• Wind and Structures
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• 제22권2호
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• pp.133-160
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• 2016

Hurricanes are among the most costly natural hazards to impact buildings in coastal regions. Building roofs are designed using the wind load provisions of building codes and standards and, in the case of large buildings, wind tunnel tests. Wind permeable roof claddings like roof pavers are not well dealt with in many existing building codes and standards. The objective of this paper is to develop simple guidance in code format for design of loose-laid roof pavers. Large-scale experiments were performed to investigate the wind loading on concrete roof pavers on the flat roof of a low-rise building in Wall of Wind, a large-scale hurricane testing facility at Florida International University. They included wind blow-off tests and pressure measurements on the top and bottom surfaces of pavers. Based on the experimental results simplified guidelines are developed for design of loose-laid roof pavers against wind uplift. The guidelines are formatted so that use can be made of the existing information in codes and standards such as American Society of Civil Engineering (ASCE) 7-10 standard's pressure coefficients for components and cladding. The effects of the pavers' edge-gap to spacer height ratio and parapet height to building height ratio are included in the guidelines as adjustment factors.

• 한국표면공학회지
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• 제49권4호
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• pp.389-394
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• 2016

The operation method of nuclear power plants is currently changing to high burn-up and long period that can enhance economics and efficiency of the plant. Since nuclear plant operation environment has been becoming severe, the amount of absorbed hydrogen also has increased. Absorbed hydrogen can be fatal securing safety of nuclear fuel cladding in case of Loss of Coolant Accidents(LOCA). In order to examine the impact of hydride on high-temperature oxidation, high-temperature oxidation experiment was performed on normal Zry-4 cladding and on Zry-4 cladding where hydrogen is charged in air pressure steam atmosphere under the $950^{\circ}C$ and $1000^{\circ}C$. According to the results, while oxidation acceleration due to charged hydrogen was not observed prior to breakaway oxidation creation, oxidation began to accelerate in cladding where hydrogens charged as soon as the breakaway oxidation started. If so much hydrogen are charged in the cladding, equiaxial monoclinic phase to unstable of stress is formed and it is presumed that oxidation is accelerated because nearby stress caused a crack in equiaxial phase, and that makes corrosion resistance decline sharply.

• Earthquakes and Structures
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• 제10권1호
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• pp.163-178
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• 2016

Composite Lightweight (CL) insulated walls have gained wide adoption recently because the exterior claddings of steel building frames have their cost effectiveness, good thermal and structural efficiency. To investigate the seismic behavior, lateral stiffness, ductility and energy dissipation of steel frames with the CL infill walls, five one-story one-bay steel frames were fabricated and tested under cyclic loads. Test results showed that the bolted connections allow relative movement between CL infill walls and steel frames, enabling the system to exhibit satisfactory performance under lateral loads. Additionally, it is found that the addition of diagonal steel straps to the CL infill wall significantly increases the initial lateral stiffness, load-carrying capacity, ductility and energy dissipation capacity of the system. Furthermore, the test results indicate that the lateral stiffness values of the frames with the CL infill wall are similar to those of the bare steel frames in large lateral displacement.

• 한국재료학회지
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• 제29권8호
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• pp.483-490
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• 2019

Currently, the Korean nuclear industry uses ZIRLO as material for nuclear fuel cladding(zirconium alloy). KEPCO Nuclear Fuel is in the process of developing a HANA alloy to enable domestic production of cladding. Cladding manufacture involves multistage heat treatments and pickling processes, the latter of which is vital for the removal of defects and impurities on the cladding surface. SMUT that forms on the cladding surface during such pickling process is a source of surface defects during heat treatment and post-treatment processes if not removed. This study analyzes ZIRLO, HANA-4, and HANA-6 alloy claddings to extensively study the SEM/EDS, XRD, and particle size characteristics of SMUT, which are second phase particles that are formed on the cladding surface during pickling processes. Using the analysis results, this study observes SMUT formation characteristics according to Nb concentration in Zr alloys during the washing process following the pickling process. In addition, this study observes SMUT removal characteristics on cladding surfaces according to concentrations of nitric acid and hydrofluoric acid in the acid solution.

