The Precipitation and redistribution of alloying element in Zircaloy-4 cladding tube oxidized in high-temperature steam

天谷 政樹

High Temperature Corrosion of Materials, 15 Pages, 2024/00

https://doi.org/10.1007/s11085-023-10216-9

Zirconium (Zr)-based alloys are widely used as fuel cladding material for light water reactors. Under a loss-of-coolant accident (LOCA) condition, the oxidation of fuel cladding by high-temperature steam induces the degradation of mechanical properties of the cladding and would affect the integrity of fuel rods and/or assemblies, etc., during LOCA. In this study, the distribution of the elements (zirconium, oxygen, tin, iron and chromium) in Zircaloy-4 cladding specimens oxidized in the temperature range of 1350- 1700 K in steam was analyzed along the radial direction of the specimens by using SEM/EPMA, and the cause of element distribution in the specimens was discussed in consideration of the morphology of precipitates in the specimens and hypothesized phase diagrams related to the elements contained in the specimens. The form of the particles precipitated and the comparison between SEM/EPMA results and hypothesized phase diagrams of Zr-Sn-O system suggested that the liquefaction of tin-rich material and/or Zr-(Fe,Cr) compounds occurred during the oxidation test. The results obtained indicate that Zircaloy-4 cladding tubes would start melting at the melting point of tin-oxide and the eutectic point of Zr-(Fe,Cr)compounds, which is much lower than the melting point of Zr, -Zr(O), or zirconium oxide (ZrO).

Microstructural evolution of intermetallic phase precipitates in Cr-coated zirconium alloy cladding in high-temperature steam oxidation up to 1400C

Mohamad, A. B.; 根本 義之; 古本 健一郎*; 岡田 裕史*; 佐藤 大樹*

Corrosion Science, 224, p.111540_1 - 111540_15, 2023/11

https://doi.org/10.1016/j.corsci.2023.111540

The steam oxidation test on the Cr-coated Zry cladding was studied up to 1400C to understand the oxidation behavior under the accidental conditions. The double-sided oxidation test study showed that Cr coating can protect Zry cladding at 1200C within 5 min. Cr coating has a protective effect on the Zry cladding up to 1200C in a steam environment. However, in the oxidation test up to 1200C/30 min and 1300C/5 min, Cr coating can no longer protect Zry cladding. Furthermore, at 1300C, the intermetallic phase of the Zr(Cr, Fe) phase that precipitated within the Zry substrate formed as globule microstructures with Fe enrichment. In addition, the transition of the intermetallic phase within the Zry substrate from the solid to the pre-liquid and liquid phases was observed, where it was determined at 1350C/60 min and 1400C/30 min within the ZrO phase (outer side region). The oxidation of the Zr(Cr, Fe) interlayer was also determined in this study, where it resulted in the formation of the oxide phase of Cr, Zr, and Fe. It is worth mentioning that further experiments, such as mechanical testing and modeling, should be considered to support the degradation of the Cr-coated Zry cladding mainly when the liquid phase of the intermetallic phase is obtained for beyond design-basis accident environment.

Anaerobic methane-oxidizing activity in a deep underground borehole dominantly colonized by Methanoperedenaceae

西村 大樹*; 幸塚 麻里子*; 福田 朱里*; 石村 豊穂*; 天野 由記; 別部 光里*; 宮川 和也; 鈴木 庸平*

Environmental Microbiology Reports (Internet), 15(3), p.197 - 205, 2023/06

https://doi.org/10.1111/1758-2229.13146

地下深部の地下水は、微生物活動などにより酸素が消費され、一般に還元性になっている。幌延の深部地下水では、微生物活動による二酸化炭素還元反応により強還元雰囲気が維持されている。一方で、幌延深地層研究センター地下研究施設の一部のボーリング孔では、嫌気的環境にも関わらずメタン酸化機能を持つ微生物の存在が明らかにされている。局所的ではあるものの、地下深部の強還元雰囲気において進行する酸化反応機構の解明を目的として、本研究では、原位置の水質・水圧を模擬し、この嫌気的メタン酸化微生物の培養を行った。その結果、地下水中の懸濁物に含まれる非晶質鉄あるいは2八面体型スメクタイトに含まれる3価鉄が酸化剤として機能していることが分かった。このような酸化剤が地下深部に存在する要因の一つとして、ボーリングの掘削泥水などの掘削に伴う人為的影響が挙げられる。高レベル放射性廃棄物の地層処分において閉鎖後の処分場坑道周辺の酸化性環境は、周辺母岩中の鉱物との反応等により本来の還元性に戻ると考えられている。本研究で得られた知見は、この処分場閉鎖後の回復過程についてより正確な理解に繋がる成果である。

