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論文

An Analytical model to decompose mass transfer and chemical process contributions to molecular iodine release from aqueous phase under severe accident conditions

Zablackaite, G.; 塩津 弘之; 城戸 健太朗; 杉山 智之

Nuclear Engineering and Technology, 56(2), p.536 - 545, 2024/02

Radioactive iodine is a representative fission product to be quantified for the safety assessment of nuclear facilities. In integral severe accident analysis codes, the iodine behavior is usually described by a multi-physical model of iodine chemistry in aqueous phase under radiation field and mass transfer through gas-liquid interface. The focus of studies on iodine source term evaluations using the combination approach is usually put on the chemical aspect, but each contribution to the iodine amount released to the environment has not been decomposed so far. In this study, we attempted the decomposition by revising the two-film theory of molecular-iodine mass transfer. The model involves an effective overall mass transfer coefficient to consider the iodine chemistry. The decomposition was performed by regarding the coefficient as a product of two functions of pH and the overall mass transfer coefficient for molecular iodine. The procedure was applied to the EPICUR experiment and suppression chamber in BWR.

論文

OECD/NEA ARC-F Project; Summary of fission product transport

Lind, T.*; Kalilainen, J.*; Marchetto, C.*; Beck, S.*; 中村 康一*; 木野 千晶*; 丸山 結; 城戸 健太朗; Kim, S. I.*; Lee, Y.*; et al.

Proceedings of 20th International Topical Meeting on Nuclear Reactor Thermal Hydraulics (NURETH-20) (Internet), p.4796 - 4809, 2023/08

The OECD/NEA ARC-F project was established to investigate the accidents at Fukushima Daiichi nuclear power station with the aim of consolidating the observations for deeper understanding of the severe accident progression and the status of reactors and containment vessels. Additionally, the project formed an information sharing framework in reactor safety between Japan and international experts. In order to achieve these objectives, the project focused on three tasks: i) to refine analysis for accident scenarios and associated fission-product transport and dispersion, ii) to compile and manage data on the Fukushima Daiichi NPS accident, and iii) to discuss future long-term projects relevant to the Fukushima Daiichi NPS accident. The work was carried out by 22 partners from 12 countries. In the fission product group, ten organizations worked on five topics which were ranked with a high significance as open issues based on the BSAF project and were thereby selected for further investigations. The five fission product related topics were: i) fission product speciation, ii) iodine chemistry, iii) pool scrubbing, iv) fission product transport and behavior in the buildings, and v) uncertainty analysis and variant calculations. In this paper, the work carried out to investigate these five fission product release and transport topics of special interest in the ARC-F project will be described and summarized.

論文

Conformation, hydration, and ligand exchange process of ruthenium nitrosyl complexes in aqueous solution; Free-energy calculations by a combination of molecular-orbital theories and different solvent models

城戸 健太朗; 金子 政志

Journal of Computational Chemistry, 44(4), p.546 - 558, 2023/02

AA2022-0469.pdf:1.33MB

 被引用回数:0 パーセンタイル:0(Chemistry, Multidisciplinary)

Distribution of solvent molecules near transition-metal complex is key information to comprehend the functionality, reactivity and so on. However, polarizable continuum solvent models still are the standard and conventional partner of molecular-orbital (MO) calculations in the solution system including transition-metal complex. In this study, we investigate the conformation, hydration structure and ligand substitution reaction between NO$$_2^-$$ and H$$_2$$O in aqueous solution for [Ru(NO)(OH)(NO$$_2$$)$$_4$$]$$^{2-}$$ (${bf A}$), [Ru(NO)(OH)(NO$$_2$$)$$_3$$(ONO)]$$^{2-}$$ (${bf B}$) and [Ru(NO)(OH)(NO$$_2$$)$$_3$$(H$$_2$$O)]$$^-$$ (${bf C}$) using a combination method of MO theories and a state-of-the-art molecular solvation technique (NI-MC-MOZ-SCF). In the complexes, the treatment is essentially required because except for nitrosyl ligand, a strong hydrogen bond is formed between the ligand and solvent water. These results are complementary to the data previously obtained by $$^{15}$$N NMR experiment. A dominant species is found in the complex ${bf B}$ conformers and, as expected, different between the solvent models, which reveals that molecular solvation beyond continuum media treatment are required for a reliable description of solvation near transition-metal complex. In the stability constant evaluation of ligand substitution reaction, similar to the previous reports, an assumption that considers the direct association between the dissociated nitrite anion and complex ${bf C}$ is useful to obtain a reliable stability constant.

