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論文

Steam oxidation of silicon carbide at high temperatures for the application as accident tolerant fuel cladding, an overview

Pham, V. H.; 倉田 正輝; Steinbrueck, M.*

Thermo (Internet), 1(2), p.151 - 167, 2021/09

Since the nuclear accident at Fukushima Daiichi Nuclear Power Station in 2011, a considerable number of studies have been conducted to develop accident tolerant fuel (ATF) claddings for safety enhancement of light water reactors. Among many potential ATF claddings, silicon carbide is one of the most promising candidates with many superior features suitable for nuclear applications. In spite of many potential benefits of SiC cladding, there are some concerns over the oxidation/corrosion resistance of the cladding, especially at extreme temperatures (up to 2000$$^{circ}$$C) in severe accidents. However, the study of SiC steam oxidation in conventional test facilities in water vapor atmospheres at temperatures above 1600$$^{circ}$$C is very challenging. In recent years, several efforts have been made to modify existing or to develop new advanced test facilities to perform material oxidation tests in steam environments typical of severe accident conditions. In this article, the authors outline the features of SiC oxidation/corrosion at high temperatures, as well as the developments of advanced test facilities in their laboratories, and, finally, give some of the current advances in understanding based on recent data obtained from those advanced test facilities.

論文

The Kinetics and mechanism of H$$_{2}$$O$$_{2}$$ decomposition at the U$$_{3}$$O$$_{8}$$ 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

 被引用回数:0

The rate of U release is affected by bicarbonate (HCO$$_{3}$$$$^{-}$$) concentrations in the groundwater, as well as H$$_{2}$$O$$_{2}$$ produced by water radiolysis. To understand the dissolution of U$$_{3}$$O$$_{8}$$ by H$$_{2}$$O$$_{2}$$ in bicarbonate solution (0.1 - 50 mM), dissolved U concentrations were measured upon H$$_{2}$$O$$_{2}$$ addition (300 $$mu$$M) to U$$_{3}$$O$$_{8}$$/bicarbonate mixtures. As the H$$_{2}$$O$$_{2}$$ decomposition mechanism is integral to U dissolution, the kinetics and mechanism of H$$_{2}$$O$$_{2}$$ decomposition at the U$$_{3}$$O$$_{8}$$ surface was investigated. The dissolution of U increased with bicarbonate concentration which was attributed to a change in the H$$_{2}$$O$$_{2}$$ decomposition mechanism from catalytic at low bicarbonate ($$leq$$ 5 mM HCO$$_{3}$$$$^{-}$$) to oxidative at high bicarbonate ($$geq$$ 10 mM HCO$$_{3}$$$$^{-}$$). Catalytic H$$_{2}$$O$$_{2}$$ at low bicarbonate was attributed to the formation of an oxidised surface layer.

報告書

軽水型動力炉の非常用炉心冷却系の性能評価指針の技術的根拠と高燃焼度燃料への適用性

永瀬 文久; 成川 隆文; 天谷 政樹

JAEA-Review 2020-076, 129 Pages, 2021/03

JAEA-Review-2020-076.pdf:3.9MB

軽水炉においては、冷却系配管破断等による冷却材喪失事故(LOCA)時にも炉心の冷却可能な形状を維持し放射性核分裂生成物の周辺への放出を抑制するために、非常用炉心冷却系(ECCS)が設置されている。ECCSの設計上の機能及び性能を評価し、評価結果が十分な安全余裕を有することを確認するために、「軽水型動力炉の非常用炉心冷却系の性能評価指針」が定められている。同指針に規定されている基準は1975年に定められた後、1981年に当時の最新知見を参考に見直しが行われている。その後、軽水炉においては燃料の高燃焼度化及びそれに必要な被覆管材料の改良や設計変更が進められたが、それに対応した指針の見直しは行われていない。一方、高燃焼度燃料のLOCA時挙動や高燃焼度燃料への現行指針の適用性に関する多くの技術的な知見が取得されてきている。本報告においては、我が国における指針の制定経緯及び技術的根拠を確認しつつ、国内外におけるLOCA時燃料挙動に係る最新の技術的知見を取りまとめる。また、同指針を高燃焼度燃料に適用することの妥当性に関する見解を述べる。

論文

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

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$$_{2}$$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。

論文

Oxidation kinetics of silicon carbide in steam at temperature range of 1400 to 1800$$^{circ}$$C studied by laser heating