溶融鉛ビスマス共晶合金中における9Cr-1Mo鋼の腐食抑制のための適切な酸素濃度の検討

小松 篤史

JAEA-Research 2021-019, 24 Pages, 2022/05

JAEA-Research-2021-019.pdf:1.53MB

溶融鉛ビスマス共晶合金(LBE)中における材料の腐食速度を低減するには、酸素濃度を調整することが重要とされており、過去の報告を見ると約1010wt%程度に酸素濃度を調整していることが多い。しかし、どの程度の濃度が最適と明言されているわけではなく、この濃度範囲内であっても激しく腐食したとの報告もある。本研究では9Cr-1Mo鋼を例にとり、酸化被膜中およびLBE中の拡散を考慮した腐食モデルを作成し、腐食モデルから推定される腐食抑制策について検討を行った。また、LBE中における9Cr-1Mo鋼の腐食を低減しつつ、流れのあるループ環境における低温部での流路閉塞を防止するための最適酸素濃度の算出を試みた。その結果、LBE中における9Cr-1Mo鋼の腐食形態は、酸化被膜厚さと拡散層厚さの比や、LBE中の鉄濃度、温度等により変化し、その成膜挙動から、緻密被膜生成、沈殿被膜生成、被膜溶解の三つに分類した。腐食を抑制するには、緻密被膜生成条件を維持できるように酸素濃度を調整することが重要であり、そのためにはLBEに浸漬する前に10m程度以上の予備酸化被膜をつける必要がある。従来報告でよく用いられる約1010wt%という酸素濃度は、酸化被膜がある程度成長した場合の最適酸素濃度であり、被膜が薄い場合は、より高い酸素濃度が必要になると推察された。

Oxidative decomposition of ammonium ion with ozone in the presence of cobalt and chloride ions for the treatment of radioactive liquid waste

粟飯原 はるか; 渡部 創; 柴田 淳広; Mahardiani, L.*; 大友 亮一*; 神谷 裕一*

Progress in Nuclear Energy, 139, p.103872_1 - 103872_9, 2021/09

https://doi.org/10.1016/j.pnucene.2021.103872

To prevent unexpected accidents at nuclear facilities caused by accumulated ammonium nitrate in an aqueous liquid waste containing ammonium salts and nitric acid, NH in the liquid waste must be decomposed under mild reaction conditions. In this study, we investigated the oxidative decomposition of NH with O at 333 K in the presence of a homogeneous Co catalyst and Cl in the wide pH range of the test solution. The reaction behavior was greatly affected by pH of the test solution. In a basic solution at pH 12, high conversion of NH was obtained even in the absence of Co and Cl and the main product was NO . However, Co and Cl in the solution greatly enhanced the decomposition rate of NH in acidic to mild basic solutions (pH 1-8), while only low conversion of NH was observed unless both Co and Cl were present. For the reaction with Co and Cl in the solutions, NH was transformed mainly into chloramines (NH Cl , x = 1-3) by the reaction with HClO, which was formed by the reaction of Cl with O catalyzed by the homogeneous Co catalyst, and led to the high decomposition rate of NH . Cl suppressed the formation of the precipitate CoO(OH) during the reaction and consequently the Co catalyst stably existed in the reaction solution, which was another reason for the high decomposition rate of NH in the presence of Cl. Owing to the swift decomposition of NH under mild reaction conditions and small formation of secondary waste, the oxidative decomposition of NH in the presence of the homogeneous Co catalyst and Cl is suitable and applicable for the treatment of the aqueous liquid waste containing ammonium salts and nitric acid.

The Kinetics and mechanism of HO decomposition at the UO surface in bicarbonate solution

McGrady, J.; 熊谷 友多; 渡邉 雅之; 桐島 陽*; 秋山 大輔*; 北村 暁; 紀室 辰伍

RSC Advances (Internet), 11(46), p.28940 - 28948, 2021/08

https://doi.org/10.1039/D1RA05580A

The rate of U release is affected by bicarbonate (HCO) concentrations in the groundwater, as well as HO produced by water radiolysis. To understand the dissolution of UO by HO in bicarbonate solution (0.1 - 50 mM), dissolved U concentrations were measured upon HO addition (300 M) to UO/bicarbonate mixtures. As the HO decomposition mechanism is integral to U dissolution, the kinetics and mechanism of HO decomposition at the UO surface was investigated. The dissolution of U increased with bicarbonate concentration which was attributed to a change in the HO decomposition mechanism from catalytic at low bicarbonate ( 5 mM HCO) to oxidative at high bicarbonate ( 10 mM HCO). Catalytic HO at low bicarbonate was attributed to the formation of an oxidised surface layer.

Corrosion property of container using hybrid material for thermal decomposition process of sulfuric acid

井岡 郁夫; 栗木 良郎*; 岩月 仁; 川井 大輔*; 横田 博紀*; 稲垣 嘉之; 久保 真治

Proceedings of 2020 International Conference on Nuclear Engineering (ICONE 2020) (Internet), 5 Pages, 2020/08

https://doi.org/10.1115/ICONE2020-16783

熱化学水素製造法ISプロセスは、大規模水素製造法の候補の一つとして研究開発が進められている。ISプロセスの硫酸を蒸発・ガス化し、熱分解する工程の腐食環境は過酷であり、その環境に耐える機器材料にはセラミックスが用いられている。本研究では、脆性材料であるセラミックスを代替し得る、表面改質技術を用いて耐硫酸性と延性を兼ね備えるハイブリッド材料の開発を狙いとしている。プラズマ溶射技術とレーザー溶融処理技術の組み合わせより試作したハイブリッド材料試験片は、95%沸騰硫酸中で十分な耐食性を示した。これは、表面に形成させた高Si濃度の耐食層が硫酸環境で酸化し、接液面にSiO層が生成したためと考えられる。本技術による容器状構造体を試作した。本報では、容器状構造体の耐食性を向上するための実施した熱処理の結果と容器状構造体の耐食性評価方法について報告する。