論文

A Theoretical investigation on the intermolecular potential curve between ruthenium tetroxide and NO$$_{rm X}$$ (X = 1, 2)

城戸 健太朗

International Journal of Quantum Chemistry, 121(21), p.e26781_1 - e26781_15, 2021/11

 被引用回数:1 パーセンタイル:18.25(Chemistry, Physical)

Ruthenium tetroxide (RuO$$_4$$) is one of chemical species of fission products assumed to be released to the environment during a severe accident of nuclear facilities and a target compound to assess the amount produced, reactivity, mobility and release timing. In this article, the NO$$_{rm X}$$ (X = 1, 2) adduct formation of RuO$$_4$$ has been investigated, based on the potential energy curve (PEC) evaluated by UM06, UTPSSh, CASSCF, and CASPT2 methods. At several stationary points, CCSD and LR-CCSD(T) energies are also computed for a comparison. The PEC shows that there is an activation barrier to form the NO$$_{rm X}$$ adduct and that the process is endothermic in terms of free energy. In the system, the electron transfer occurs from NO$$_{rm X}$$ to RuO$$_4$$ when the bond between the nitrogen and oxo ligand is formed. It has been discussed in detail using active orbitals, weight of electron configurations and spin population obtained by CASSCF.

論文

A Noniterative mean-field QM/MM-type approach with a linear response approximation toward an efficient free-energy evaluation

城戸 健太朗

Journal of Computational Chemistry, 40(24), p.2072 - 2085, 2019/09

 被引用回数:2 パーセンタイル:8.93(Chemistry, Multidisciplinary)

Mean-field treatment of solvent provides an efficient technique to investigate chemical processes in solution in QM/MM framework. In the algorithm, an iterative calculation is required to obtain the self-consistency between QM and MM regions, which is a time-consuming step. In the present study, we have proposed a non-iterative approach by introducing a linear response approximation (LRA) into the solvation term in the one-electron part of Fock matrix in a hybrid approach between MO calculations and a three-dimensional integral equation theory for molecular liquids (MC-MOZ-SCF; Kido $textit{et al.}$, J. Chem. Phys. $textbf{143}$, 014103 (2015)). To save the computational time, we have also developed a fast method to generate electrostatic potential map near solute and the solvation term in Fock matrix, using Fourier transformation (FT) and real spherical harmonics expansion (RSHE). To numerically validate the LRA and FT-RSHE method, we applied the present approach to water, carbonic acid and their ionic species in aqueous solution. Molecular properties of the solutes were evaluated by the present approach with four different types of initial wave function and compared with those by the original (MC-MOZ-SCF). From the averaged speed up ratio, the present approach is 13.5 times faster than MC-MOZ-SCF.

論文

Effects of constituents of seawater on formation of volatile iodine by aqueous phase radiation chemistry

端 邦樹; 城戸 健太朗; 西山 裕孝; 丸山 結

NEA/CSNI/R(2016)5 (Internet), p.196 - 203, 2016/05

Model calculations of radiolysis of seawater with iodide were carried out to predict effects of seawater constituents on iodine chemistry at the time of a severe accident. Through the calculations based on $$gamma$$-radiolysis of solutions at ambient temperature, it was found that the production of molecular iodine (I$$_{2}$$) was promoted by the addition of seawater constituents. Especially, Br$$^{-}$$ and HCO$$_{3}$$$$^{-}$$ had a large impact on its production. The production yields of radiolytic species were affected by the addition of I$$^{-}$$ as well. It was also shown that the pH of seawater is an important parameter to determine radiolytic conditions of iodine. These results imply the potential that the injection of seawater has significant impact on iodine source term.

論文

Formation and release of molecular iodine in aqueous phase chemistry during severe accident with seawater injection

城戸 健太朗; 端 邦樹; 丸山 結; 西山 裕孝; 星 陽崇*

NEA/CSNI/R(2016)5 (Internet), p.204 - 212, 2016/05

Seawater injection into the degraded core is one of the measures of accident management as it has been performed at Fukushima Daiichi Nuclear Power Plant. The constituents of seawater deeply relates to the iodine chemistry in the water pool of the suppression chamber, which indicates that it is important to assess their effect on the source term in a severe accident. In the present study, by employing a four-component seawater (SW) model we try to simulate the I$$_2$$ molecules yielding in aqueous solution as the function of time, based on several datasets about chemical reaction kinetics and to evaluate its fraction of the initial inventory released from the solution to gas phase. The amount of I$$_2$$ molecule in gas phase was in proportion as the SW mixing ratio. The combination of bromide and hydrogen-carbonate anions considerably contributes to the behavior of the history of producing I$$_2$$ gas. The oxygen molecules solved from air drastically reduced yielding I$$_2$$ gas by catalytically consuming hydroxyl radicals, while the I$$_2$$ gas increased by the carbon dioxide gas contained in air. The effects of SW and carbon dioxide gas are recommended to be considered in the quantitative discussion about I$$_2$$ gas released from aqueous solution.