Pham, V. H.; 永江 勇二; 倉田 正輝; Bottomley, D.; 古本 健一郎*

Journal of Nuclear Materials, 529, p.151939_1 - 151939_8, 2020/02

AA2019-0197.pdf:1.61MB

 被引用回数:2 パーセンタイル:62.87(Materials Science, Multidisciplinary)

As expected for accident tolerant fuels, investigation of steam oxidation for silicon carbide under the conditions beyond design basis accident scenarios is needed. Many studies focused on steam oxidation of SiC at temperatures up 1600$$^{circ}$$C have been conducted and reported in the literature. However, behavior of SiC in steam at temperatures above 1600$$^{circ}$$C still remains unclear. To complete this task, we have designed and manufactured a laser heating facility for steam oxidation at extreme temperatures. With the facility, we report the first results on the steam oxidation behavior of SiC at temperatures range of 1400-1800$$^{circ}$$C for short term exposure of 1-7 h under atmospheric pressure. Based on the mass change of SiC samples, parabolic oxidation rate and linear volatilization rate were calculated. The oxidation layer appears to be maintained at 1800$$^{circ}$$C in steam, but the bubble formation phenomenon suggests other volatilization reactions may limit its life.

論文

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

 被引用回数:1 パーセンタイル:23.13(Nuclear Science & Technology)

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$textsuperscript{texttrademark}$, low-tin ZIRLO$textsuperscript{texttrademark}$, M5$textsuperscript{textregistered}$, 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.

論文

Behavior of high-burnup advanced LWR fuel cladding tubes under LOCA conditions

成川 隆文; 天谷 政樹

Proceedings of International Nuclear Fuel Cycle Conference / Light Water Reactor Fuel Performance Conference (Global/Top Fuel 2019) (USB Flash Drive), p.912 - 921, 2019/09

To evaluate behavior of high-burnup advanced light-water-reactor fuel cladding tubes under loss-of-coolant accident conditions, laboratory-scale isothermal oxidation tests and integral thermal shock tests were performed using the following advanced fuel cladding tubes with burnups of 73-85 GWd/t: M-MDA$textsuperscript{texttrademark}$, low-tin ZIRLO$textsuperscript{texttrademark}$, M5textregistered, and Zircaloy-2 (LK3). The isothermal oxidation tests were performed in steam-flowing conditions at temperatures ranging from 1173 to 1473 K for durations between 120 and 4000 s. The oxidation kinetics of the high-burnup advanced fuel cladding tube specimens was comparable to or slower than that of the unirradiated Zircaloy-4 cladding tube and was slower than that given by the Baker-Just oxidation rate equation. Therefore, the oxidation kinetics is considered to be not significantly accelerated by extending the burnup and changing the alloy composition. During the integral thermal shock tests, the high-burnup advanced fuel cladding tube specimens did not fracture under the oxidation condition equivalent to or lower than the fracture limit of the unirradiated Zircaloy-4 cladding tube. Therefore, the fracture limit of fuel cladding tubes is considered to be not significantly reduced by extending the burnup and changing the alloy composition, though it may slightly decrease with increasing initial hydrogen concentration.

論文

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 1600$$^{circ}$$C 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 1400-1800$$^{circ}$$C 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$$_{2}$$) 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$$^{-1}$$. 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 1800$$^{circ}$$C, a unique bubble formation on sample surface was recorded.

論文

Oxidation behavior of high-burnup advanced fuel cladding tubes in high-temperature steam

成川 隆文; 天谷 政樹

Journal of Nuclear Science and Technology, 56(7), p.650 - 660, 2019/07

 被引用回数:6 パーセンタイル:81.33(Nuclear Science & Technology)

To evaluate the oxidation behavior of high-burnup advanced fuel cladding tubes in high-temperature steam, laboratory-scale isothermal oxidation tests were conducted using the following advanced fuel cladding tubes with burnups of up to 85 GWd/t: M-MDA$textsuperscript{texttrademark}$, low-tin ZIRLO$textsuperscript{texttrademark}$, M5$textsuperscript{textregistered}$, and Zircaloy-2 (LK3). These oxidation tests were performed in steam-flowing conditions at temperatures ranging from 1173 to 1473 K for durations between 120 and 4000 s, and the oxidation kinetics was evaluated. The oxidation kinetics of the high-burnup advanced fuel cladding tube specimens estimated by assuming the parabolic rate law was comparable to or slower than that of the unirradiated Zircaloy-4 cladding tube specimens reported in a previous study. It is considered that the protective effect of the corrosion layer hindered oxidation. Furthermore, no increase in the oxidation kinetics because of the pre-hydriding was observed. The onset times of the breakaway oxidations of these cladding tube specimens were comparable to those of the unirradiated Zircaloy-4 cladding tubes reported in previous studies. Therefore, it is considered that the burnup extension up to 85 GWd/t and the use of the advanced fuel cladding tubes do not significantly increase the oxidation kinetics and do not significantly reduce the onset time of the breakaway oxidation.

