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Negyesi, M.*; 長谷川 邦夫
Journal of Pressure Vessel Technology, 148(4), p.044501_1 - 044501_4, 2026/08
When flaws are detected in power plant pipes, they are evaluated to determine their impact on component integrity. According to the ASME Code Section XI, the maximum allowable flaw depth should be less than 75% of the pipe wall thickness. The current code maximum allowable flaw length is defined as the length at which through-wall flawed piping fails due to applied stress. Therefore, the current length is irrespective of flaw depth. This paper uses a flat plate model with surface flaws to examine the characteristics of flaw lengths and propose a new methodology for determining the maximum allowable flaw lengths.
河 侑成; Negyesi, M.*; 長谷川 邦夫; Lacroix, V.*
Proceedings of the ASME 2025 Pressure Vessels & Piping Conference (PVP2025) (Internet), 7 Pages, 2025/07
原子力発電用配管に欠陥が認められた場合、欠陥寸法に対する許容応力を求め、規格上の問題がないことを確認する必要がある。配管の許容応力は、配管材の降伏応力と引張強度を平均した流動応力が用いられるが、実機配管の引張特性が明確でない場合にはASME codeに定められる当該材料の下限値を用いることにより、保守的な評価が行われる。本研究では、計装化押込み試験により求めた引張特性を用いて許容応力及び許容欠陥寸法を評価した。計装化押込み試験による引張特性はやや高いものの、その流動応力は従来の引張試験結果から求めた場合の約3%の誤差範囲であった。また、計装化押込み試験及び従来の引張試験による許容応力及び許容欠陥寸法は、ASME codeによる当該材料の下限値で求められた値より大きく許容された。本研究の結果により、配管を用いた直接的な引張試験が困難な実機において、計装化押込み試験による引張特性を取得することにより、実際の引張特性に近い値が得られることを明らかにした。
Negyesi, M.*; 河 侑成; 長谷川 邦夫; Lacroix, V.*
Proceedings of the ASME 2025 Pressure Vessels & Piping Conference (PVP2025) (Internet), 6 Pages, 2025/07
When evaluating allowable flaw sizes for high toughness ductile pipes, failure stresses are necessary for the flawed pipes. The failure stresses are estimated by flow stresses for the pipe materials and the flow stress is usually given by the average of the yield strength and the ultimate tensile strength. The American Society of Mechanical Engineers (ASME) Code Section XI states to use the yield strengths and the ultimate tensile strengths defined by the ASME Code Section II Materials. The ASME Code Section XI also states that actual measured yield strengths and ultimate tensile strengths are possible to be used, alternatively. Values of yield strength and ultimate tensile strength given by the ASME Code Section II are conservative, corresponding to lower bound values. Then, it is easily inferred that the allowable stresses calculated by the code given flow stress are lower than the allowable stresses calculated by the actual measured flow stress. The objective of this paper is to compare the allowable stresses and allowable flaw lengths based on both the code and actual measured flow stresses for pipes with axial flaws subjected to internal pressure. It is further demonstrated that allowable flaw lengths based on the code flow stress are substantially shorter than those calculated based on the actual measured flow stress.
山口 義仁; 長谷川 邦夫; Negyesi, M.*
Proceedings of the ASME 2025 Pressure Vessels & Piping Conference (PVP2025) (Internet), 8 Pages, 2025/07
Reference fatigue crack growth rates da/dN for ferritic steels in boiling water reactor environment are provided in Appendix Y of the ASME Code Section XI. Therein, da/dN are expressed as a function of stress intensity factor range 
K and as a function of the stress ratio parameter R. Different expressions apply for low K and high K. Current expressions result in lower fatigue crack growth rates in the water environment compared to the air environment for certain values of K. Therefore, the ASME Code states that reference fatigue crack growth rates in water environments are given by the expressions which results in higher fatigue crack growth rates. Sample calculations on fatigue crack growth under water and air environments were compared. The applicability of the current standard was confirmed.
