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Radiocesium distribution in the sediments of the Odaka River estuary, Fukushima, Japan

萩原 大樹; 中西 貴宏; 小西 博巳*; 鶴田 忠彦; 御園生 敏治; 藤原 健壮; 北村 哲浩

Journal of Environmental Radioactivity, 220-221, p.106294_1 - 106294_9, 2020/09

Radiocesium that originated from the Fukushima Daiichi Nuclear Power Plant accident was deposited on the ground surface and has been transported via fluvial discharge, primarily in the form of particulates, to downstream areas and eventually to the ocean. During transportation, some of the radiocesium accumulated on the riverbed. In this study, we quantified the radiocesium deposition on the riverbed in the Odaka River estuary and investigated the radiocesium sedimentation process of the river bottom. Our results show that the radiocesium inventory in the seawater intrusion area is larger than those in the freshwater and marine parts of the estuary. Moreover, the particle-size distribution in the seawater intrusion area shows a high proportion of silt and clay particles compared with the distribution in other areas. The increased radiocesium inventory in this area is attributed to the sedimentation of fine particles caused by hydrodynamic factors (negligible velocity of the river flow) rather than flocculation factor by salinity variation.


A Cubic CeBr$$_{3}$$ gamma-ray spectrometer suitable for the decommissioning of the Fukushima Daiichi Nuclear Power Station

冠城 雅晃; 島添 健次*; 大鷹 豊*; 上ノ町 水紀*; 鎌田 圭*; Kim, K. J.*; 吉野 将生*; 庄子 育宏*; 吉川 彰*; 高橋 浩之*; et al.

Nuclear Instruments and Methods in Physics Research A, 971, p.164118_1 - 164118_8, 2020/08


Our work focused on the passive gamma-ray analysis (PGA) of the nuclear fuel debris based on measuring gamma rays with an energy greater than 1 MeV for the decommissioning of the Fukushima Daiichi Nuclear Power Station (FDNPS). The PGA requires gamma-ray spectrometers to be used under the high dose rates in the FDNPS, then we fabricated a small cubic CeBr$$_{3}$$ spectrometer with dimensions of 5 mm $$times$$ 5 mm $$times$$ 5 mm, coupled to a Hamamatsu R7600U-200 photomultiplier tube (PMT). The performance at dose rates of 4.4 to 750 mSv/h in a $$^{60}$$Co field was investigated. The energy resolution (FWHM) at 1333 keV ranged from 3.79% to 4.01%, with a standard deviation of 6.9%, which met the narrow gamma decay spectral lines between $$^{154}$$Eu (1274 keV) and $$^{60}$$Co (1333 keV). However, the spectra shifted to a higher energy level as the dose rate increase, there was a 51% increase at the dose rates of 4.4 to 750 mSv/h, which was caused by the PMT gain increase.



澤口 拓磨; 高井 静霞; 武田 聖司

JAEA-Research 2020-005, 47 Pages, 2020/06



Development of microwave-assisted, laser-induced breakdown spectroscopy without a microwave cavity or waveguide

大場 正規; 宮部 昌文; 赤岡 克昭; 若井田 育夫

Japanese Journal of Applied Physics, 59(6), p.062001_1 - 062001_6, 2020/06




Vertical and horizontal distributions of $$^{137}$$Cs on paved surfaces affected by the Fukushima Dai-ichi Nuclear Power Plant accident

吉村 和也; 渡辺 貴善; 操上 広志

Journal of Environmental Radioactivity, 217, p.106213_1 - 106213_6, 2020/06

 被引用回数:0 パーセンタイル:100(Environmental Sciences)

To obtain the important information and parameter for the measurement of $$^{137}$$Cs activity per unit area on paved ground, this study evaluated their vertical and horizontal distributions. This study showed that $$^{137}$$Cs was retained in extremely surface layer with mass depth less than 0.5 cm$$^{-2}$$. This study also demonstrated the horizontal variations of $$^{137}$$Cs activity and the relation of the variation with grid size.


Remote detection of radioactive hotspot using a Compton camera mounted on a moving multi-copter drone above a contaminated area in Fukushima

佐藤 優樹; 小澤 慎吾*; 寺阪 祐太; 峯本 浩二郎*; 田村 智志*; 新宮 一駿*; 根本 誠*; 鳥居 建男

Journal of Nuclear Science and Technology, 57(6), p.734 - 744, 2020/06

 被引用回数:0 パーセンタイル:100(Nuclear Science & Technology)

The Fukushima Daiichi Nuclear Power Station, operated by Tokyo Electric Power Company Holdings, Inc., suffered a meltdown as a result of a large tsunami triggered by the Great East Japan Earthquake on March 11, 2011. To proceed with the environmental recovery by decontamination, drawing a radiation distribution map that can indicate the distribution of radioactive substances is extremely important to establish detailed decontamination plans. We developed a remote radiation imaging system consisting of a lightweight Compton camera and a multi-copter drone to remotely measure the distribution of the radioactive substances. This system can perform radiation imaging using a Compton camera while flying and moving. In addition, it is also possible to draw the distribution of radioactive substances three-dimensionally by projecting the radiation image measured with the Compton camera on a three-dimensional topography model separately acquired by a 3D-LiDAR. We conducted a survey of radioactive hotspots in difficult-to-return zone in the coastal area of Fukushima, Japan. The drone system succeeded in three-dimensional visualization of several hotspots deposited on the ground. Such remote technology would be useful not only for monitoring the difficult-to-return zone, but also for monitoring distribution of radioactive substances inside the site of the FDNPS where decommissioning work is ongoing.


