• Progress in Superconductivity
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• 제3권2호
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• pp.198-212
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• 2002

The effects of reacted precursors on phase evolution, microstructure, $J_{c}$ and junctional characteristic of the inter-granular contacts were investigated in Ag-sheathed T1-1223 tapes prepared using three kinds of reacted precursors, and compared to those in the tape prepared using an unreacted precursor The precursors were prepared by heat-treating a mixture of Sr-Ba-Ca-Cu-O, $Tl_2$$O_3$, PbO and $Bi_2$$O_3$ powders at $805^{\circ}C$ (precursor I ), $840^{\circ}C$ (precursor II ) and $905^{\circ}C$(precursor III) for 20 min. Tl-1223 phase content, grain size and J\ulcorner in the tapes appeared to increase in an order of precursors I, II and III Compared to tapes prepared using an unreacted precursor, the tapes prewar ed using precursors II and III revealed reduced pore and impurity densities and an enhanced texture. Also characteristic of inter -granular contacts and fraction of strong-links were improved. The improved properties are attributed to enhanced densification resulting from using the reacted precursors.s.

• Nuclear Engineering and Technology
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• 제55권7호
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• pp.2504-2515
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• 2023

In past years the Paul Scherrer Institute (PSI, Switzerland) and the Karlsruhe Institue of Technology (KIT, Germany)) collaborated to develop a model to account for the active role of nitrogen in the air oxidation of a Zircalloy cladding. The "PSI-KIT Nitriding Model for Zirconium based Fuel Cladding" model was implemented at PSI into PSI-MELCOR 1.8.6. In order to make a preliminary evaluation of the effect of the new model on the evolution of full-scale spent fuel pool accidents, one spent fuel pool event was analyzed using the PSI research version of PSI-MELCOR 1.8.6, which includes the nitriding model. To adapt an existing input deck for the calculations, a sensitivity study was conducted to find an optimal nodalization for the analyses. The nitriding model results were compared to those calculated with the MELCOR 1.8.6-PSI without the new nitriding model. The results demonstrate the effect of the nitriding reactions in spent fuel pool accident progression. Moreover, they confirm the impact of ZrN formation during cladding oxidation in air when the oxidation reactions lead to oxygen starvation inside the fuel assemblies. The nitriding reaction led to higher chemical heat generation during the accident and to an earlier failure of the cladding than when the effect of nitrogen reactions was not considered. It should be noted that the nitriding model, as implemented in the PSI version of MELCOR 1.8.6 has not yet been conclusively validated. Thereby the results presented in this paper should be treated as a preliminary demonstration of the capabilities of the model.

• 한국분말재료학회지
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• 제31권2호
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• pp.137-145
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• 2024

In order to predict the process window of laser powder bed fusion (LPBF) for printing metallic components, the calculation of volumetric energy density (VED) has been widely calculated for controlling process parameters. However, because it is assumed that the process parameters contribute equally to heat input, the VED still has limitation for predicting the process window of LPBF-processed materials. In this study, an explainable machine learning (xML) approach was adopted to predict and understand the contribution of each process parameter to defect evolution in Ti alloys in the LPBF process. Various ML models were trained, and the Shapley additive explanation method was adopted to quantify the importance of each process parameter. This study can offer effective guidelines for fine-tuning process parameters to fabricate high-quality products using LPBF.

• Nuclear Engineering and Technology
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• 제56권2호
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• pp.688-706
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• 2024

Austenitic stainless steels (ASS) are extensively employed in various sectors such as nuclear, power, petrochemical, oil and gas because of their excellent structural strength and resistance to corrosion. SS304 and SS316 are the predominant choices for piping, pressure vessels, heat exchangers, nuclear reactor core components and support structures, but they are susceptible to stress corrosion cracking (SCC) in chloride-rich environments. Over the course of several decades, extensive research efforts have been directed towards evaluating SCC using diverse methodologies and models, albeit some uncertainties persist regarding the precise progression of cracks. This review paper focuses on the application of Acoustic Emission Technique (AET) for assessing SCC damage mechanism by monitoring the dynamic acoustic emissions or inelastic stress waves generated during the initiation and propagation of cracks. AET serves as a valuable non-destructive technique (NDT) for in-service evaluation of the structural integrity within operational conditions and early detection of critical flaws. By leveraging the time domain and time-frequency domain techniques, various Acoustic Emission (AE) parameters can be characterized and correlated with the multi-stage crack damage phenomena. Further theories of the SCC mechanisms are elucidated, with a focus on both the dissolution-based and cleavage-based damage models. Through the comprehensive insights provided here, this review stands to contribute to an enhanced understanding of SCC damage in stainless steels and the potential AET application in nuclear industry.