論文

A Hybrid framework of first principles molecular orbital calculations and a three-dimensional integral equation theory for molecular liquids; Multi-center molecular Ornstein-Zernike self-consistent field approach

城戸 健太朗; 笠原 健人*; 横川 大輔*; 佐藤 啓文*

Journal of Chemical Physics, 143(1), p.014103_1 - 014103_9, 2015/07

 被引用回数:3 パーセンタイル:11.05(Chemistry, Physical)

In this study, we reported the development of a new quantum mechanics/molecular mechanics (QM/MM)-type framework to describe chemical processes in solution by combining standard molecular-orbital calculations with a three-dimensional formalism of integral equation theory for molecular liquids (MC-MOZ method). The theoretical procedure is very similar to the 3D-RISM-SCF approach. Since the MC-MOZ method is highly parallelized for computation, the present approach has the potential to be one of the most efficient procedures to treat chemical processes in solution. Benchmark tests to check the validity of this approach were performed for two solute (solute water and formaldehyde) systems and a simple S$$rm_N$$2 reaction (Cl$$^-$$ + CH$$_3$$Cl $$rightarrow$$ ClCH$$_3$$ + Cl$$^-$$) in aqueous solution. The results for solute molecular properties and solvation structures obtained by the present approach were in reasonable agreement with those obtained by other hybrid frameworks and experiments. In particular, the results of the proposed approach are in excellent agreements with those of 3D-RISM-SCF.

論文

再処理廃液の沸騰乾固模擬ツールの開発

石川 淳; 城戸 健太朗; 吉田 一雄

日本原子力学会和文論文誌, 12(2), p.165 - 174, 2013/06

日本原子力研究開発機構では、核燃料施設の確率論的安全評価手法の整備の一環として事故影響評価手法の開発を進めている。その中で再処理廃液の沸騰時の物性値の変化を模擬するツールを開発した。再処理施設の機器では、万一、冷却機能が喪失した場合、放射性物質の崩壊熱により内包する溶液(廃液を含む)の温度が上昇し沸騰に至る。このような場合、機器内気相部への放射性物質の移行量が増加し、その移行挙動を評価するには、廃液の温度、硝酸濃度など廃液の物性値が必要となる。開発したツールは、モル沸点上昇の原理に基づき再処理廃液の沸騰、濃縮過程を模擬している。モデルの検証のため実廃液を用いた沸騰実験を解析した。その結果、廃液の硝酸濃度及び温度等の挙動を比較的精度よく計算できることを確認した。

口頭

3次元溶媒和理論(MC-MOZ法)と量子化学の結合による溶液内化学過程に対する新規方法論の開発

城戸 健太朗; 笠原 健人*; 佐藤 啓文*; 横川 大輔*

no journal, , 

分子性液体論に基づく3次元溶媒和理論(MC-MOZ法)は、溶液内の化学過程を理解する上で鍵となる自由エネルギープロファイルや溶媒和構造を分子レベルで評価する強力な方法論である。しかし、化学結合の解離と生成を伴う過程への適用は難しかった。本研究では、MC-MOZ法と標準的な第一原理分子軌道法との結合によってこの点を解決し、より広範な溶液内化学過程に応用可能な新規方法論を開発した。この新規方法論はQM/MM法と同等の物理量を与え、溶液のマルチフィジックスモデルの一つと位置付けられる。水溶液内の水分子やホルムアルデヒド分子、及びSN2反応へ適用した。関連した方法との比較を行い、その有効性を議論した。

口頭

Development and application of methodologies for source term analysis

丸山 結; 石川 淳; Zheng, X.; 城戸 健太朗; 松本 俊慶; 塩津 弘之; 伊藤 裕人; 玉置 等史

no journal, , 

An integral code system for severe accident analysis in light water reactors, THALES2/KICHE, has been developed at Japan Atomic Energy Agency (JAEA). The core melt progression and the transportation of radioactive materials within reactor coolant system (RCS) and containment vessel (CV) are analyzed with THALES2 code in conjunction with KICHE code for the iodine reaction kinetics in aqueous phase. The applications of THALES2/KICHE code have been made in various analytical studies for severe accident progression, including analyses for the accident at the Fukushima Daiichi Nuclear Power Plant (1F) in order to obtain technical knowledge on the source term into the environment and the core damage state.

