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Rizaal, M.; 中島 邦久; 斉藤 拓巳*; 逢坂 正彦; 岡本 孝司*
ACS Omega (Internet), 7(33), p.29326 - 29336, 2022/08
被引用回数：2 パーセンタイル：29.84(Chemistry, Multidisciplinary)Here we report an investigation of the gas-solid reaction between cesium hydroxide (CsOH) and siliceous (calcium silicate) thermal insulation at high temperature, which was postulated as the origin for the formation mechanism of cesium-bearing material emitted from the Fukushima Daiichi Nuclear Power Plant. A developed reaction furnace consisting of two heating compartments was used to study the reaction at temperatures of 873, 973, and 1073 K. Under the influence of hydrogen-steam atmospheric conditions (H
/HO = 0.2), the reaction between cesium hydroxide vapor and solid thermal insulation was confirmed to occur at temperatures of 973 and 1073 K with the formation of dicalcium silicate (CaSiO
) and cesium aluminum silicate (CsAlSiO). Water-dissolution analyses of the reaction products have demonstrated their stability, in particular, the CsAlSiO. Constituents similarity of the field-observed cesium-bearing materials near the Fukushima Daiichi Nuclear Power Plants with CsAlSiO suggests for the first time that gaseous reaction between CsOH with calcium silicate thermal insulation could be one of the original formation mechanisms of the cesium-bearing materials.
Rizaal, M.; 三輪 周平; 鈴木 恵理子; 井元 純平; 逢坂 正彦; Gou
llo, M.*
ACS Omega (Internet), 6(48), p.32695 - 32708, 2021/12
被引用回数：1 パーセンタイル：6.77(Chemistry, Multidisciplinary)This paper presents our investigation on cesium and iodine compounds revaporization from cesium iodide (CsI) deposits on the surface of stainless steel type 304L, which were initiated by boron and/or steam flow. A dedicated basic experimental facility with a thermal gradient tube (TGT) was used for simulating the phenomena. The number of deposits, the formed chemical compounds, and elemental distribution were analyzed from samples located at temperature range 1000-400 K. In the absence of boron in the gas flow, it was found that the initial deposited CsI at 850 K could be directly re-vaporized as CsI vapor/aerosol or reacted with the carrier gas and stainless steel (CrO layer) to form CsCrO on the former deposited surface. The latter mechanism consequently gave a release of gaseous iodine that was accumulated downstream. After introducing boron to the steam flow, a severe revaporization of iodine deposit at 850 K occurred (more than 70% initial deposit). This was found as a result of the formation of two kinds of cesium borates (CsBO
5HO and CsB
O
4HO) which contributed to a large release of gaseous iodine that was capable of reaching outlet of TGT (
400 K). In the case of nuclear severe accident, our study have demonstrated that gaseous iodine could be expected to increase in the colder region of a reactor after late release of boron or a subsequent steam flow after refloods of the reactor, thus posing its near-term risk once leaked to the environment.
Rizaal, M.; 中島 邦久; 斉藤 拓巳*; 逢坂 正彦; 岡本 孝司*
Journal of Nuclear Science and Technology, 57(9), p.1062 - 1073, 2020/09
被引用回数：8 パーセンタイル：71.58(Nuclear Science & Technology)福島第一原子力発電所2号機においてペデスタル内よりもペデスタル外で線量が高くなっている現象が見つかっている。この線量の上昇については、原子炉格納容器内の配管に使用されている保温材(ケイ酸カルシウム)がガス状あるいは粒子状となって沈着したセシウム(Cs)と化学反応を起こして固着するとともに破損してペデスタル外に堆積することで線量が上昇した可能性があると考えている。そこで、本研究では、化学反応の有無を調べるため、反応温度等を調べることのできる熱重量示差熱分析装置(TG-DTA)を用いて、水素-水蒸気含有雰囲気下、最高1100
Cまで温度を上昇させて、主なセシウム化合物の一つである水酸化セシウムと保温材との混合物に対して分析を行った。その結果、575-730Cの範囲で反応が起こり、試験後試料のX線回折パターンや元素分析機能付き走査型電子顕微鏡(SEM/EDS)による試料表面の元素分布の結果から、保温材の構成物質であるケイ素(Si)に加え、不純物として含まれるアルミニウム(Al)と安定な化合物(CsAlSiO)を形成することが分かった。したがって、ペデスタル外で見つかった高線量の原因として、保温材が関係する可能性があることが分かった。
Rizaal, M.; 斉藤 拓巳*; 岡本 孝司*; Erkan, N.*; 中島 邦久; 逢坂 正彦
Mechanical Engineering Journal (Internet), 7(3), p.19-00563_1 - 19-00563_10, 2020/06
福島第一原子力発電所2号機において、一次冷却系の断熱材に使用されているケイ酸カルシウムへのセシウム(Cs)の吸着がペデスタル領域の高線量に影響している可能性が指摘されている。本研究では、高温でのCs吸着挙動評価の前段階として、室温でCsをケイ酸カルシウムに吸着させる試験を行った。吸着速度の解析の結果、基本的な吸着機構は化学吸着であることが示唆された。また、等温における吸着量のCs濃度依存性の評価により、単分子層の形成の後に多分子層が形成することが示唆された。
Rizaal, M.; 中島 邦久; 逢坂 正彦; 斉藤 拓巳*; Erkan, N.*; 岡本 孝司*
no journal, ,
水蒸気や水素雰囲気下でのカルシウムシリケート保温材と水酸化セシウムとの化学的な相互作用の有無を調べるため、熱重量測定とX線回折測定を行った。この化学的相互作用が、福島第一原子力発電所2号機におけるペデスタル周辺部での高線量の原因の一つになっているのではないかと考えている。試験の結果、雰囲気に関わらず575-730Cの温度範囲において、何らかの化学的相互作用を引き起こすことが示唆された。
Rizaal, M.; 中島 邦久; 逢坂 正彦; 斎藤 拓巳*; 岡本 孝司*
no journal, ,
Here we report an experimental study to investigate cesium retention on calcium silicate insulation following the gas-solid chemical interaction at high temperature. This study used cesium hydroxide and was performed at 800C under the oxidizing atmosphere. The experimental results showed that water-insoluble CsAlSiO and dicalcium silicate/larnite (CaSiO) were formed, which evidences that the insulation material could retain Cs in a stable form.