• Nuclear Engineering and Technology
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• 제56권4호
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• pp.1296-1309
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• 2024

The aim of this work is to explore the effect of the double subdiffusion on the stability in BWRs. A BWR novel reduced order model with double subdiffusion effects: reduced order fractional model (DS-F-ROM) to describe the neutron and heat transfer processes was proposed for this study. The double subdiffusion was developed with a fractional-order two-equation model, and with different fractional-orders and relaxation times. The stability analysis was carried out using the root-locus method and change from the s to the W domain and were confirmed using the time-domain evolution of neutron flux for a unit step change in reactivity. The results obtained using the reduced fractional-order model are presented for different anomalous diffusion coefficient values. Results are compared with normal diffusion and P1 equations, which are obtained straightforwardly with DS-ROM when relaxation time tends to zero, and when the anomalous diffusion coefficient tends to one, respectively.

• 한국재료학회지
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• 제10권9호
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• pp.629-635
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• 2000

본 논문에서는 Ti-48.5at%Al-0.96at%Mo 조성을 갖는 $\gamma$-TiAl 합금은 항온 단조 및 후속 조직제어 열처리 시 발달하는 집합조직의 변화에 관해 연구하였다. 특히, 동적 재결정 후 조직제어 열처리 온도와 시간의 증가에 따라 발생하는 lamellar volume fraction 과 집합조직 변화에 주목하여 관찰하였다. 동적 재결정 후 집합조직은 ND$\perp${302) 선분과, TD$\perp${100) 및 이에 비해 상대적으로 다소 약한{111) 성분들이 발달하였으며, 열처리 온도와 시간이 증가함에 따라 lameller volume fraction 은 증가했고 동일한 성분의 집합조직도 점차 강하게 발달함을 알 수 있었다. 하지만 상온인장시험 결과는 lameller volume fraction이 증가할수록 낮은 상온 연신을 보였는데, 이는 준 층상조직을 갖는 $\gamma$-TiAl합금의 상온인장특성이 집합조직의 영향보다는 lameller volume fractio과 같은 미세조직 특성에 더욱 강하게 영향을 받기 때문인 것으로 판단된다.

• 수산해양교육연구
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• 제5권1호
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• pp.31-44
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• 1993

Through the early 1900's, the evolution of the surface condenser was closely tied to the development of steam engine and the turbine. As the chemical and petroleum industries evolved in the 1900's, the use of surface condensers in many different processes. Today, industry uses condensers in many shapes and sizes. The actual condensation process occurs on the outside surface of tubes. The nature of this surface geometry affects the condenser's heat transfer performance. The first condensers were built with plain tubes. As tube manufacturing techniques advanced, manufacturers started making tubes with integral fins. In the 1940's, fin densities were limited to about 600 to 700 fins per meter(fpm) because of manufacturing procedure. Today new manufacturing techniques allow production of tubes with fin densities ranging from 750 to 1600 fpm. The integral-fin tubes investigated in this paper are nominally 19 mm diameter. Eight tubes have been used with trapezodially shaped integral-fins having fin density from 748 to 1654 fpm and 10, 30 grooves. For comparison, tests are made using a plain tube having the same inside diameter and an outside diameter equal to that at the root of the fins for the finned tubes. Betty and Katz's theoretical modelis is used to predict the R-11 condensation coefficient on horizontal integral-fin tubes having 748, 1024 and 1299 fpm. Experiments are carried out using R-11 as working fluid. The refrigerant condensates at a saturation state of $30^{\circ}C$ on the outside tube surface cooled by coolant. The amount of noncondensable gases present in the test loop is reduced to a negligible value by repeated purging. For a given heat input to the boiler and given cooling water flow rate, all test data are taken at steady state. The observed heat transfer enhancement for the finned and grooved tubes significantly exceeded that to be expected on grounds of increased area. For the eight fin tubes and one plain tube tested, the best performance has been obtained with a tube having a fin density of 1299 fpm, and a fin bight of 1.2mm and 30 grooves.

• 한국결정성장학회지
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• 제29권5호
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• pp.203-207
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• 2019