口頭

水溶液内のヨウ素化学に及ぼす海水成分の影響

城戸 健太朗; 端 邦樹; 丸山 結

no journal, , 

海水は様々な化学種を含み、放射線によって引き起こされる一連の化学反応は純水の場合よりも複雑になり、原子炉のシビアアクシデントの評価に影響しうるが、十分に議論されていない。本研究では海水成分の関与する化学反応を追加した格納容器内ヨウ素化学計算コードKICHEを用いて、ヨウ素分子生成に対する海水成分の効果を検討した。海水成分、特に臭化物イオンによってヨウ素分子の生成が顕著に促進されることが示唆された。純水の場合と同様に、二酸化炭素によってもヨウ素分子の生成が増加した。

口頭

線形応答近似を用いて3次元溶媒和理論(MC-MOZ法)とMO法を非反復に結合する方法の開発

城戸 健太朗

no journal, , 

QM/MM型の溶液内モデルを採用し、自由エネルギーなどの熱力学量に基づきながら化学的挙動を記述することは依然として容易でなく、溶媒の分子論的描像を保持し、尚かつコストの低い枠組みを構築する努力が続けられている。これを実現する有望な方法の一つは、多数の分子から成る溶媒を平均場として取り合うことであるが、MO計算と溶媒が作る平均場との間で溶媒の応答関数が自己無撞着になるまで計算を反復しなければならない。これが計算時間を増大させる要因になっている。一次元の溶媒和理論とMO計算を結合したRISM-SCF-SEDD法では、QM領域に及ぼす溶媒の応答を線形と仮定し、繰り返し計算を経由せずにFock演算子の溶媒和項を評価する方法が考案され、その仮定が良好に成立することが示されている。本研究では、3次元溶媒和理論とMO計算の組み合わせ(MC-MOZ-SCF法)を用いて、この枠組みを3次元に拡張するとともに、複雑な3次元溶媒和においてその線形応答近似がどの程度成立するかについても議論する。

口頭

シビアアクシデントの影響評価における不確かさの低減を目指して

城戸 健太朗

no journal, , 

シビアアクシデント(SA)時のソースターム(放射性物質の種類や放出量など)評価には不確かさが伴う。この不確かさの定量化及び低減を目的とし、SA時の重要現象に係るデータの取得、重要現象解析コードの開発、これらの成果を集約したSA総合解析コードTHALES2の改良を行うとともに、同コードを活用したソースターム評価技術の開発を進めている。個別トピックとして、代替統計モデルを用いたFP化学計算機能の導入、溶融炉心/冷却材相互作用解析コードJASMINEによる溶融炉心冷却性評価、福島第一原子力発電所(1F)原子炉建屋・格納容器内情報分析(OECD/ARC-F計画)を取り上げ、概要を紹介する。

口頭

Analysis of mass transfer effect on chemically produced iodine release from aqueous phase

Zablackaite, G.; 塩津 弘之; 城戸 健太朗; 杉山 智之

no journal, , 

To investigate the mass transfer effect on the overall iodine release to the environment, we propose a chemical-process-reduced mass transfer (CPR-MT) model for iodine that is based on two-film theory and includes an effective mass transfer coefficient ($$tilde{k}_{mt}$$). By formally decomposing $$tilde{k}_{mt}$$ into iodine chemistry and mass transfer contributions, we discuss their impact for a variety of severe accident conditions of nuclear power station.

口頭

A Sensitivity analysis of the iodine production using a reaction kinetics dataset

Zablackaite, G.; 塩津 弘之; Zheng, X.; 城戸 健太朗

no journal, , 

An extensive sensitivity analysis with Monte Carlo sampling of correlated Arrhenius parameters was performed by coupling KICHE and RAPID codes under conditions corresponding to a severe accident. The effect on the released amount of iodine by chemical reactions listed in the reaction kinetics dataset was investigated as well as the uncertain-ties induced by elevated temperature and varied pH.

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