scheme holds significant promise for future monitoring and decommissioning projects in both operational and damaged nuclear facilities.

Experimental study of AESOP code for aerosol removal behavior from a rising gas bubble in water pool and parametric study for application to sodium pool system

宮原 信哉*; 鯉江 竜輔*; 宇埜 正美*; 河口 宗道*; 佐藤 理花; 清野 裕

Nuclear Engineering and Design, 446(Part A), p.114523_1 - 114523_14, 2026/01

https://doi.org/10.1016/j.nucengdes.2025.114523

In a postulated accident of fuel pin failure of a sodium-cooled fast reactor, a fission product of cesium will be released from the failed pin as an aerosol such as cesium iodide and/or cesium oxide together with a fission product noble gas such as xenon and krypton. The xenon and krypton released with the cesium aerosols into the sodium coolant as bubbles have an influence on the removal of cesium aerosols by the sodium pool in a period of bubble rising to the sodium pool surface. Then, the cesium aerosols could transfer into the containment vessel as an initial inventory of a source term. To meet this phenomenon, the computer program AESOP (AErosol scrubbing in SOdium Pool) has been developed to deal with the expansion and the deformation of the bubble together with the aerosol absorption considering the effects of the particle size distribution and the agglomeration in aerosols. In this study, simulation experiments have been conducted using simulant particles under the condition of room temperature in water pool and nitrogen gas bubble systems and the experimental results were compared with the analysis results calculated under the same condition by the AESOP code. Furthermore, to examine the applicability of the AESOP code to the sodium pool system, the sensitivities of the physical parameters on decontamination factor (DF) of fission product aerosols such as the initial bubble diameter, the sodium pool depth and the temperature, the aerosol particle diameter and the density, the initial aerosol concentration in the bubble had been studied and the analysis results were discussed for the sensitivities of the parameter as same as DF of the aerosol.

最新の知見に基づいた研究施設等廃棄物浅地中埋設施設からの浸出水量の評価

北谷 光; 小曽根 健嗣; 仲田 久和

JAEA-Technology 2025-011, 57 Pages, 2025/12

JAEA-Technology-2025-011.pdf:5.32MB

日本原子力研究開発機構は、研究施設等廃棄物の埋設処分の実施主体として、現在低レベル放射性廃棄物を対象としたトレンチ処分及びピット処分の2通りの検討を行っている。埋設施設の安全評価における被ばく線量評価には、埋設施設の浸透水量データが必要となる。浸透水量の評価には、廃棄物条件や埋設環境などによる不確実性を考慮する必要がある。そのため、本報告では、研究施設等廃棄物浅地中処分施設の概念設計の設計条件等を基にリファレンスモデルを設定し、先行事業者の申請書を参考に、最新の知見に基づいた安全評価に反映する浅地中埋設施設からの浸出水量を地下水流動解析により算出した。これにより、埋設施設の各層及び周辺土壌の透水係数が浸出水量に及ぼす影響を評価した。具体的には、有限要素法による二次元地下水流動解析コード(MIG2DF)を用いて、トレンチ埋設施設については、覆土層の経年劣化を想定した評価を行うとともにコンクリートピット埋設施設については、廃棄体に含まれる塩類の影響を想定した評価を行った。解析の結果、トレンチ埋設施設では、粘土層の透水性が劣化すると廃棄体層への浸入水量が増加し、特に排水層の透水性が低下した場合にはその傾向が一層顕著となった。これは、排水層による水平流路が機能せず、水の粘土層への浸入が促進されるためである。一方、コンクリートピット埋設施設では、粘土層の破断により周辺の流速が上昇し、廃棄体層を通過する水量が増加する現象が確認された。これらの結果は、施設の各層ごとの透水性の変化が、浸出水量にどのような影響を及ぼすかを定量的に示しており、安全評価におけるシナリオ設定や埋設施設の維持管理の方針策定に資する有効な知見といえる。