Negyesi, M.*; 長谷川 邦夫
Proceedings of the ASME 2025 Pressure Vessels & Piping Conference (PVP2025) (Internet), 5 Pages, 2025/07
When flaws are detected in power plants, the flaws are evaluated for their impact on the integrity of the components. There are three conditions imposed on the flawed components to ascertain safe operation. The first one is the allowable stress, the second is the allowable flaw depth, and the third is allowable flaw length. The ASME Code Section XI provides the allowable flaw lengths based on the lengths of through-wall flaws. However, the failure stress for shallow flaw is higher than that for through-wall flaw. Besides, when the length for the shallow flaw is beyond the allowable length determined by through-wall flaw, the shallow flaw is not acceptable. The current third condition of the allowable flaw length is inconsistent in that it does not allow for shallow and long flaws that are not actually detrimental for piping integrity. This paper examines the characteristics of flaw lengths using a flat plate model with surface flaws, and proposes new allowable flaw lengths.
Lacroix, V.*; Dulieu, P.*; 長谷川 邦夫; 河 侑成; Negyesi, M.*
Proceedings of the ASME 2025 Pressure Vessels & Piping Conference (PVP2025) (Internet), 8 Pages, 2025/07
When a subsurface flaw is detected close to the free surface of a pressure retaining component, it must be assessed whether the subsurface flaw shall be transformed into a surface flaw in accordance the ASME Code Sec. XI. The threshold of this proximity rule depends on the ratio between the subsurface flaw to the free surface distance and the flaw depth. This limit value depends neither on the flaw aspect ratio, nor on the thickness and the curvature of the component, nor on the type of loading. After demonstrating in a previous paper that the interaction between the subsurface flaw and the free surface of the component highly depends on the flaw aspect ratio, the present paper highlights the influence of the thickness and the curvature of the component. Consequently, the current flaw-to-surface proximity rule should account for those parameters as well. Then, the paper proposes an interaction limit criterion and presents a new limit value for the flaw-to-surface proximity factor.
for negative stress ratios R based on trend in experimental data for fatigue crack growth rates of austenitic stainless steels for ASME code Section XINegyesi, M.*; 山口 義仁; 長谷川 邦夫; Lacroix, V.*; Morley, A.*
Journal of Pressure Vessel Technology, 147(2), p.021201_1 - 021201_7, 2025/04
被引用回数：0 パーセンタイル：0.00(Engineering, Mechanical)Fatigue crack growth rates for stainless steels in air environment are provided by the ASME Code Section XI. When the stress ratio R is positive from 0 to 1, the scaling parameter SR increases with the increasing ratio R, and the crack growth rates increase with the increasing stress ratio R. When R is less than 0, the parameter SR=1. Hence, fatigue crack growth rates under negative R ratios are independent of stress ratios R according to the ASME Code Section XI. However, from the literature survey, experimental data reveal that the fatigue crack growth rates decreases with decreasing R ratios below zero. The objective of this paper is to assess fatigue crack growth rates under such negative stress ratios for stainless steels in air environment. An equation determined from trends in experimental data surveyed in this study is proposed for negative stress R ratios to calculate the parameter SR for the ASME Code Section XI.
Negyesi, M.*; 山口 義仁; 長谷川 邦夫; Lacroix, V.*; Morley, A.*
Proceedings of the ASME 2024 Pressure Vessels & Piping Conference (PVP 2024) (Internet), 8 Pages, 2024/07
Fatigue crack growth rates da/dN for stainless steels in air environment are provided by the ASME Code Section XI. The fatigue crack growth rates are given by da/dN = C
(K)
, where C is the fatigue crack growth rate coefficient, n is the fatigue crack growth rate exponent and K is the stress intensity factor range. The coefficient C contains a temperature parameter S
, and a scaling parameter, S
, which is a function of the stress ratio R. When the stress ratio R is positive from 0 to 1, the parameter S increases with increasing the ratio R, and da/dN increases with increasing stress ratio R. When R is less than 0, the parameter is given by S = 1.0. Accordingly, fatigue crack growth rates under negative stress ratio are always constant, independent of stress ratios. This means that a cyclic stress state with a minimum that is compressive is considered to cause the same degree of crack growth as one with the same range but a zero minimum. However, from the results of literature survey, experimental data reveal that the fatigue crack growth rates decrease with decreasing R ratios below zero, i.e. negative stress ratios. The objective of this paper is to assess fatigue crack growth rates under such negative stress ratios for stainless steels in air environment. An equation determined from trends in experimental data is proposed for negative R ratios for calculating the parameter S for the ASME Code Section XI, Appendix Y, based on the literature surveyed in this study.
