• Title/Summary/Keyword: Helium impurities

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Theoretical studies on the stabilization and diffusion behaviors of helium impurities in 6H-SiC by DFT calculations

  • Obaid Obaidullah;RuiXuan Zhao;XiangCao Li;ChuBin Wan;TingTing Sui;Xin Ju
    • Nuclear Engineering and Technology
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    • v.55 no.8
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    • pp.2879-2888
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    • 2023
  • In fusion environments, large scales of helium (He) atoms are produced by a radical transformation along with structural damage in structural materials, resulting in material swelling and degradation of physical properties. To understand its irradiation effects, this paper investigates the stability, electronic structure, energetics, charge density distribution, PDOS and TDOS, and diffusion processes of He impurities in 6HSiC materials. The formation energy indicates that a stable, favorable position for interstitial He is the HR site with the lowest energy of 2.40 eV. In terms of vacancy, the He atom initially prefers to substitute at pre-existing Si vacancy than C vacancy due to lower substitution energy. The minimum energy paths (MEPs) with migration energy barriers are also calculated for He impurity by interstitial and vacancy-mediated diffusion. Based on its calculated energy barriers, the most possible diffusion path includes the exchange of interstitial and vacancy sites with effective migration energies ranging from 0.101 eV to 1.0 eV. Our calculation provides a better understanding of the stabilization and diffusion behaviors of He impurities in 6H-SiC materials.

Investigation of Oxidation Behavior of Alloy 617 under Air/Helium Environments at 950℃ (니켈기 합금 Alloy 617의 950℃ 대기/헬륨 분위기에서 산화거동 고찰)

  • Jung, Sujin;Lee, Gyeong-Geun;Kim, Dong-Jin
    • Corrosion Science and Technology
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    • v.17 no.5
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    • pp.218-224
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    • 2018
  • 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 $H_2O$, $H_2$, CO and $CH_4$ 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 $950^{\circ}C$ of operating temperature of a VHTR, leading to material corrosion aggravation. In this circumstance, high-temperature corrosion tests were performed at $950^{\circ}C$ in air and impure helium environments, up to 250 hours in this study. Oxidation rate of $950^{\circ}C$ 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.

INVESTIGATION ON MATERIAL DEGRADATION OF ALLOY 617 IN HIGH TEMPERATURE IMPURE HELIUM COOLANT

  • Kim, Dong-Jin;Lee, Gyeong-Geun;Jeong, Su-Jin;Kim, Woo-Gon;Park, Ji-Yeon
    • Nuclear Engineering and Technology
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    • v.43 no.5
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    • pp.429-436
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    • 2011
  • The corrosion of materials exposed to high temperature helium in a very high temperature reactor is caused by interaction with the impurities in the helium. This interaction then induces high temperature mechanical deterioration. By considering the effect of the impurity concentration on material corrosion, a long-term coolant chemistry guideline can be determined for the range of impurity concentration at which the material is stable for a long time. In this work, surface reactions were investigated by analyzing the thermodynamics and the experimental results for Alloy 617 exposed to controlled impure helium at $950^{\circ}C$. Moreover, the surfaces were examined for the Alloy 617 crept in air and in uncontrolled helium, which was explained by possible surface reactions.

Development of a Monitoring and Control System in Gas Purification Process (가스 정제공정의 감시 제어시스템 연구)

  • 조택선;양종화
    • Proceedings of the Korean Society of Precision Engineering Conference
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    • 1996.11a
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    • pp.313-317
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    • 1996
  • This work deals with description of gas purifing system to product high pure helium gas using low temperature absorption. The system controls temperature of heaters, open/close of solenoid valves and levels of liquid nitrogen to purify a raw gas and continuously products purified gas with perfoming alternatively purification and regeneration. We develop the monitoring and control program to monitor the gas purification process on real-time and control the process time with checking the impurities in purified gas. From the result of system operation, the developed monitoring and control system continuously products high pure helium gas with reducing impurities in raw gas to permitted limits(less than 0.01 ~ 0.05 ppm)

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Microstructural Investigation of Alloy 617 Creep-Ruptured in Pure Helium Environment at 950℃ (950℃ 순수헬륨 분위기에서 크리프 파단된 Alloy 617의 미세구조적 고찰)

  • Lee, Gyeong-Geun;Jung, Su-Jin;Kim, Dae-Jong;Kim, Woo-Gon;Park, Ji-Yeon;Kim, Dong-Jin
    • Korean Journal of Materials Research
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    • v.21 no.11
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    • pp.596-603
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    • 2011
  • 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.

High Temperature Oxidation Behavior of Nickel and Iron Based Superalloys in Helium Containing Trace Impurities

  • Tsai, C.J.;Yeh, T.K.;Wang, M.Y.
    • Corrosion Science and Technology
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    • v.18 no.1
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    • pp.8-15
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    • 2019
  • A high-temperature gas-cooled reactor (HTGR) is recognized as the best candidate reactor for next generation nuclear reactors. Helium is used to be the coolant in the core of the HTGR with temperature expected to exceed $900^{\circ}C$ at the core outlet. Several iron- and nickel-based superalloys, including Alloy 800H, Hastelloy X, and Alloy 617, are potential structural materials for intermediate heat exchanger (IHX) in an HTGR. Oxidation behaviors of three selected alloys (Hastelloy X, Alloy 800H, and Alloy 617) were investigated at four different temperatures from $650^{\circ}C$ to $950^{\circ}C$ under helium environments with various concentrations of $O_2$ and $H_2O$. Preliminary results showed that chromium oxide as the primary protective layer was observed on surfaces of the three tested alloys. Based on results of mass gain and SEM analyses, Hastelloy X alloy exhibited the best corrosion resistance in all corrosion tests. Further details on the oxidation mechanism of these alloys are presented in this study.

