• Nuclear Engineering and Technology
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• v.54 no.1
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• pp.152-161
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• 2022

We analyzed mitigation strategies for steam generator tube rupture (SGTR) accidents using MARS code under both full-power and low-power and shutdown (LPSD) conditions. In general, there are two approaches to mitigating SGTR accidents: supplementing the reactor coolant inventory using safety injection systems and depressurizing the reactor coolant system (RCS) by cooling it down using the intact steam generator. These mitigation strategies were compared from the viewpoint of break flow from the ruptured steam generator tube, the core integrity, and the possibility of the main steam safety valves opening, which is associated with the potential release of radiation. The "cooldown strategy" is recommended for break flow control, whereas the "RCS make-up strategy" is better for RCS inventory control. Under full power, neither mitigation strategy made a significant difference except for on the break flow while, in LPSD modes, the RCS cooldown strategy resulted in lower break and discharge flows, and thus less radiation release. As a result, using the cooldown strategy for an SGTR under LPSD conditions is recommended. These results can be used as a fundamental guide for mitigation strategies for SGTR accidents according to the operational mode.

• Nuclear Engineering and Technology
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• v.48 no.5
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• pp.1273-1279
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• 2016

Production of iodine-131 by neutron activation of tellurium in tellurium dioxide ($TeO_2$) material requires a target that meets the safety requirements. In a radiopharmaceutical production unit, a new lid for a can was designed, which permits tight sealing of the target by using tungsten inert gaswelding. The leakage rate of all prepared targets was assessed using a helium mass spectrometer. The accepted leakage rate is ${\leq}10^{-4}mbr.L/s$, according to the approved safety report related to iodine-131 production in the TRIGA Mark II research reactor (TRIGA: Training, Research, Isotopes, General Atomics). To confirm the resistance of the new design to the irradiation conditions in the TRIGA Mark II research reactor's central thimble, a study of heat effect on the sealed targets for 7 hours in an oven was conducted and the leakage rates were evaluated. The results show that the tightness of the targets is ensured up to $600^{\circ}C$ with the appearance of deformations on lids beyond $450^{\circ}C$. The study of heat transfer through the target was conducted by adopting a one-dimensional approximation, under consideration of the three transfer modes-convection, conduction, and radiation. The quantities of heat generated by gamma and neutron heating were calculated by a validated computational model for the neutronic simulation of the TRIGA Mark II research reactor using the Monte Carlo N-Particle transport code. Using the heat transfer equations according to the three modes of heat transfer, the thermal study of I-131 production by irradiation of the target in the central thimble showed that the temperatures of materials do not exceed the corresponding melting points. To validate this new design, several targets have been irradiated in the central thimble according to a preplanned irradiation program, going from4 hours of irradiation at a power level of 0.5MWup to 35 hours (7 h/d for 5 days a week) at 1.5MW. The results showthat the irradiated targets are tight because no iodine-131 was released in the atmosphere of the reactor building and in the reactor cooling water of the primary circuit.

• Tribology and Lubricants
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• v.33 no.3
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• pp.85-91
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• 2017

This work studies the flow behaviors in the gap between the friction pad and separator in wet-clutch systems. The fluid volume of the lubricant is modeled using the entire system of wet-clutch pack of a dual clutch transmission that has larger outer radius of odd gear shifts and smaller inner radius of even gear shifts. Flow behaviors in the gap of the clutch pad are computed using the gear shift modes that consider the real relative velocities between the friction pad and separator. Flow behaviors in the gap of the disengaged clutch pad are mainly investigated for the wet-clutch system, whereas the engaged clutch pad is modeled with no fluid rate through the contacting surfaces. The developed hydrodynamic fluid pressures and velocity fields in the clutch pad gap are computed to obtain the relevant information for managing flow rates in wet-clutch packs under dual operating conditions during gear shifts. These hydrodynamic pressures and velocity fields are compared on the basis of each gear level and gap location, which is necessary to determine the effects of groove patterns on the friction pad. Shear stresses in the gap locations are also computed on the basis of the gear level for the inner and outer clutch pads. The computed results are compared and used for the design of cooling capacity against frictional heat generation in wet-clutch pack systems.

