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유동가속부식에 대한 통계적 모델링 해석방법 개발: 유속, 온도, pH 및 Cr 함량의 효과
이경근,이은희,김성우,김동진,Lee, Gyeong-Geun,Lee, Eun Hee,Kim, Sung-Woo,Kim, Dong-Jin 한국압력기기공학회 2016 한국압력기기공학회 논문집 Vol.12 No.2
Flow accelerated corrosion (FAC) of the carbon steel piping has been a significant problem in nuclear power plants. FAC occurs under certain hydrodynamic, environmental, and material conditions, and extensive research into the factors of FAC has been conducted. The basic process of FAC is now relatively well understood; however, a full mechanistic model has not yet been established. Recently, the Korea Atomic Energy Research Institute (KAERI) has built a large experiment loop system for FAC. To produce significant experimental results using this system, the factors affecting on FAC should be analyzed quantitatively, and a model needs to be developed. In this work, a statistical modeling methodology to develop an empirical model is described in detail, and a preliminary model is suggested. Firstly, FAC data were collected from the research literature in Japan and the results of domestic experiments. The flow rate, water temperature, pH at room temperature, and the Cr content are selected as major factors, and nonlinear regression is used to find the best fit of the available data. An iterative procedure between suggesting and evaluating a model is used until an optimum model is obtained. The developed model gives the FAC rate comparable to the measured FAC rate. The developed model is going to be refined using additional laboratory data in the future.
멀티스케일 모델링을 이용한 압력용기강의 조사손상 정량예측
이경근,권준현,Lee, Gyeong-Geun,Kwon, Junhyun 한국압력기기공학회 2014 한국압력기기공학회 논문집 Vol.10 No.1
In this work, an integrated model including molecular dynamics and chemical rate theory was implemented to calculate the growth of point defect clusters(PDC) and copper-rich precipitates(CRP) which could change the mechanical properties of reactor pressure vessel(RPV) steels in a nuclear power plant. A number of time-dependent differential equations were established and numerically integrated to estimate the evolution of irradiation defects. The calculation showed that the concentration of the vacancies was higher than that of the self-interstitial atoms. The higher concentration of vacancies induced a formation of the CRPs in the later stage. The size of the CRPs was used to estimate the mechanical property changes in RPV steels, as is the same case with the PDCs. The calculation results were compared with the measured values of yield strength change and Charpy V-notch transition temperature shift, which were obtained from the surveillance test data of Korean light water reactors(LWRs). The estimated values were in fair agreement with the experimental results in spite of the uncertainty of the modeling parameters.
950℃ 순수헬륨 분위기에서 크리프 파단된 Alloy 617의 미세구조적 고찰
이경근,정수진,김대종,김우곤,박지연,김동진,Lee, Gyeong-Geun,Jung, Su-Jin,Kim, Dae-Jong,Kim, Woo-Gon,Park, Ji-Yeon,Kim, Dong-Jin 한국재료학회 2011 한국재료학회지 Vol.21 No.11
The very high temperature gas reactor (VHTR) is one of the next generation nuclear reactors for its safety, long-term stability, and proliferation-resistance. The high operating temperature of over 800$^{\circ}C$ enables various applications with high energy efficiency. Heat is transferred from the primary helium loop to the secondary helium loop through the intermediate heat exchanger (IHX). The IHX material requires creep resistance, oxidation resistance, and corrosion resistance in a helium environment at high operating temperatures. A Ni-based superalloy such as Alloy 617 is considered as a primary candidate material for the intermediate heat exchanger. In this study, the microstructures of Alloy 617 crept in pure helium and air environments at 950$^{\circ}C$ were observed. The rupture time in helium was shorter than that in air under small applied stresses. As the exposure time increased, the thickness of outer oxide layer of the specimens clearly increased but delaminated after a long creep time. The depth of the carbide-depleted zone was rather high in the specimens under high applied stress. The reason was elucidated by the comparison between the ruptured region and grip region of the samples. It is considered that decarburization caused by minor gas impurities in a helium environment caused the reduction in creep rupture time.
기계학습을 이용한 유동가속부식 모델링: 랜덤 포레스트와 비선형 회귀분석과의 비교
이경근 ( Gyeong-geun Lee ),이은희 ( Eun Hee Lee ),김성우 ( Sung-woo Kim ),김경모 ( Kyung-mo Kim ),김동진 ( Dong-jin Kim ) 한국부식방식학회(구 한국부식학회) 2019 Corrosion Science and Technology Vol.18 No.2
Flow-Accelerated Corrosion (FAC) is a phenomenon in which a protective coating on a metal surface is dissolved by a flow of fluid in a metal pipe, leading to continuous wall-thinning. Recently, many countries have developed computer codes to manage FAC in power plants, and the FAC prediction model in these computer codes plays an important role in predictive performance. Herein, the FAC prediction model was developed by applying a machine learning method and the conventional nonlinear regression method. The random forest, a widely used machine learning technique in predictive modeling led to easy calculation of FAC tendency for five input variables: flow rate, temperature, pH, Cr content, and dissolved oxygen concentration. However, the model showed significant errors in some input conditions, and it was difficult to obtain proper regression results without using additional data points. In contrast, nonlinear regression analysis predicted robust estimation even with relatively insufficient data by assuming an empirical equation and the model showed better predictive power when the interaction between DO and pH was considered. The comparative analysis of this study is believed to provide important insights for developing a more sophisticated FAC prediction model.