• Wind and Structures
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• 제35권1호
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• pp.67-81
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• 2022

While the aerodynamics of solid bluff bodies is reasonably well-understood and methodologies for their reliable numerical simulation are available, the aerodynamics of porous bluff bodies formed by assembling perforated plates has received less attention. The topic is nevertheless of great technical interest, due to their ubiquitous presence in applications (fences, windbreaks and double skin facades to name a few). This work follows previous investigations by the authors, aimed at verifying the consistency of numerical simulations based on the explicit modelling of the perforated plates geometry and their representation by means of pressure-jumps. In this work we further expand such investigations and, contextually, we provide insight into the flow arrangement and its sensitivity to important modelling and setup configurations. To this purpose, Unsteady Reynolds-Averaged Navier-Stokes (URANS) and Large-Eddy Simulations (LES) are performed for a 5:1 rectangular cylinder at null angle of attack. Then, using URANS, porosity and attack angle are simultaneously varied. To the authors' knowledge this is the first time in which LES are used to model a porous bluff body and compare results obtained using the explicit modelling approach to those obtained relying on pressure-jumps. Despite the flow organization often shows noticeable differences, good agreement is found between the two modelling strategies in terms of drag force.

• Wind and Structures
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• 제36권1호
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• pp.41-60
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• 2023

The effect of cladding panel size on the size reduction factor (SRF) of extreme area-averaging wind pressure (EAWP) on the facades of a high-rise building is often ignored in previous studies. Based on wind tunnel tests, this study investigated the horizontal and vertical correlations of wind pressure on the facade claddings of square-section high-rise buildings. Then, the influencing parameters on the SRF of the EAWP on the cladding panels were analyzed, which were the panel area, panel width, panel length and building width. The results show clear regional distinctions in the correlation of wind pressures on the building facades and the rules of the horizontal and vertical correlations are remarkably different, which causes the cladding size ratio to impact the SRF significantly. Therefore, this study suggests the use of the non-dimensional comprehensive size parameter b^𝜶h^1-𝜶/B (𝜶 is the fitting parameter) determined by the cladding panel horizontal size b, cladding panel vertical size h and the building width B rather than the cladding panel area to describe the variation of the EAWP. Finally, some empirical formula for the SRF of the EAWP on the cladding of a high-rise building is proposed with the nondimensional comprehensive size parameter.

• 한국강구조학회 논문집
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• 제28권3호
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• pp.139-149
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• 2016

건물의 지붕 외장재는 강한 풍압 또는 태풍으로 인한 부압으로 지붕 외장재와 지붕 골조 간에 접합철물인 스크류가 인발되어 지붕외장재의 박리, 비산을 초래함으로써 인명, 재산적 피해가 발생함으로서 스크류 인발에 대한 대책이 요구되고 있다. 본 연구에서는 '인서트 너트'라는 철물을 지붕외장재가 설치되는 프레임에 체결하여 스크류의 관통 깊이를 증가시켜 인발 저항력을 높였으며 시험편 인장실험을 통한 단순 스크류 접합과 인서트 너트 접합의 인발 강도를 비교하였고 Solid Work를 이용한 모델링 해석을 통해 실제 사용되는 지붕외장재와 동일한 조건의 실험체를 제작하여 정적실험 및 가상의 풍하중을 이용한 동적실험을 통해 일반적인 스크류 접합 지붕외장재와 인서트 너트를 사용한 지붕외장재를 비교하였다.

