• Nuclear Engineering and Technology
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• 제53권4호
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• pp.1100-1108
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• 2021

Supercritical water has great potential as a coolant for nuclear reactor. Its use will lead to higher thermal efficiency of Rankine cycle. However, in certain conditions heat transfer may get deteriorated which may lead to undesirable high clad surface temperature. It is necessary to estimate the operating conditions in which heat transfer deterioration (HTD) will take place, so as to establish thermal margins for safe reactor operation. In the present work, the heat flux corresponding to onset of HTD for vertically upward flow of supercritical water in a pipe is obtained over a wide range of system parameters, namely pressure, mass flux, and pipe diameter. This is done by performing large number of simulations using an in-house CFD code, which is especially developed and validated for this purpose. The identification of HTD is based on observance of one or more peak/s in the computed wall temperature profile. The existing correlations for predicting the onset of HTD are compared against the results obtained by present simulations as well as available sets of experimental data. It is found that the prediction accuracy of the correlation proposed by Dongliang et al. is best among the existing correlations.

• Nuclear Engineering and Technology
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• 제55권4호
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• pp.1518-1527
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• 2023

A method was proposed for in-situ evaluating the thickness and resistivity of the oxide/hydroxide film formed on the surface of aluminum alloy exposed to sump water formed in the containment after a loss-of-coolant accident. The evaluation entailed fitting a model for the film impedance, which has film thickness and other variables describing the resistivity profile of the film along its thickness direction as fitting parameters, to the practically measured electrochemical impedance data. The obtained resistivity profiles implied that the films formed at pHs_25℃ 7, 8, 9, 10, and 11 all had a duplex structure; compared to the outer layer in contact with the solution, the inner layer of the film had a much higher resistivity and was inferred to be denser and provide most of the protectiveness of the film. Both the thickness and the total resistance of the film decreased with the increasing solution pH_25℃, suggesting that the films formed in more alkaline solutions had less protectiveness against corrosion, consistent with the increasing aluminum alloy corrosion rates previously identified.

• Nuclear Engineering and Technology
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• 제55권12호
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• pp.4431-4446
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• 2023

A Multiphysics coupling framework, MPCORE, has been developed to analyze safety parameters using the best estimate codes. The framework contains neutron kinetics (NK), thermal hydraulics (TH), and fuel performance (FP) codes to analyze fuel burnup, radial power distribution, and coolant temperature (T_bc). Shuffling and rotation capabilities have been verified on the Watts Bar reactor for three cycles. This study focuses on two coupling approaches for TH and FP modules. The one-way coupling approach involves coupling the FP code with the NK code, providing no data to the TH modules but getting T_bc as boundary condition from TH module. The two-way coupling approach exchanges information from FP to TH modules, so that the simplified heat conduction solver of the TH module is not used. The power profile in both approaches does not differ significantly, but there is an impact on coolant and cladding parameters. The one-way coupling approach tends to over-predict the cladding hydrogen concentration (CHC). This research highlights the difference between one-way and two-way coupling on critical boron concentration, T_bc, CHC, oxide surface temperature, and pellet centerline temperature. Overall, MPCORE framework with two-way coupling provides a more accurate and reliable analysis of safety parameters for nuclear reactors.

• 한국컴퓨터정보학회논문지
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• 제15권11호
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• pp.1-9
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• 2010

집적회로 공정기술이 급속도로 발달하면서 멀티코어 프로세서를 설계하는데 있어서 내부 연결망 (interconnection)은 성능 향상을 방해하는 주요 원인이 되고 있다. 멀티코어 프로세서의 내부 연결망에서 발생하는 병목 (bottleneck) 현상을 해결하기 위한 방안으로 최근에는 2D 평면 구조에서 3D 적층 구조로 설계 방식을 변경하는 기법이 주목을 받고 있다. 3D 구조는 칩 내부의 와이어 길이를 크게 감소시킴으로써 성능 향상과 전력 소모 감소의 큰 이점을 가져오지만, 전력 밀도 증가로 인한 온도 상승의 문제를 발생시킨다. 따라서 효율적인 3D 구조 멀티코어 프로세서를 설계하기 위해서는 내부의 온도 문제를 해결할 수 있는 설계 기법이 우선적으로 고려되어야 한다. 본 논문에서는 실험을 통해 다양한 측면에서 3D 구조 멀티코어 프로세서 내부의 온도 분포를 분석하고자 한다. 3D 구조 멀티코어 프로세서에서 수행되는 프로그램의 특성, 냉각 효과, 동적 주파수 조절 기법 적용에 따른 각 코어의 온도 분포를 상세하게 분석함으로써 저온도 3D 구조 멀티코어 프로세서 설계를 위한 가이드라인을 제시하고자 한다. 실험 결과, 3D 구조 멀티코어 프로세서의 온도를 효과적으로 관리하기 위해서는 더 높은 냉각 효과를 갖는 코어를 상대적으로 더 높은 동작 주파수로 작동 시켜야 하고 온도에 영향을 많이 주는 작업 또한 더 높은 냉각 효과를 갖는 코어에 할당해야 함을 알 수 있다.