초고온가스로 헬륨 분위기에서 Alloy 617의 고온 부식 거동
이경근 ( Gyeong Geun Lee ),정수진 ( Su Jin Jung ),김대종 ( Dae Jong Kim ),정용환 ( Yong Whan Jeong ),김동진 ( Dong Jin Kim ) 대한금속재료학회(구 대한금속학회) 2012 대한금속·재료학회지 Vol.50 No.9
Alloy 617 is a Ni-base superalloy and a candidate material for the intermediate heat exchanger (IHX) of a very high temperature gas reactor (VHTR) which is one of the next generation nuclear reactors under development. The high operating temperature of VHTR enables various applications such as mass production of hydrogen with high energy efficiency. Alloy 617 has good creep resistance and phase stability at high temperatures in an air environment. However, it was reported that the mechanical properties decreased at a high temperature in an impure helium environment. In this study, high-temperature corrosion tests were carried out at 850℃-950℃ in a helium environment containing the impurity gases H2, CO, and CH4, in order to examine the corrosion behavior of Alloy 617. Until 250 h, Alloy 617 specimens showed a parabolic oxidation behavior at all temperatures. The activation energy for oxidation in helium environment was 154 kJ/mol. The SEM and EDS results elucidated a Cr-rich surface oxide layer, Al-rich internal oxides and depletion of grain boundary carbides. The thickness and depths of degraded layers also showed a parabolic relationship with time. A normal grain growth was observed in the Cr-rich surface oxide layer. When corrosion tests were conducted in a pure helium environment, the oxidation was suppressed drastically. It was elucidated that minor impurity gases in the helium would have detrimental effects on the high- temperature corrosion behavior of Alloy 617 for the VHTR application.
초고온가스로용 Alloy 617의 불순물 함유 헬륨/공기 중에서 고온부식 특성
정수진 ( Su Jin Jung ),이경근 ( Gyeong Geun Lee ),김동진 ( Dong Jin Kim ),김대종 ( Dae Jong Kim ) 한국부식방식학회(구 한국부식학회) 2013 Corrosion Science and Technology Vol.12 No.2
A very high-temperature gas reactor (VHTR) is one of the next generation nuclear reactors owing to its safety, high energy efficiency, and proliferation-resistance. Heat is transferred from the primary helium loop to the secondary helium loop through an intermediate heat exchanger (IHX). Under VHTR environment Alloy 617 is being considered a candidate Ni-based superalloy for the IHX of a VHTR, owing to its good creep resistance, phase stability and corrosion resistance at high temperature. In this study, high-temperature corrosion tests were carried out at 850 - 950℃ in air and impure helium environments. Alloy 617 specimens showed a parabolic oxidation behavior for all temperatures and environments. The activation energy for oxidation was 154 kJ/mol in helium environment, and 261 kJ/mol in an air environment. The scanning electron microscope (SEM) and energy-dispersive x-ray spectroscopy (EDS) results revealed that there were a Cr-rich surface oxide layer, Al-rich internal oxides and depletion of grain boundary carbide after corrosion test. The thickness and depths of degraded layers also showed a parabolic relationship with the time. A corrosion rate of 950℃ in impure helium was higher than that in an air environment, caused by difference in the outer oxide morphology.
니켈기 합금 Alloy 617의 950℃ 대기/헬륨 분위기에서 산화거동 고찰
정수진 ( Sujin Jung ),이경근 ( Gyeong-geun Lee ),김동진 ( Dong-jin Kim ) 한국부식방식학회(구 한국부식학회) 2018 Corrosion Science and Technology Vol.17 No.5
Alloy 617 is a candidate Ni-based superalloy for intermediate heat exchanger (IHX) of a high-temperature gas reactor (VHTR), because of its good creep strength and corrosion resistance at high temperature. Small amount of impurities such as H<sub>2</sub>O, H<sub>2</sub>, CO and CH<sub>4</sub> are introduced inevitably in helium, as a coolant during operation of a VHTR. Reactions of material and impurities are accelerated with increase of temperature to 950℃ of operating temperature of a VHTR, leading to material corrosion aggravation. In this circumstance, high-temperature corrosion tests were performed at 950℃ in air and impure helium environments, up to 250 hours in this study. Oxidation rate of 950℃ in an air environment was higher than that of impure helium, explained by difference in outer oxide morphology and microstructure as a function of oxygen partial pressure. An equiaxed Cr-rich surface oxide layer was formed in an air environment, and a columnar Cr-rich oxide was formed in an impure helium environment.
Alloy 617의 850℃ 에서 공기 및 헬륨 분위기 크리프 파단 특성
김우곤(Woo-Gon Kim),박재영(Jae-Young Park),이경근(Gyeong-Geun Lee),홍성덕(Sung-Deok Hong),김용완(Yong-Wan Kim) 대한기계학회 2012 대한기계학회 춘추학술대회 Vol.2012 No.4-2
Experimental creep data for Alloy 617 in the air and He environments at 850°C was obtained under different applied stresses. Alloy 617 followed Norton’s power law and the Monkman-Grant relationship well. There were no large differences in the shapes of the creep curves between the air and He environments. The minimum creep rate and rupture time in the He environment were almost similar to those in the air one. As the stress decreased, the creep ductility decreased significantly. The oxide layer thickness in the He environment was a little higher than that in the air one. However, it was found that creep rupture time was closely related to oxide layer thickness, because the thickness of the outer and internal oxide layers between the air and He environments did not reveal a large difference.