Gamma detector response simulation inside the pedestal of Fukushima Daiichi Nuclear Power Station

Riyana, E. S.; 奥村 啓介; 寺島 顕一; 松村 太伊知; 坂本 雅洋

Mechanical Engineering Journal (Internet), 7(3), p.19-00543_1 - 19-00543_8, 2020/06

Prediction of the fuel debris location and distribution inside the primary containment vessel (PCV) of the Fukushima Daiichi Nuclear Power Plant is important to decide further decommissioning step and strategy. The radiation measurements in the past internal investigations have not yet provided enough information to predict fuel debris location and its distribution inside PCV. To support further measurement efforts, we simulate the detector response inside the PCV. The calculation result could provide a base on deciding suitable detector systems to assist the efforts on searching, localizing and defining distributions of the fuel debris.


Development of fission product chemistry database ECUME for the LWR severe accident

三輪 周平; 中島 邦久; 宮原 直哉; 西岡 俊一郎; 鈴木 恵理子; 堀口 直樹; Liu, J.; Miradji, F.; 井元 純平; Mohamad, B. A.; et al.

Mechanical Engineering Journal (Internet), 7(3), p.19-00537_1 - 19-00537_11, 2020/06



Study on chemisorption model of cesium hydroxide onto stainless steel type 304

中島 邦久; 西岡 俊一郎*; 鈴木 恵理子; 逢坂 正彦

Mechanical Engineering Journal (Internet), 7(3), p.19-00564_1 - 19-00564_14, 2020/06



Expansion of high temperature creep test data for failure evaluation of BWR lower head in severe accident

山口 義仁; 勝山 仁哉; 加治 芳行; 逢坂 正彦; Li, Y.

Mechanical Engineering Journal (Internet), 7(3), p.19-00560_1 - 19-00560_12, 2020/06



Evaluation of an ultra-thin plastic scintillator to detect alpha and beta particle contamination

森下 祐樹; 星 勝也; 鳥居 建男

Nuclear Instruments and Methods in Physics Research A, 966, p.163795_1 - 163795_8, 2020/06

これまでに、福島第一原子力発電所(FDNPS)の原子炉建屋でアルファおよび高$$beta$$粒子エミッターが検出された。FDNPS原子炉建屋の$$beta$$放射線レベルは非常に高いため、ガイガーミュラー(GM)カウンターなどの市販のベータ測量計では、$$beta$$汚染レベルを測定できない。この問題を解決するために、極薄のプラスチックシンチレータを使用して、アルファおよびベータ汚染の検出器を開発した。検出器の評価のため、7, 22, 24, 31, 39、および55$$mu$$mのさまざまな厚さの超薄型プラスチックシンチレータを準備した。それらの感度をテストするために、各シンチレータをガラスプレートと2インチの位置に敏感な光電子増倍管に光学的に結合し、アルファ, ベータ、またはガンマ線源に曝露した。アルファ分光法の結果は、厚さ55$$mu$$mのプラスチックシンチレータのみがアルファ粒子(5.5MeV)を完全に吸収し、半値全幅が16.7%であった。高ベータバックグラウンド下でのアルファイメージングの場合、厚さ7$$mu$$mのプラスチックシンチレータが最良の選択であり、アルファ対ベータ比が652であることがわかった。また、厚さ7$$mu$$mのプラスチックシンチレータは1MBq $$^{90}$$Sr/$$^{90}$$Y線源に直接コンタクトして測定しても41.74$$pm$$0.93cpsのカウントレートであり、非常に低感度であるため飽和せずに測定を行うことが可能である。したがって、厚さ7$$mu$$mのプラスチックシンチレータを使用して、以前の方法では不可能だったFDNPSサイトのベータの表面汚染レベルをリアルタイムで直接測定することができる。




JAEA-Review 2020-004, 140 Pages, 2020/05




Toward technological contributions to remote operations in the decommissioning of the Fukushima Daiichi Nuclear Power Station

川端 邦明

Japanese Journal of Applied Physics, 59(5), p.050501_1 - 050501_9, 2020/05




山本 正弘

材料と環境2020講演集(CD-ROM), p.9 - 16, 2020/05



Development of experimental technology for simulated fuel-assembly heating to address core-material-relocation behavior during severe accident

阿部 雄太; 山下 拓哉; 佐藤 一憲; 中桐 俊男; 石見 明洋

Journal of Nuclear Engineering and Radiation Science, 6(2), p.021113_1 - 021113_9, 2020/04