• 아시안잔디학회지
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• 제23권2호
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• pp.229-240
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• 2009

여름철 고온 다습한 우리나라 기온에서 토양 온도와 수분은 잔디의 생리학적 변화를 초래하며, 특히 한지형 잔디생육은 우리나라 여름철 기후의 특이성에 많은 영향을 받는다. 본 실험은 크리핑 벤트그래스(Agrostispalustris Huds.)를 이용하여 고온의 조건에서 각기 다른 토양 수분함량 조건에서의 한지형 잔디생육을 관찰하였다. 구체적인 실험 방법으로는 수분함량과 온도에 따른 bentgrass의 생육 및 생리적 스트레스 반응을 평가하였다. 모의 USGA(United State of Golf Association) 그린 조건에서의 토양 온도 및 열 특성 실험 결과 수분이 거의 없는 건조 상태에서의 토양 온도는 $34^{\circ}C$로 발열 했을 때 토양 표면 온도가 $80^{\circ}C$까지 올라가는 것을 알 수 있었다. 반면, 과수분 조건에서 $34^{\circ}C$ 발열을 했을 때에는 건조 조건에 비해 상대적으로 $10^{\circ}C$가 낮은 것을 알 수 있었다. 실험 포장에서의 온도와 수분에 따른 열 특성 변화는 관수시기와 무관하게 처리구 모두에서 관수 후에 열전도도(thermal conductivity), 열 확산성(thermal diffusivity), 및 토양 온도가 증가하였다. 이는 수분이 공기에 비해 상대적으로 높은 열전도도를 갖기 때문으로 사료된다. 또한, 본 실험에서 관수 초기에는 과수분 조건에 비해 수분 결핍 조건에서 토양 온도의 증가를 보였으며, 시간이 지남에 따라 과수분 조건에서 더 높은 토양 온도의 증가를 확인하였다. 즉, 토양 온도는 과수분 조건에서 열전도에 의해 높아져 잔디의 생육에 영향을 미친다. 이는 잔디 표면부 온도가 높은 시각대의 과도한 관수는 여름철 잔디의 생육에 부정적 영향을 미치며, 소나기 등에 의한 일시 침수 시 지반배수의 불량은 잔디 생육에 치명적인 요인이 될 수 있다는 것을 의미한다.

• 한국진공학회:학술대회논문집
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• 한국진공학회 2012년도 제43회 하계 정기 학술대회 초록집
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• pp.380-381
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• 2012

Bioinspired sea urchin-like structures were fabricated on silicon by inductively coupled plasma (ICP) etching using lens-like shape hexagonally patterned photoresist (PR) patterns and subsequent metal-assisted chemical etching (MaCE) [1]. The lens-like shape PR patterns with a diameter of 2 ${\mu}m$ were formed by conventional lithography method followed by thermal reflow process of PR patterns on a hotplate at $170^{\circ}C$ for 40 s. ICP etching process was carried out in an SF6 plasma ambient using an optimum etching conditions such as radio-frequency power of 50 W, ICP power of 25 W, SF6 flow rate of 30 sccm, process pressure of 10 mTorr, and etching time of 150 s in order to produce micron structure with tapered etch profile. 15 nm thick Ag film was evaporated on the samples using e-beam evaporator with a deposition rate of 0.05 nm/s. To form Ag nanoparticles (NPs), the samples were thermally treated (thermally dewetted) in a rapid thermal annealing system at $500^{\circ}C$ for 1 min in a nitrogen environment. The Ag thickness and thermal dewetting conditions were carefully chosen to obtain isolated Ag NPs. To fabricate needle-like nanostructures on both the micron structure (i.e., sea urchin-like structures) and flat surface of silicon, MaCE process, which is based on the strong catalytic activity of metal, was performed in a chemical etchant (HNO3: HF: H2O = 4: 1: 20) using Ag NPs at room temperature for 1 min. Finally, the residual Ag NPs were removed by immersion in a HNO3 solution. The fabricated structures after each process steps are shown in figure 1. It is well-known that the hierarchical micro- and nanostructures have efficient light harvesting properties [2-3]. Therefore, this fabrication technique for production of sea urchin-like structures is applicable to improve the performance of light harvesting devices.