論文

Experimental determination of the photooxidation of aqueous I$$^{-}$$ as a source of atmospheric I$$_{2}$$

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

 被引用回数:2 パーセンタイル:26.9(Chemistry, Multidisciplinary)

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$$^{-}$$$$_{(aq)}$$)の光酸化によって気相へヨウ素分子(I$$_{2}$$$$_{(g)}$$)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$$^{-}$$$$_{(aq)}$$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$$^{-}$$$$_{(aq)}$$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$$^{-}$$$$_{(aq)}$$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$$^{-}$$$$_{(aq)}$$の光酸化によるI$$_{2}$$$$_{(g)}$$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。

論文

Effect of re-oxidation rate of additive cations on corrosion rate of stainless steel in boiling nitric acid solution

入澤 恵理子; 山本 正弘; 加藤 千明; 本岡 隆文; 伴 康俊

Journal of Nuclear Science and Technology, 56(4), p.337 - 344, 2019/04

 被引用回数:0 パーセンタイル:0.01(Nuclear Science & Technology)

The boiling nitric acid solution containing highly oxidizing cations dissolved from the spent nuclear fuels corrodes stainless steels because of the nobler corrosion potential and their fast reduction rate. The cations themselves are re-oxidized to higher oxidizing states in a bulk solution after the corrosion reaction. In this paper, the re-oxidation rate constants of typical cations, such as Cr, V, Pu, and Np, were analyzed, and discussed about the effect on time dependencies of the corrosion rate. It was indicated that the cations with a large re-oxidation rate constant, such as Np, could keep the corrosion rate at high level continuously for the long immersion duration.

論文

Effects of ballooning and rupture on the fracture resistance of Zircaloy-4 fuel cladding tube after LOCA-simulated experiments

湯村 尚典; 天谷 政樹

Annals of Nuclear Energy, 120, p.798 - 804, 2018/10

 被引用回数:3 パーセンタイル:45.99(Nuclear Science & Technology)

To investigate the relationship between the fracture resistance of a cladding tube and the amount of deformation of the cladding tube due to ballooning and rupture during a loss-of-coolant accident (LOCA), four-point-bending tests were performed using non-irradiated Zircaloy-4 cladding tubes which experienced a LOCA-simulated sequence (ballooning, rupture, high temperature oxidation and quench). According to the obtained results, it was found that the maximum bending stress of the cladding tube after the LOCA-simulated sequence, which was defined as the fracture resistance, correlated to the average thickness of prior-$$beta$$ layer in the cladding tube. Based on the average thickness of prior-$$beta$$ layer, the fracture resistance of the cladding tube with ballooning and rupture was expressed as functions of isothermal oxidation time and temperature and the maximum circumferential strain on the cladding tube.

論文

Improvements in drill-core headspace gas analysis for samples from microbially active depths

宮川 和也; 奥村 文章*

Geofluids, 2018, p.2436814_1 - 2436814_11, 2018/10

 被引用回数:1 パーセンタイル:15.51(Geochemistry & Geophysics)

地下深部の炭化水素ガスの濃度組成や同位体組成を調べる事により、炭化水素ガスの起源や長期的な流体移動に関する知見が得られる。地上からのボーリング孔を用いてガス採取を行う場合によく用いられる方法として、ボーリングのカッティングスあるいはコアに吸着したガスをサンプル容器のヘッドスペースに追い出して分析する方法がある。しかしながら、この方法により得られた結果は、しばしば大きなばらつきを示す。本研究では、上述の手法により採取・分析した結果を、地下施設を利用して得られた結果と比較し、ばらつきの原因が試料の採取・保管方法に起因することを明らかにし、正しい値を得るための要点を指摘した。

論文

Oxidation kinetics of Zry-4 fuel cladding in mixed steam-air atmospheres at temperatures of 1273 - 1473 K

Negyesi, M.; 天谷 政樹

Journal of Nuclear Science and Technology, 54(10), p.1143 - 1155, 2017/10

 被引用回数:5 パーセンタイル:58.67(Nuclear Science & Technology)

This paper deals with the oxidation behavior of Zry-4 nuclear fuel cladding tubes in mixed steam_air atmospheres at temperatures of 1273 and 1473 K. The main goal is to study the oxidation kinetics of Zry-4 fuel cladding in dependence on the air fraction in steam in the range from 0 up to 100%. The purpose of this study is to provide experimental data suitable for an oxidation correlation applicable for thermomechanical analysis codes of nuclear power reactor under severe accidents. The influence of the air addition in steam on parameters of Zry-4 kinetic equation has been quantified using the results of weight gain measurements. At 1273 K, both pre-transient and post-transient regimes were treated. The results of weight gain measurements showed a strong dependence of the Zry-4 oxidation kinetics on the air fraction in steam, especially at 1473 and at 1273 K in the post-transient regime.