The authors are developing an experimental technology for simulating severe accident (SA) conditions using simulate fuel material (ZrO$$_{2}$$) that would contribute, not only to Fukushima Daiichi (1F) decommissioning, but also to enhance the safety of worldwide existing and future nuclear power plants through clarification of accident progression behavior. Nontransfer (NTR) type plasma, which has been in practical use with a large torch capacity as high as 2 MW, has the potential to heat subject materials to very high temperatures without selecting the target to be heated. When simulating 1F with SA code, the target of this core-material-melting and relocation (CMMR) experiment was to confirm that NTR plasma has a sufficient heating performance realizing large temperature gradients ($$>$$ 2000 K/m) expected under 1F conditions. The authors selected NTR-type plasma-heating technology that has the advantage of continuous heating in addition to its high-temperature level. The CMMR-2 experiments were carried out in 2017 applying the improved technology (higher heating power and controlled oxygen concentration). The CMMR-2 experiment adopted a 30-min heating period, wherein the power was increased to a level where a large temperature gradient was expected at the lower part of the core under actual 1F accident conditions. Most of the control blade and channel box migrated from the original position. After heating, the simulated fuel assembly was measured by X-ray computed tomography (CT) technology and by electron probe micro-analyzer (EPMA). CT pictures and elemental mapping demonstrated its excellent performance with rather good precision. Based on these results, an excellent perspective, in terms of applicability of the NTR-type plasma-heating technology to the SA experimental study, was obtained.


New research programme of JAEA/CLADS to reduce the knowledge gaps revealed after an accident at Fukushima-1; Introduction of boiling water reactor mock-up assembly degradation test programme

Pshenichnikov, A.; 倉田 正輝; Bottomley, D.; 佐藤 一憲; 永江 勇二; 山崎 宰春

Journal of Nuclear Science and Technology, 57(4), p.370 - 379, 2020/04

 被引用回数:2 パーセンタイル:27.34(Nuclear Science & Technology)

The new research and development programme of JAEA/CLADS tests complement the previous investigations related to BWR severe accidents. A series of tests aiming at closing the gaps in understanding of the Fukushima Daiichi degradation sequence at each unit. The paper emphasises the problem of control blade degradation, which influences the accident progression at an early stage and shows the approach for thorough investigation of this problem.


Predictability of a short-term emergency assessment system of the marine environmental radioactivity

川村 英之; 上平 雄基; 小林 卓也

Journal of Nuclear Science and Technology, 57(4), p.472 - 485, 2020/04

 被引用回数:0 パーセンタイル:100(Nuclear Science & Technology)



Degradation prediction of a gamma-ray radiation dosimeter using InGaP solar cells in a primary containment vessel of the Fukushima Daiichi Nuclear Power Station

奥野 泰希; 山口 真史*; 大久保 成彰; 今泉 充*

Journal of Nuclear Science and Technology, 57(4), p.457 - 462, 2020/04

 被引用回数:0 パーセンタイル:100(Nuclear Science & Technology)

優れた高耐放射線性を備えたリン化インジウムガリウム(InGaP)太陽電池は、高放射線量率環境に適用可能な線量計の有力な候補材料になると予想されている。本研究では、InGaP太陽電池を用いた線量計の寿命を予測するために、照射試験及び経験的計算により、InGaP太陽電池の線量信号としての放射線誘起電流に対する少数キャリア拡散長($$L$$)の影響を明らかした。照射試験では、$$gamma$$線線量率の関数としての短絡電流密度($$J_{rm sc}$$)を測定することでInGaP太陽電池の$$L$$を推定した。また、様々な線量率でInGaP太陽電池を検出器として使用した際の動作寿命を、$$L$$と吸収線量の関係に基づく経験式を用いて推定した。この計算結果から、InGaP太陽電池を用いた線量計が福島第一原子力発電所の原子炉格納容器で10時間以上使用可能であり、廃炉に貢献する耐放射線性を有した線量計である可能性が高いことを明らかにした。


Leaching behavior of prototypical Corium samples; A Step to understand the interactions between the fuel debris and water at the Fukushima Daiichi reactors

仲吉 彬; Jegou, C.*; De Windt, L.*; Perrin, S.*; 鷲谷 忠博

Nuclear Engineering and Design, 360, p.110522_1 - 110522_18, 2020/04

 被引用回数:0 パーセンタイル:100(Nuclear Science & Technology)

Simulated in-vessel and ex-vessel fuel debris, fabricated in the Colima experimental facility set up in the PLINIUS platform at CEA Cadarache, were selected and leaching experiments were carried out under oxidizing conditions. In parallel, geochemical modeling was performed to better understand the experimental concentrations, pH evolutions and secondary phase's formation. Finally, the Fractional Release Rates of the (U, Zr)O$$_{2}$$ matrix for the two kinds of samples (in-vessel and ex-vessel) were found to be close to or one order of magnitude lower than that of SF under oxidizing conditions (from 10$$^{-6}$$ to 10$$^{-7}$$ per day), but the release processes are different.


Effectiveness of decontamination by litter removal in Japanese forest ecosystems affected by the Fukushima nuclear accident

小嵐 淳; 安藤 麻里子; 西村 周作; 武藤 琴美*

Scientific Reports (Internet), 10(1), p.6614_1 - 6614_11, 2020/04



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