Morley, A.*; Negyesi, M.*; 長谷川 邦夫
Proceedings of the ASME 2024 Pressure Vessels & Piping Conference (PVP 2024) (Internet), 7 Pages, 2024/07
大気中におけるステンレス鋼の疲労亀裂進展速度はASME (米国機械学会)規格Section XIに規定されている。この亀裂進展速度は温度の上昇とともに増加し、この温度効果はパラメータSTで表されている。現在のこのパラメータSTの式は扱いにくく、温度の関数とした根拠に疑問があり、かつ、ASME規格の対象としている機器の適用温度から外れた実験データも用いて作成されている。そこで、文献調査による実験データをもとに、大気中におけるステンレス鋼の温度パラメータSTの代替式を提案する。
河 侑成; 山口 義仁; 長谷川 邦夫; Negyesi, M.*
Proceedings of the ASME 2024 Pressure Vessels & Piping Conference (PVP 2024) (Internet), 6 Pages, 2024/07
If a flaw in a high-toughness ductile pipe of a power plant is detected during periodic in-service inspection, stress applied at the flaw location of the pipe is compared with an allowable stress. When the applied stress is less than the allowable stress, the plant can operate continuously for a certain evaluation period in accordance with ASME Code Section XI. The flow stress given by the average of yield strength and ultimate tensile strength is an important material parameter for allowable stress. Recently, many fitness-for-service codes and technical reports have adopted conversions from hardness measurement values to yield strength and ultimate tensile strength. In this paper, we introduced the flow stress obtained from converted tensile properties from Vickers hardness using the presented equations for austenitic stainless steel. The allowable stress estimated by the Vickers hardness was compared with the allowable stress determined by actual tensile properties. As a result, the flow stress converted from hardness was about 1.48 times larger than that obtained by actual flow stress. The allowable flaw sizes calculated by the flow stress converted from hardness gave an appropriate indication when the allowable or applied stress was very low. However, the flow stress converted from hardness gave unconservative allowable stress, when the applied stress was large.
Negyesi, M.*; 河 侑成; 長谷川 邦夫; Lacroix, V.*
Proceedings of the ASME 2024 Pressure Vessels & Piping Conference (PVP 2024) (Internet), 6 Pages, 2024/07
Allowable stresses for pipes are determined by combining failure stresses and safety factors. When predicting the plastic collapse failure stresses for high toughness ductile pipes, flow stresses of the pipe materials are indispensable. The flow stress is usually given by the average of the yield strength and the ultimate tensile strength. Inservice Inspection of the American Society of Mechanical Engineers (ASME) Code Section XI states to use the yield strengths and the ultimate tensile strengths defined by the ASME Code Section II Materials. The ASME Code Section XI also states that actual measured yield strengths and ultimate tensile strengths are possible to be used, alternatively. Values of yield strength and ultimate tensile strength given by the ASME Code Section II are conservative, corresponding to lower bound values. Then, it is easily inferred that the allowable stresses calculated by the code given flow stress are lower than the allowable stresses calculated by the actual measured flow stress. The objective of this paper is to compare the plastic collapse failure and allowable stresses based on both the code and actual flow stresses for pipes with circumferential flaws subjected to bending and tensile loading. In addition, it is demonstrated that allowable flaw sizes based on both flow stresses do not differ much at low allowable stress. However, when the allowable stress is large, the allowable flaw size based on the code flow stress is substantially lower compared to that based on the actual flow stress.