論文

Behavior of high-burnup advanced LWR fuels under design-basis accident conditions

天谷 政樹; 宇田川 豊; 成川 隆文; 三原 武; 谷口 良徳

Proceedings of 2017 Water Reactor Fuel Performance Meeting (WRFPM 2017) (USB Flash Drive), 10 Pages, 2017/09

JAEA has conducted a research program called ALPS-II program for advanced fuels of LWRs. In this program, the tests simulating a RIA and a LOCA have been performed on the high burnup advanced fuels irradiated in European commercial reactors. The failure limits of the high-burnup advanced fuels under RIA conditions have been obtained by the pulse irradiation tests at the NSRR in JAEA. The information about pellet fragmentation etc. during the pulse irradiations was also obtained from post-test examinations on the test rods after the pulse irradiation tests. As for the simulated LOCA test, integral thermal shock tests and high-temperature oxidation tests have been performed at the RFEF in JAEA. The fracture limits under LOCA and post-LOCA conditions etc. of the high-burnup advanced fuel cladding have been investigated, and it was found that in terms of these materials the fracture boundaries do not decrease and the oxidation does not significantly accelerate in the burnup level examined.

論文

Oxidation and reduction behaviors of a prototypic MgO-PuO$$_{2-x}$$ inert matrix fuel

三輪 周平; 逢坂 正彦

Journal of Nuclear Materials, 487, p.1 - 4, 2017/04

 被引用回数:4 パーセンタイル:50.55(Materials Science, Multidisciplinary)

MgOを母材としたPuO$$_{2-x}$$含有イナートマトリックス原型燃料の酸化還元挙動を熱分析により調べ、MgO-PuO$$_{2-x}$$試料の酸化還元速度を実験的に決定した。MgO-PuO$$_{2-x}$$試料の酸化還元速度はPuO$$_{2-x}$$試料に比べ低くなった。また、高い酸素分圧条件において、MgO-PuO$$_{2-x}$$試料の定比からのO/Pu比の変化はPuO$$_{2-x}$$試料に比べ小さくなることがわかった。これより、MgOを母材にすることによりイナートマトリックス燃料からの酸素供給及び放出が抑制されることがわかり、被覆管酸化等の観点で懸念となる高い酸素ポテンシャルを有するマイナーアクチニド酸化物の母材としてMgOが有効であることがわかった。

論文

Corrosion test of HTGR graphite with SiC coating

Chikhray, Y.*; Kulsartov, T.*; Shestakov, V.*; Kenzhina, I.*; Askerbekov, S.*; 角田 淳弥; 植田 祥平; 柴田 大受; 坂場 成昭; Abdullin, Kh.*; et al.

Proceedings of 8th International Topical Meeting on High Temperature Reactor Technology (HTR 2016) (CD-ROM), p.572 - 577, 2016/11

高温ガス炉(HTGR)に用いられる黒鉛について、耐腐食性を有するSiC被覆の適用は重要な課題である。本研究は、高密度のIG-110黒鉛にSiCを傾斜被覆した材料について、1673Kまでの温度で蒸気との化学反応の結果を示したものである。100Paの蒸気との反応に関する実験では、SiC酸化のパッシブ反応により表面にSiO$$_{2}$$膜が形成されることを確認した。また、1Paの蒸気とのアクティブ反応では、表面に様々なカーボン複合材が形成されることを確認した。

論文

In situ X-ray diffraction study of the oxide formed on alloy 600 in borated and lithiated high-temperature water

渡邉 真史*; 米澤 利夫*; 菖蒲 敬久; 城 鮎美; 庄子 哲雄*

Corrosion, 72(9), p.1155 - 1169, 2016/09

 被引用回数:0 パーセンタイル:0.01(Materials Science, Multidisciplinary)