• 한국추진공학회지
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• 제18권1호
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• pp.97-104
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• 2014

고속 비행체에 연료를 공급하고, 내/외부의 열부하를 처리하기 위한 연료공급 및 냉각계통의 예비 열설계를 수행하였다. 이를 위해 임의의 임무형상에 대한 해석모델을 구성하고 성능해석을 진행하였다. 산출된 연료소모율과 내부의 유동 상태량을 이용하여 시스템의 각부 경계조건에 대한 열부하량을 계산하고 검증하였다. 이를 연료의 흡열반응을 이용한 시스템의 냉각성능과 비교하여 설계 요구조건을 충족시키는 것을 확인하였다.

• 한국재료학회지
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• 제22권12호
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• pp.711-716
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• 2012

Tungsten bronze structure $Sr_{1-x}Ba_xNb_2O_6$ (SBN) single crystals were grown primarily using the Czochralski method, in which several difficulties were encountered: striation formation and diameter control. Striation formation occurred mainly because of crystal rotation in an asymmetric thermal field and unsteady melt convection driven by thermal buoyancy forces. To optimize the growth conditions, bulk SBN crystals were grown in a furnace with resistance heating elements. The zone of $O_2$ atmosphere for crystal growth is 9.0 cm and the difference of temperature between the melt and the top is $70^{\circ}C$. According to the growth conditions of the rotation rate, grown SBN became either polycrystalline or composed of single crystals. In the case of as-grown $Sr_{1-x}Ba_xNb_2O_6$ (x = 0.4; 60SBN) single crystals, the color of the crystals was transparent yellowish and the growth axis was the c-axis. The facets of the crystals were of various shapes. The length and diameter of the single crystals was 50~70 mm and 5~10 mm, respectively. Tungsten bronze SBN growth is affected by the temperature profile and the atmosphere of the growing zone. The thermal expansion coefficients on heating and on cooling of the grown SBN single crystals were not matched. These coefficients were thought to influence the phase transition phenomena of SBN.

• 한국결정성장학회지
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• 제17권6호
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• pp.256-263
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• 2007

For $P_B=50Torr,\;P_T=5401Torr,\;T_S=450^{\circ}C,\;{\Delta}T=20K$, Ar=5, Pr=3.34, Le=0.01, Pe=4.16, Cv=1.05, adiabatic and linear thermal profiles at walls, the intensity of solutal convection (solutal Grashof number $Grs=7.86{\times}10^6$) is greater than that of thermal convection (thermal Grashof number $Grt=4.83{\times}10^5$) by one order of magnitude, which is based on the solutally buoyancy-driven convection due to the disparity in the molecular weights of the component A ($Hg_2Cl_2$) and B (He). With increasing the partial pressure of component B from 20 up to 800 Torr, the rate is decreased exponentially. It is also interesting that as the partial pressure of component B is increased by a factor of 2, the rate is approximately reduced by a half. For systems under consideration, the rate increases linearly and directly with the dimensionless Peclet number which reflects the intensity of condensation and sublimation at the crystal and source region. The convective transport decreases with lower g level and is changed to the diffusive mode at $0.1g_0$. In other words, for regions in which the g level is $0.1g_0$ or less, the diffusion-driven convection results in a parabolic velocity profile and a recirculating cell is not likely to occur. Therefore a gravitational acceleration level of less than $0.1g_0$ can be adequate to ensure purely diffusive transport.

• 한국세라믹학회지
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• 제41권9호
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• pp.677-683
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• 2004

레이저 모체재료용 Nd:LLM (Nd:LiLa(MoO$_4$)$_2$) 단결정을 쵸크랄스키법으로 성장시켰다 성장된 Nd:LLM 단결정은 균열 등이 쉽게 발생하였는데, 균열의 원인은 상전이, 불합치용융, 구성성분의 화학적 불균질, 열적구조의 불균형 및 성장방향 등이 있다. 성장된 단결정의 TG-DTA 열분석 결과 상전이는 없었으며, XRD 회절분석에 의해 합치 용융체임을 확인하였다. Li$_2$O 성분의 휘발은 화학적 불균질에 중요한 원인이었다. 자체 제작된 저항발열로의 온도프로파일은 도가니 높이로 조절하였다 또한, Nd:LLM 결정은 성장방향에 따라 단결정의 성장에 영향을 받았으며, (101)의 방향의 성장에서 단결정의 품질이 가장 우수하였다. 성장된 단결정의 N$d^{3+}$ 이온의 분포 및 유효편석계수은 PIXE분석에 의해 수행되었다.