Negyesi, M.; 天谷 政樹
Oxidation of Metals, 94(3-4), p.283 - 299, 2020/10
被引用回数：0 パーセンタイル：0.00(Metallurgy & Metallurgical Engineering)Oxidation tests of Zry-4 fuel cladding in steam at 1273 K were carried out in this study. The effect of specimen surface roughness on the oxidation behavior was investigated. Steam was applied either at room temperature or at experimental temperature. Weight gain kinetics was evaluated by post-test weight measurement. Metallographic analysis was conducted using optical microscopy. Hydrogen pick-up was measured by gas extraction technique. The effect of specimen surface roughness on the oxidation kinetics as well as on the hydrogen absorption has not clearly been evidenced. The breakaway oxidation was suppressed significantly when the steam was applied at RT. The oxide breakaway was related to grain size of the base metal. Higher hydrogen absorption before the kinetic transition in the condition when steam was applied at 1273 K suggested enhanced oxide porosity.
Negyesi, M.; 天谷 政樹
Oxidation of Metals, 92(5-6), p.439 - 455, 2019/12
被引用回数：1 パーセンタイル：4.40(Metallurgy & Metallurgical Engineering)The paper deals with the effect of air fraction in steam on the embrittlement of Zry-4 fuel cladding exposed under steam-air atmospheres (air fractions of 10-100%) in the temperature range of 1273-1573 K. Ring compression tests were carried out in order to evaluate the embrittlement of fuel cladding. Furthermore, the microhardness of prior 
-phase was measured and fractured surfaces were observed under SEM. The degree of the embrittlement was discussed against the results of metallographic and hydrogen analyses. The microstructure and the hydrogen pick-up were substantially affected by nitride formation. Accelerated oxidation kinetics enhanced shrinking of the prior -region. The enhanced hydrogen absorption resulted in the increased microhardness of prior -phase. The degree of fuel cladding embrittlement, expressed by the plastic strain at failure and the maximum load, correlated well with the microhardness and the thickness of prior -phase.
Negyesi, M.; 天谷 政樹
Journal of Nuclear Materials, 524, p.263 - 277, 2019/10
被引用回数：4 パーセンタイル：30.77(Materials Science, Multidisciplinary)The study deals with the oxidation behavior of fuel cladding under mixed steam-air atmospheres. Oxidation tests of Zry-4 were carried out at temperatures of 1273-1573 K. Post-test weight gain measurement along with metallographic examination were conducted to study separately the kinetics of the region where nitrides formed and the nitride-free region. The weight gain coming from the nitride-free region was estimated employing one-dimensional finite difference oxygen diffusion model and measured thicknesses of the metallic part of the oxidized specimen, the columnar oxide and the oxygen stabilized 
-Zr(O) as well as the fraction of the columnar oxide at the oxide/metal interface. Consequently, the weight gain related to the nitride formation has been assessed.
Negyesi, M.; 天谷 政樹
Proceedings of 27th International Conference on Nuclear Engineering (ICONE-27) (Internet), 8 Pages, 2019/05
This work deals with oxidation behavior of Zry-4 fuel cladding exposed to steam at 1273 K. The condition corresponds to LOCA. The effect of the specimen surface roughness and experimental setting on the oxidation behavior was investigated by employing two experimental techniques for oxidation tests and metallographic analysis along with hydrogen pick-up measurement. Slower heating rate under steam flow led to significantly slower oxidation rate during the subsequent isothermal exposure. As a consequence, the breakaway was delayed substantially. The effect of the specimen surface roughness on the oxidation behavior seemed to be rather minor under the investigated conditions. On the other hand, hydrogen uptake was found to be substantially affected by both the specimen surface roughness and the tested experimental setting.
-Zr(O) layer on Zry-4 fuel cladding at 1273 and 1473 KNegyesi, M.; 天谷 政樹
Annals of Nuclear Energy, 114, p.52 - 65, 2018/04
被引用回数：7 パーセンタイル：48.92(Nuclear Science & Technology)The growth kinetics of the columnar oxide and -Zr(O) layers of Zry-4 under mixed steam-air conditions at temperatures of 1273 and 1473 K were investigated in this study be means of post-test metallographic measurements. The hydrogen uptake was also determined by the inert gas fusion technique. The kinetics of the columnar oxide layer obeyed a parabolic law for all air fractions at both temperatures. The kinetics of -Zr(O) layer appeared to deviate slightly from the parabolic law. The parabolic oxidation rate constant of the columnar oxide increased with increasing air fraction, whereas the parabolic oxidation rate constant of -Zr(O) layer seemed to be independent of the air fraction. Mixed steam-air conditions appeared to enhance hydrogen absorption substantially, especially after the columnar oxide lost its protectiveness.