Fracture limit of high-burnup advanced fuel cladding tubes under loss-of-coolant accident conditions

成川 隆文; 天谷 政樹

Journal of Nuclear Science and Technology, 57(1), p.68 - 78, 2020/01

https://doi.org/10.1080/00223131.2019.1659873

To evaluate the fracture limit of high-burnup advanced fuel cladding tubes under loss-of-coolant accident (LOCA) conditions, laboratory-scale integral thermal shock tests were performed using the following advanced fuel cladding tubes with burnups of 73 - 85 GWd/t: M-MDA, low-tin ZIRLO, M5, and Zircaloy-2 (LK3). In total eight integral thermal shock tests were performed for these specimens, simulating LOCA conditions including ballooning and rupture, oxidation, hydriding, and quenching. During the tests, the specimens were oxidized to 10% - 30% equivalent cladding reacted (ECR) at approximately 1473 K and were quenched under axial restraint load of approximately 520 - 530 N. The effects of burnup extension and use of the advanced fuel cladding tubes on the ballooning and rupture, oxidation, and hydriding under LOCA conditions were inconsiderable. Further, the high-burnup advanced fuel cladding tube specimens did not fracture in the ECR values equal to or lower than the fracture limits of the unirradiated Zircaloy-4 cladding tube reported in previous studies. Therefore, it can be concluded that the fracture limit of fuel cladding tubes is not significantly reduced by extending the burnup to approximately 85 GWd/t and using the advanced fuel cladding tubes, though it slightly decreases with increasing initial hydrogen concentration.

Oxidation of silicon carbide in steam studied by laser heating

Pham, V. H.; 永江 勇二; 倉田 正輝; 古本 健一郎*; 佐藤 寿樹*; 石橋 良*; 山下 真一郎

Proceedings of International Nuclear Fuel Cycle Conference / Light Water Reactor Fuel Performance Conference (Global/Top Fuel 2019) (USB Flash Drive), p.670 - 674, 2019/09

Silicon carbide (SiC) has recently attracted much attention as a potential material for accident tolerant fuel cladding. To investigate the performance of SiC in severe accident conditions, study of steam oxidation at high temperatures is necessary. However, the study focusing on steam oxidation of SiC at temperatures above 1600C is still certainly limited due to lack of test facilities. With the extreme oxidation/corrosion environment in steam at high temperatures, current refractory materials such as alumina and zirconia would not survive during the tests. Application of laser heating technique could be a great solution for this problem. Using laser heating technique, we can localize the heat and focus them on the test sample only. In this study, we developed a laser heating facility to investigate high-temperature oxidation of SiC in steam at temperature range of 1400-1800C for 1-7 h. The oxidation kinetics is then being studied based on the weight gain and observation on cross-sectioned surface of tested sample using field emission scanning electron microscope. Off-gas measurement of hydrogen (H) and carbon monoxide (CO) generated during the test is also being conducted via a sensor gas chromatography. Current results showed that the SiC sample experienced a mass loss process which obeyed paralinear laws. Parabolic oxidation rate constant and linear volatilization rate constant of the process were calculated from the mass change of the samples. The apparent activation energy of the parabolic oxidation process was calculated to be 85 kJ.mol. The data of the study also indicated that the mass change of SiC under the investigated conditions reached to its steady stage where hydrogen generation became stable. Above 1800C, a unique bubble formation on sample surface was recorded.

Experimental determination of the photooxidation of aqueous I as a source of atmospheric I

渡辺 耕助*; 松田 晶平; Cuevas, C. A.*; Saiz-Lopez, A.*; 薮下 彰啓*; 中野 幸夫*

ACS Earth and Space Chemistry (Internet), 3(4), p.669 - 679, 2019/04

https://doi.org/10.1021/acsearthspacechem.9b00007

ヨウ素の物質循環において、海洋から大気への輸送が起こる要因の一つに、海面付近での太陽光による光酸化過程がある。これまでに液相のヨウ化物イオン(I)の光酸化によって気相へヨウ素分子(I)が放出されることが報告されている。この過程の全球的な影響を評価するため、溶液中のIの光酸化を実験的に詳細に調べ、290-500nmの波長域におけるIのモル吸光係数と光酸化の量子収率を決定した。さらに、I光酸化量子収率のpHおよび溶存酸素依存性を調べた。これらから海洋条件でのIの光酸化によるI放出速度を推定した。全球化学気候モデルを用いてシミュレーションを行った結果、低緯度域では1年あたり約8%の寄与があることが明らかになった。本研究は、シビアアクシデント時において環境中へ飛散される放射性ヨウ素の挙動を予測するために必要な基礎的知見を与える。