In situ X-ray diffraction (XRD) measurements of the oxide film formed on Alloy 600 in borated and lithiated high-temperature water were conducted to demonstrate a capability to investigate rapid changes in oxide films during transient water chemistry conditions. In the presence of dissolved hydrogen (DH) = 30 cm$$^{3}$$/kg [H$$_{2}$$O] and dissolved oxygen (DO) $$<$$ 0.06 ppm, only spinel oxides were detected and no significant NiO peak was found even after 1,220 h exposure. By contrast, once the DO was increased to 8 ppm, a NiO peak grew rapidly. Within 7 h, the amount of NiO became comparable to that of spinel oxide. However, when DO was decreased again below 0.3 ppm and DH was increased up to 30 cm$$^{3}$$/kg [H$$_{2}$$O], the ratio of NiO to spinel did not change during 10 h. Thus, the rate of dissolution of NiO in DH = 30 cm$$^{3}$$/kg water seemed to be lower than the growth rate of NiO in high DO conditions.

論文

The Effect of crystal textures on the anodic oxidization of zirconium in a boiling nitric acid solution

加藤 千明; 石島 暖大; 上野 文義; 山本 正弘

Journal of Nuclear Science and Technology, 53(9), p.1371 - 1379, 2016/09

AA2015-0626.pdf:1.2MB

 被引用回数:2 パーセンタイル:25.63(Nuclear Science & Technology)

硝酸水溶液でジルコニウムの応力腐食割れに関して、ジルコニウムの陽極酸化に及ぼす結晶配向の影響を調査した。酸化ジルコニウム膜の成長挙動は、脱不動態化電位(1.47V vs. SSE)で、劇的に変化した。1.5Vでは、酸化皮膜は速く成長し、その平均皮膜厚さは電荷量に比例して増加した。酸化皮膜は周期的な3乗則に従った成長挙動を示した。周期的な3乗則に従った成長挙動への遷移前では、酸化皮膜の成長挙動に結晶配向の影響を及ぼさなかった。しかし、皮膜成長の遷移後は、酸化皮膜の平均皮膜厚さと厚い酸化皮膜の下に生じたき裂は結晶配向の影響を受けた。(0002)面が配向している圧延面では、その平均皮膜厚さは減少し、皮膜内のき裂により皮膜の剥離が生じた。(0002)面に垂直面である圧延方向面では、厚い酸化皮膜の下には、圧延方向に深く進展したき裂が観察された。き裂は酸化物層の中をジルコニウム母材の(0002)面に沿って進展した。酸化物層は、ひも状の酸化ジルコニウムとジルコニウム水素化物からなり、ひも状の酸化ジルコニウムは、単斜晶ZrO$$_{2}$$に加えて斜方晶ZrO$$_{2}$$を含んでいた。外部応力が無い条件でき裂が発生・進展した機構の1つの仮定として、(0002)面に析出した水素化物の酸化と、その後に生じる斜方晶ZrO$$_{2}$$から単斜晶ZrO$$_{2}$$への相変態がその原因の一つと考えられた。

報告書

Zircaloy-4の高温酸化挙動に及ぼす固体ホウ酸の影響

小宮山 大輔; 天谷 政樹

JAEA-Research 2016-013, 20 Pages, 2016/08

JAEA-Research-2016-013.pdf:6.05MB

PWRの冷却材喪失事故(LOCA)において、流路の閉塞等により燃料棒の冷却が十分に行われない場合、燃料被覆管表面に冷却材中のホウ酸が析出する可能性が考えられる。通常運転温度域では、実機での実績からホウ酸水はZircaloy-4の酸化挙動に影響を及ぼさないと考えられるが、LOCAを想定した高温域におけるホウ酸とZircaloy-4との反応に係る知見は十分に得られていない。本研究では、固体ホウ酸を載せたZircaloy-4の板材を900$$^{circ}$$Cまでの温度及び複数の雰囲気で酸化させることにより、固体ホウ酸の高温時挙動、ホウ酸とZircaloy-4との反応の有無、及びホウ酸がZircaloy-4の酸化挙動に及ぼす影響を調べた。実験結果から、高温酸化雰囲気においてZircaloy-4表面に固体ホウ酸の脱水により生成する無水ホウ酸が存在すると、この無水ホウ酸がZircaloy-4と雰囲気との接触を断つことでZircaloy-4の酸化を抑制することが示唆された。また、酸化膜付きZircaloy-4の表面に固体ホウ酸が付着し高温まで加熱された場合は、形成している酸化膜の空隙に無水ホウ酸が浸透することでその後の酸化を抑制することがうかがえた。

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