Negyesi, M.; 天谷 政樹
Journal of Nuclear Science and Technology, 54(10), p.1143 - 1155, 2017/10
被引用回数：9 パーセンタイル：58.07(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.
Negyesi, M.; 天谷 政樹
Proceedings of 25th International Conference on Nuclear Engineering (ICONE-25) (CD-ROM), 10 Pages, 2017/07
Zry-4 fuel cladding tubes were exposed in mixtures of oxygen and nitrogen at temperatures of 800-1380 
C. The influence of various flow rates of oxygen and nitrogen as well as specimen height on the weight gain was examined. The overall weight gain was substantially affected by both the applied flow rates and the height of specimens. The oxidation kinetics in air was assessed based on the results of weight gain measurements. A transition in the kinetics was observed at 800 and 1000 C. The kinetics in the post-transient regimes was rather accelerated than linear. The equation proposed in this study for air condition was in good agreement with the Leistikow-Berg correlation and the Baker-Just correlation. Prior -phase shrinked when the oxide scale along with the -Zr(O) layer progressed. Eventually, both the specimen plastic strain and maximum load decreased due to the shrinkage and increasing embrittlement of the prior -phase.
Negyesi, M.; 天谷 政樹
Proceedings of Annual Topical Meeting on LWR Fuels with Enhanced Safety and Performance (TopFuel 2016) (USB Flash Drive), p.1065 - 1074, 2016/09
This study dealt with oxidation behavior of Zry-4 nuclear fuel cladding under a severe nuclear reactor accident scenario. Influence of the reaction extent on cladding mechanical properties was also treated. Zry-4 fuel cladding segments were exposed to steam atmosphere containing different amount of air in composition ranging from 0 to 100 vol.%. Exposure times covered both pre- and post-breakaway regimes. Metallographic observations along with microhardness testing were carried out to study the effect of air fraction on the material microstructure. Hydrogen pick-up was measured and residual mechanical properties were assessed employing Ring Compression Tests. The results show that the addition of air in steam can lead to more than 300% increase of weight gain in case of 
50% or higher air fraction after the exposure of 60 min., namely, "late post-breakaway regime". Approximately three times thicker oxide scales were observed in such cases, indicating that most of the weight gain was due to the oxide layer increase. Nitride phase was preferentially observed close to the interface between oxide and metal layers. The -Zr(O) sublayer thickness decreases with the increasing fraction of air in steam. Microhardness, measured within the prior -phase region, slightly depends on the air fraction. Hydrogen uptake, on the other hand, depends strongly on the air fraction in steam. In the case of shorter exposure times, the effect of air addition in steam was suppressed. Eventually, specimen plastic strains drastically reduced in the late post-breakaway regime.
Negyesi, M.
no journal, ,
The presentation deals with oxidation behavior of Zry-4 alloy during severe accidents of LWR. Zry-4 fuel cladding was exposed in steam-air mixtures at temperatures of 1000-1200 C. Specimen weight gain was measured, metallographic observations were carried out, hydrogen uptake was determined and ring compression tests were conducted. The oxidation kinetics of Zry-4 was significantly affected by the air fraction in steam at 1000 C in the post-transient regime and at 1200 C. The effect of air fraction was diminished at 1000 C in the pre-transient regime. Metallographic examination revealed severe oxide layer cracking and nitride formation. Substantial hydrogen pick-ups were measured, especially at 1000 C after 60 min, strongly depending on the air fraction. They probably led to the significant decrease in plastic strain at 1000 C after 60 min. The effect of air fraction on plastic strain has been hardly observed at 1200 C.