• 한국재료학회지
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• 제9권4호
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• pp.378-385
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• 1999

핵 연료 피복관 재료로 샤용되고 었는 Zr 합금의 부식특성을 개선하기 위하여 Zn-xSn 2원계 합금과 Zr-0.4Nb-xSn 3원계 합금을 제조하여 부식특성을 평가하였다. 부식 실험은 $360^{\circ}C$ 물 분위기의 mini-autoclave에서 수행하였다. 2원계 합금의 경우 Sn이 0.5, 0.8, 1.5wt.% 첨가된 합금에서는 15일에서 속도천이가 발생한 후 급격한 부식 가속 현상이 나타녔으나, 2.0wt.% 가 첨가된 합금의 경우 80일까지의 부식 실험에서도 천이 현상을 보이지 않는 매우 높은 부식 저항성을 보였다. 3원계 합금에서는 Sn 함량이 증가할수록 부식이 증가하는 경향을 보였다. 이러한 경향은 2원계 합금과 3원계 합금에서 Sn의 고용도 차, 미량 첨가된 Nb의 영향 및 석출물의 특성과 관련이 잇는 것으로 생각된다. 또한 Zr-xSn, Zr-O.4Nb-xSn 합금의 부식은 tetragonal-$ZrO_2$의 분율과 수소 흡수율에 의해서 제어될 수도 있다.

• Nuclear Engineering and Technology
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• 제52권3호
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• pp.597-609
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• 2020

Accident Tolerant Fuels have been widely studied since the Fukushima-Daiichi accident in 2011 as one of the options on how to further enhance the safety of nuclear power plants. Deposition of protective coatings on nuclear fuel claddings has been considered as a near-term concept that will reduce the high-temperature oxidation rate and enhance accidental tolerance of the cladding while providing additional benefits during normal operation and transients. This study focuses on experimental testing of Zr-based alloys coated with Cr-based coatings using Physical Vapour Deposition. The results of long-term corrosion tests, as well as tests simulating postulated accidents, are presented. Zr-1%Nb alloy used as nuclear fuel cladding serves as a substrate and Cr, CrN, Cr_xN_y layers are deposited by unbalanced magnetron sputtering and reactive magnetron sputtering. The deposition procedures are optimized in order to improve coating properties. Coated as well as reference uncoated samples were experimentally tested. The presented results include standard long-term corrosion tests at 360℃ in WWER water chemistry, burst (creep) tests and mainly single and double-sided high-temperature steam oxidation tests between 1000 and 1400℃ related to postulated Loss-of-coolant accident and Design extension conditions. Coated and reference samples were characterized pre- and post-testing using mechanical testing (microhardness, ring compression test), Thermal Evolved Gas Analysis analysis (hydrogen, oxygen concentration), optical microscopy, scanning electron microscopy (EDS, WDS, EBSD) and X-ray diffraction.

• Nuclear Engineering and Technology
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• 제52권3호
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• pp.508-519
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• 2020

During transients or accidents, the reactor core is uncovered, and droplets entrained above the quench front collides with the uncovered fuel rod surface. Droplet impact cooling can reduce the peak cladding temperature. Besides zirconium-based cladding, versatile accidental tolerant fuel (ATF) claddings, including FeCrAl, have been proposed to increase the accident coping time. In order to investigate the effect of surface properties on droplet impact cooling of cladding surfaces, the droplet impact phenomena are photographed on the FeCrAl and zircaloy-4 (Zr-4) surfaces under different conditions. On the oxidized FeCrAl surface, the Leidenfrost phenomenon is not observed even when the surface temperature is as high as 550 ℃ with We > 30. Comparison of the impact behaviors observed on different materials shows that nucleate and transition boiling is more intensive on surfaces with larger thermal conductivity. The Leidenfrost point temperature (LPT) decreases with the solid thermal effusivity (${\sqrt{k{\rho}C_p}}$). However, the CHF temperature is relatively insensitive to the surface oxidation and Weber number. Droplet spreading diameter is analyzed quantitatively in the film boiling stage. Based on the energy balance a correlation is proposed for droplet maximum spreading factor. A mechanistic model is also developed for the LPT based on homogeneous nucleation theory.