Rizaal, M.; 三輪 周平; 鈴木 恵理子; 井元 純平; 逢坂 正彦; Goullo, M.*
no journal, ,
We experimentally investigated the revaporization behavior of deposited CsI on the surface of structural material such as stainless steel in a subsequent high-temperature steam flow (i.e., reflood of the reactor at severe accident). The experimental results showed that the deposited CsI at 850 K could be vaporized, leaving the formation of CsCrO on that once-deposited surface and consequently giving an increase of iodine release observed below 400 K.
Rizaal, M.; 三輪 周平; 鈴木 恵理子; 井元 純平; 逢坂 正彦; Goullo, M.*
no journal, ,
The gaseous species interaction with the early FP deposits (mostly Cs and I) on the surface of reactor coolant system (RCS) material is considered as one of the factors of gaseous iodine generation at late phase and could affect the containment atmosphere. In this study, we investigated the reaction of CsI deposit formed on stainless steel type 304 L with steam-boron flow. The experiments for reproducing the RCS temperature gradient in the range of 1000-400 K showed a significant depletion of CsI at 850 K, resulted in formation of low volatile cesium borates on the former surface and simultaneously released gaseous iodine that could reach a range below 400 K.
唐澤 英年; Rizaal, M.; 三輪 周平; 木野 千晶*
no journal, ,
SA解析において、FP挙動に関する化学反応の重要性が指摘されている。このため、先回(23春)、CsI挙動の温度依存性を評価したTeRRa実験を対象にし、ガス状ヨウ素の生成量をECUMEデータベースを用いた気相化学反応モデルにより評価した。今回、CsI挙動に及ぼすBの影響を評価したCsI-BO
-TeRRa実験を対象にして気相化学反応モデルを検討した。実験では、BOを加えることによりガス状ヨウ素の生成量が約10倍増加した。この挙動を説明するため、Cs-I-H-O系反応機構にBOから生成するHBOとCsIやCsOHとの反応を追加したCs-I-B-H-O系反応機構を設定した。ECUMEデータベースの化学反応速度を用いた計算により、BOの追加によるガス状ヨウ素の増加(1,273Kで約26倍)を確認できた。高温でガス状ヨウ素が十分に生成するという結果が得られ、温度が下がっていく移行時の化学反応計算によりTeRRa実験を再現できる可能性を示した。
Rizaal, M.; 中島 邦久; 唐澤 英年; Luu, V. N.; 三輪 周平
no journal, ,
Our research focused on, but is not limited to, cesium (Cs) and iodine (I) chemistry due to their high impact on the overall source term. The retention or release of both elements is largely affected by chemical interaction with materials that are present in the reactor. To understand their chemistry during transport in the event of a nuclear severe accident (SA), we studied the interaction phenomena taking place from high- to low-temperature conditions. We have succeeded in elucidating these phenomena (particularly Cs) and summarized them in a fission product (FP) chemistry database ECUME. This database not only could deepen our understanding of the mechanism of Cs and I chemistry in an SA, but could also improve source term analysis. Improvement in the reaction between Cs vapor and stainless steel was shown by the use of the ECUME database in SA analysis code SAMPSON. Better reproducibility of Cs retention at high temperatures of the large-scale experiment was obtained, in contrast to using the MELCOR Cs interaction model (i.e. widely used model in SA code) that was worse in reproducing such phenomenon. Taking into consideration of near-term implementation of Accident Tolerant Fuel (ATF) materials such as chromium (Cr)-coated Zircaloy, further study on the interaction with FP would be important to ensure the material impact on source term because the reaction between Cs and Cr can be thermodynamically expected.