• Publications of The Korean Astronomical Society
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• v.23 no.1
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• pp.1-12
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• 2008

Understanding of the basic characteristics of an astronomical instrument is a prerequisite to obtaining reliable data from the instrument. We have analyzed more than 1,000 calibration images from the Fairchild 486 CCD (hereafter the Maidanak 4k CCD system) attached to the AZT-22 1.5m telescope at Maidanak Astronomical Observatory in Uzbekistan. The Maidanak 4k CCD system supports three readout modes through 1, 2, or 4 amplifiers. In most cases observers use 4-amplifier readout mode to save time. We have tested the stability and seasonal variation of zero levels and confirm that two quadrants of the images (Amp 1 & 2) show no appreciable seasonal variation. but the other two quadrants (Amp 3 & Amp 4) show an evident seasonal variation in the bias level. The Cryo Tiger, the cooling system used at the Maidanak 4k CCD system, maintains the CCD temperature at -108'E, and effectively suppresses the dark electrons. The mean value versus the variance plot of the flat images does not show the expected relation for an ideal Poisson noise distribution and this is attributed to the large variation in quantum efficiency between different pixels. In addition, we confirm that there is no appreciable difference in gain between readout amplifiers, but there is a large variation in quantum efficiency across CCD chip especially in U. Due to the finite length of shutter opening and closing time, the effective exposure time varies across the science images. We introduce two parameters to quantify the effect of this uneven illumination and present a method to remove these effects. We also present a method to remove the interference patterns appearing in the images obtained with longer wavelength filters and investigate the spatial variation of the point spread function.

• Journal of Welding and Joining
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• v.26 no.6
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• pp.81-91
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• 2008

The weldability of resistance spot welding of TRIP1180 steels for automobile components investigated enhance in order to achieve understanding of weld fracture during tensile-shear strength (TSS) test. The main failure modes for spot welds of TRIP1180 steels were nugget pullout and interfacial fracture. The peak load to cause a weld interfacial failure was found to be related to fracture toughness of the weld and the weld diameter. Although interfacial fracture occurred in the spot welded samples, the load-carrying capacity of the weld was high and not significantly affected by the fracture mode. Substantial part of the weld exhibits the characteristic dimple (or elongated dimple) fractures on interfacial fractured surface also, dimple fracture areas were drawmatically increased with heat input which is propotional to the applied weld current. In spite of the high hardness values associated with the martensite microstructures due to high cooling rate. The high load-carrying ability of the weld is directly associated with the area of ductile fracture occurred in weld. Therefore, the judgment of the quality of resistance spot welds in TRIP1180 steels, the load-carrying capacity of the weld should be considered as an important factor than fracture mode.

• Bulletin of the Korean Chemical Society
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• v.35 no.2
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• pp.587-596
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• 2014

A new ${\pi}$-stacked donor-acceptor (D-A) system, [Ru(1-([2,2'-bipyridine]-6-yl-methyl)-3-(2-cyclohexa-2',5'-diene-1,4-dionyl)-1H-imidazole)(2,2':6',2"-terpyridine)]$[PF_6]_2$ (ImQ_T), has been synthesized and characterized. Similar to its precedent, [Ru(6-(2-cyclohexa-2',5'-diene-1,4-dione)-2,2':6',2"-terpyridine)(2,2':6',2"-terpyridine)]$[PF_6]_2$ (TQ_T), this system has a cofacial alignment of terpyridine (tpy) ligand and quinonyl (Q) group, which facilitates an electron transfer through ${\pi}$-stacked manifold. Despite the presence of lowest-energy charge transfer transition from the Ru-based-HOMO-to-Q-based-LUMO (MQCT) predicted by theoretical calculations by using time-dependent density functional theory (TD-DFT), the experimental steady-state absorption spectrum does not exhibit such a band. The selective excitation to the Ru-based occupied orbitals-to-tpy-based virtual orbital MLCT state was thus possible, from which charge separation (CS) reaction occurred. The photo-induced CS and thermal charge recombination (CR) reactions were probed by using ultrafast visible-pump/mid-IR-probe (TrIR) spectroscopic method. Analysis of decay kinetics of Q and $Q^-$ state CO stretching modes as well as aromatic C=C stretching mode of tpy ligand gave time constants of <1 ps for CS, 1-3 ps for CR, and 10-20 ps for vibrational cooling processes. The electron transfer pathway was revealed to be Ru-tpy-Q rather than Ru-bpy-imidazol-Q.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.35 no.6
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• pp.357-365
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• 2022

Refrigerant mischarging is one of the most frequently occurring failure modes in air conditioners, and both undercharging and overcharging degrade cooling performance. Therefore, it is important to accurately determine the amount of charged refrigerant. In this study, a support vector machine (SVM) model was developed to multi-classify the refrigerant mischarge through steady-state identification via fuzzy clustering techniques. For steady-state identification, a fuzzy clustering algorithm was applied to the air conditioner operation data using the difference between moving averages. The identification results using the proposed method were compared with those using existing steady-state determination techniques studied through the inversed Fisher's discriminant ratio (IFDR). Subsequently, the main features were selected using minimum redundancy maximum relevance (mRMR) considering the correlation among candidate features, and an SVM multi-classification model was devised using the derived features. The proposed method achieves satisfactory accuracy and robustness from test data collected in the new domain.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.21 no.12
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• pp.715-732
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• 2009