Creep and Oxidation Behaviors of Alloy 617 in High Temperature Helium Environments with Various Oxygen Concentrations (산소 농도에 따른 Alloy 617의 고온헬륨환경에서의 크립 및 산화거동)

  • Koo, Jahyun;Kim, Daejong;Jang, Changheui
    • Transactions of the Korean Society of Pressure Vessels and Piping
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    • v.7 no.2
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    • pp.34-41
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    • 2011
  • Wrought nickel-base superalloys are being considered as the structural materials in very-high temperature gas-cooled reactors. To understand the effects of impurities, especially oxygen, in helium coolant on the mechanical properties of Alloy 617, creep tests were performed in high temperature flowing He environments with varying $O_2$ contents at 800, 900, and $1000^{\circ}C$. Also, creep life in static He was measured to simulate the pseudo-inert environment. Creep life was the longest in static He, while the shortest in flowing helium. In static He, impurities like $O_2$ and moisture were quickly consumed by oxidation in the early stage of creep test, which prevented further oxidation during creep test. Without oxidation, microstructural change detrimental to creep such as decarburization and internal oxidation were prevented, which resulted in longer creep life. On the other hand, in flowing He environment, surface oxides were not stable enough to act as diffusion barriers for oxidation. Therefore, extensive decarburization and internal oxidation under tensile load contributed to premature failure resulting in short creep life. Limited test in flowing He+200ppm $O_2$ resulted in even shorter creep life. The oxidation samples showed extensive spallation which resulted in severe decarburization and internal oxidation in those environments. Further test and analysis are underway to clarify the relationship between oxidation and creep resistance.

Development of nitrogen and oxygen certified reference materials in 10 μmol/mol for the purity evaluation

  • Ahn, Byung Soo;Moon, Dong Min;Lee, Jin Bok;Kim, Jin Seog;Lee, Jin-Hong;Hong, Kiryong
    • Analytical Science and Technology
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    • v.33 no.2
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    • pp.68-75
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    • 2020
  • We have developed 10 μmol/mol nitrogen and oxygen certified reference materials (CRMs) in helium, as a SI-traceable gas standard for a quantifying of impurities in pure gases for the first time in Korea. The standard gas mixtures of nitrogen and oxygen were prepared in 5000 μmol/mol and sequentially were diluted to 250 μmol/mol and 10 μmol/mol according to the gravimetric preparation. In each dilution step, two cylinders of CRMs were prepared. The verification of internal consistency among the prepared gas mixtures was performed by using GC-TCD. The amount fractions and those expanded uncertainties (k = 2) of nitrogen and oxygen in the standard gas mixtures were (10.12 ± 0.08) μmol/mol and (10.18 ± 0.08) μmol/mol for nitrogen, and (9.88 ± 0.06) μmol/mol and (9.94 ± 0.06) μmol/mol for oxygen, respectively. We have conducted a purity assessment of two commercial helium gases using developed CRMs. As the results of the purity assessment, nitrogen and oxygen were detected by (1.66 ± 0.03) μmol/mol and (0.31 ± 0.02) μmol/mol, respectively, as the impurities in one of the pure helium.

The Influence Of The Cathode Surface State On The Spark Voltage In The Low Pressure Gare Gas (저기압희유 gas중에서 불꽃전압에 미치는 음극표면상태의 영향)

  • 백용현
    • 전기의세계
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    • v.23 no.4
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    • pp.46-52
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    • 1974
  • Generally, it has been regarded that there are two kinds of the effect of the electrodes, especially of the cathode in the gas discharge, (a) the effect caused by the difference of the cathode meterial and (b) the effect by the change of the cathode surface state even in the same meterials. Thus the two effects must be investigated independently to study the roles of the cathode in gas discharges. This paper measured sparking voltage in Rare gas (Ar, He) for the change of sparking voltage in repeating sparks and for the effect of (a) and (b) mentioned above, under the condition that the desorption of impurities from the cathod can be nigligible, and it is obtained that the correlative relations of the work function, sparking voltage and secondary coefficient are comparatively simple. In addition, the interesting character of the minimum point of the paschen's curves is found. The results were as follows; 1) The value of (pd)min with minimum pint of sparking voltage, (Vs)min, is 0.7-0.9 Torr. cm in Argon, but is 5.6-7.1 Torr. cm in Helium, and Paschen's curve in Helium shows slow curve than in Argon. 2) The minimum point of the Paschen's curve is satisfied actually Townsend's self sustaining criterion in Argon, but non-satisfaction in Helium, and the Townsend's secondary coefficient .gamma. action have compound property (.gamma.$_{i}$, .gamma.$_{p}$, .gamma.$_{m}$) in Helium. 3) The dependenting character of work function in Helium is less than in Argon. 4) The minimum point of sparking voltage increase under oxidized electrode than clear electrode in Au and Ag, but minimum point decrease in Ni and Cu.

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Analysis of Trace Impurities in The Bulk $H_2$ and He Gases by a Cold Concentration Method (저온 농축법에 의한 수소와 헬륨 중의 미량가스 분석)

  • Lee, Taeck Hong;Park, Doo Seon;Son, Moo Ryong
    • Journal of the Korean Chemical Society
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    • v.42 no.5
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    • pp.526-530
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    • 1998
  • Analysis of trace impurities in the gases has been very important with the development of semi-conductor related industry. Particularly, the contamination of the gas handling systems in a semi-conductor plant by the air has been a trouble to the manufacturers. Thus, the analysis of the air components in the system has been a task to the analysts. In this study, we report the analysis data with a expanded uncertainty for the trace impurities of nitrogen and argon in the bulk helium and hydrogen. All data show a good correspondence, exhibiting reliable statistical error ranges.

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