인산칼슘(hydroxyapatite(HAp), ${\beta}$-tricalcium phosphate(${\beta}$-TCP) 및 HAp와 ${\beta}$-TCP가 혼합된 2상 인산칼슘(BCP, HA/${\beta}$-TCP))들을 수열합성법을 통해 합성하였다. 여러 수열합성 조건(최고온도 $150^{\circ}C$ 및 pH 12 이하)에서 수산화칼슘($Ca(OH)_2$) 수용액과 인산($H_3PO_4$) 용액을 반응시켜 얻은 침전물을 수열합성에 이용하였다. 초기 전구체 Ca/P 비율을 1.30, 1.50 및 1.67로 조절하였으며 이 조성비와 합성 후 열처리가 분말 및 소결체의 상합성에 미치는 영향에 대해 조사하였다. 초기 전구체 Ca/P 비율을 조절함으로써 분말 및 소결체의 상의 조절이 가능하였다. Ca/P 비율이 1.67일 경우 모든 열처리 온도에서 분말 및 소결체 모두에서 HAp 단일 상이 합성되었다. Ca/P 비율이 1.30과 1.50에서 $900^{\circ}C$ 이상 열처리 할 경우 각각 순수한 ${\beta}$-TCP와 BCP(HAp/${\beta}$-TCP)가 합성되었다. 반응성 높은 미세한 분말을 이용하여 소결함으로써 미세한 입경을 가지며 투광성을 가지는 소결체들을 저온에서 얻을 수 있었다.

• 한국광물학회지
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• 제16권3호
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• pp.265-280
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• 2003

인리형 분지인 백악기 음성분지와 단층 접촉하는 무극 광화대는 백악기 흑운모 화강암을 모암으로 하여 배태된 금$.$은광상들로 구성된다. 무극 광화대내 금$.$은광상들은 북측의 무극광산으로부터 남측의 태극광산까지 서로 다른 금$.$은 품위비, 광석광물의 종류 및 산출 빈도 등을 보이며 열수변질대 분포 특성에 있어서도 상이한 공간적 분포특성을 보인다. 비교적 높은 금$.$은 품위비를 보이는 금봉광산은 복성맥의 구조를 보이며 석영맥으로부터 견운모대 \$\longrightarrow$ 아견운모대 \$\longrightarrow$ 프로필리틱대 \$\longrightarrow$ 아프로필리틱대의 열수변질대가 수평적으로 발달하는 특징을 보인다. 광화대 최남단에 위치하는 태극광산은 상대적으로 낮은 금$.$은비를 보이며 망상 세맥을 중심으로 프로필리틱대 \$\longrightarrow$ 아프로필리틱대의 순서로 열수변질대가 발달한다. 열수변질대의 수직적 변화는 대체로 변질대 하부에서 견운모대가 우세하나, 상부에서는 프로필리틱대가 광범위하게 분포하며, 점토대가 불연속적으로 중첩되는 특징을 보인다. 이러한 열수변질대의 상이한 수평$.$수직적 분포 특성은 각 광산의 열수계의 차이를 반영한 것으로, 금봉광산은 비교적 고온$.$고염도와 물-암석 상호반응이 진행된 광화 유체로부터 견운모대가 형성되었으며, 태극광산은 상대적으로 저온 저염도와 물-암석 상호반응이 미약한 광화 유체에 의하여 프로필리틱대가 형성된 것으로 해석된다. 이런 열수계의 차이는 각 광산이 열수계의 열적 중심(무극광산)에 위치하는지 또는 외곽부(태극광산)에 위치하는지에 따라 열수변질대 분포 양상과 금$.$은비 분포 양상의 차이를 유도한다. 따라서 무극 지역 탐사시 열적 중심부에 해당하는 고온의 지온 구배가 형성되는 지역과 물-암석 상호반응에 의한 견운모 변질작용이 우세한 지역을 고품위대 탐사 지침으로 제시할 수 있다.

• 콘크리트학회논문집
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• 제15권2호
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• pp.305-313
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• 2003

본 연구에서는 외기와의 열전달을 나타내는 외기대류계수에 관한 실험을 실시하였다. 외기대류계수에 관한 기존의 모델에서 나타났던 문제점을 해결하기 위해 실험변수로 풍속외에 거푸집의 종류(목재, 철재)를 선정하였다. 실험결과를 이용하여 외기대류계수를 산정하고자 열평형 방정식을 이용한 수치해법을 사용하였으며, 이론적인 고찰을 통해 각 거푸집별로 풍속에 따른 외기대류계수의 변화를 예측할 수 있는 모델식을 제안하였다. 제안된 모델식에 의하면, 모든 경우에 풍속에 따라 외기대류계수가 증가하는 경향을 보였으나 거푸집의 사용여부나 거푸집 재료에 따라 다른 양상을 보이는 것을 알 수 있었다. 이러한 양상의 차이는 거푸집 재료의 열특성에 의해 결정되는 것으로 외기대류계수는 거푸집이 없는 경우, 철재 거푸집을 사용한 경우, 목재 거푸집을 사용한 경우의 순으로 풍속의 영향을 받는 것으로 나타났다. 제안된 모델식을 이용하면 수화열에 의한 콘크리트 구조물의 온도해석시 보다 정확한 결과를 얻을 수 있을 것으로 사료되며, 향후 이러한 열특성계수에 대한 연구가 필요할 것으로 판단된다.