This article reviews the papers published in the Korean Journal of Air-Conditioning and Refrigeration Engineering during 2008. It is intended to understand the status of current research in the areas of heating, cooling, ventilation, sanitation, and indoor environments of buildings and plant facilities. Conclusions are as follows. (1) Research trends in thermal and fluid engineering have been surveyed in the categories of general fluid flow, fluid machinery and piping, new and renewable energy, and fire. Well-developed CFD technologies were widely applied in developing facilities and their systems. New research topics include fire, fuel cell, and solar energy. Research was mainly focused on flow distribution and optimization in the fields of fluid machinery and piping. Topics related to the development of fans and compressors had been popular, but were no longer investigated widely. Research papers on micro heat exchangers using nanofluids and micro pumps were also not presented during this period. There were some studies on thermal reliability and performance in the fields of new and renewable energy. Numerical simulations of smoke ventilation and the spread of fire were the main topics in the field of fire. (2) Research works on heat transfer presented in 2008 have been reviewed in the categories of heat transfer characteristics, industrial heat exchangers, and ground heat exchangers. Research on heat transfer characteristics included thermal transport in cryogenic vessels, dish solar collectors, radiative thermal reflectors, variable conductance heat pipes, and flow condensation and evaporation of refrigerants. In the area of industrial heat exchangers, examined are research on micro-channel plate heat exchangers, liquid cooled cold plates, fin-tube heat exchangers, and frost behavior of heat exchanger fins. Measurements on ground thermal conductivity and on the thermal diffusion characteristics of ground heat exchangers were reported. (3) In the field of refrigeration, many studies were presented on simultaneous heating and cooling heat pump systems. Switching between various operation modes and optimizing the refrigerant charge were considered in this research. Studies of heat pump systems using unutilized energy sources such as sewage water and river water were reported. Evaporative cooling was studied both theoretically and experimentally as a potential alternative to the conventional methods. (4) Research papers on building facilities have been reviewed and divided into studies on heat and cold sources, air conditioning and air cleaning, ventilation, automatic control of heat sources with piping systems, and sound reduction in hydraulic turbine dynamo rooms. In particular, considered were efficient and effective uses of energy resulting in reduced environmental pollution and operating costs. (5) In the field of building environments, many studies focused on health and comfort. Ventilation. system performance was considered to be important in improving indoor air conditions. Due to high oil prices, various tests were planned to examine building energy consumption and to cut life cycle costs.

• Journal of the Korean Society of Propulsion Engineers
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• v.2 no.2
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• pp.14-23
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• 1998

The present study investigates numerically particle laden flow through compressor cascade. In general, a lot of turbine engines are affected by various particles which are suspending in the atmosphere. Especially in the case of aircraft aviating in volcanic, industrial and desert region including many particles, each components of engine system are damaged severely. That damage modes are erosion of compressor binding and rotor path components, partial or total blockage of cooling passage and engine control system degradation.. Initial damages can not be serious but cumulation of damages influences on safety of aircraft control and economical maintenance cost of engine system can be increased. When dust, materials and volcanic particles in the atmosphere flow in the compressor, it is necessary to predict damaged and deposited region of compressor blades. To the various flow inlet angle, predictions of particles trajectory in compressor cascade by Lagrangian method are presented and impulses by impaction of particles at blade surface are calculated. By the definition of particle deposition efficiency, characteristics of particles impact are considered quantitatively. With these prediction and experimental data, erosion rates are predicted for two materials - ceramic, soft metal - on compressor blade surface. Improvements like coating of blade surface could be found, by above prediction.

• Journal of Korea Water Resources Association
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• v.53 no.10
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• pp.871-876
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• 2020

Understanding the wave characteristics near the outlet of coastal power plants for cooling water in the vicinity of the dredged areas is critically important for the construction and operation of the plants. By Employing the eigenfunction expansion method, in this study, we analyzed the reflection of monochromatic water waves over (1) shear currents near the outlet and (2) multi-arrayed trenches representing dredged areas. We firstly optimized the number of grids expressing shear currents and the number of evanescent modes based on a convergence test. We then analyzed the sensitivity of the reflection coefficients depending on (1) magnitude of shear currents, (2) width of shear currents, (3) a distance between adjacent trenches, and (4) a number of trenches. The results showed that the reflection coefficient was more sensitive to the number of trenches and the distance between trenches than the velocity of shear currents and the width of shear currents. We also found that even the effect of shear currents is relatively small, the effect is not negligible in a relative water depth from shallow to near shallow water waves (0.01 < kh ≦ 0.70).