• Nuclear Engineering and Technology
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• v.34 no.1
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• pp.30-41
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• 2002

The neutron beam is fully characterized for the prompt gamma activation analysis facility at Hanaro in the Korea Atomic Energy Research Institute(KAERI). The facility uses thermal neutrons which are diffracted vertically from a horizontal beam port by a set of pyrolytic graphite(PG) crystals positioned at the Bragg angle of 45" Neutron spectra, neutron flux and Cd-ratio are determined for the three extraction modes of diffracted beam by means of the theoretical and experimental efforts. To obtain theoretical result, the reflectivity of pyrolytic graphite is calculated in the diffraction model for mosaic crystal and the angular divergence after diffraction by mosaic crystal is estimated from Monte Carlo simulation. The time-of-flight spectrometer and gold activation wire are used for measuring the neutron spectra. Both the calculated and measured spectra have proven that the unique feature of polychromatic beam obtained by PG crystals are useful for PGAA. The thermal neutron flux of 7.9$\times$107 n/cm$^2$s and the Cd-ratio of 266 for gold have been achieved at the sample position while the reactor operates at 24 MW The uniformity of beam flux is 12% in the central 1$\times$1 cm$^2$ area. Finally, the beam is briefly characterized by the effective velocity and temperature which are determined by measuring the prompt Y-ray spectra for thin and thick boron samples.ples.

• Nuclear Engineering and Technology
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• v.39 no.4
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• pp.327-336
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• 2007

The influence of density differences on the mixing of the primary loop inventory and the Emergency Core Cooling (ECC) water in the downcomer of a Pressurised Water Reactor (PWR) was analyzed at the ROssendorf COolant Mixing (ROCOM) test facility. ROCOM is a 1:5 scaled model of a German PWR, and has been designed for coolant mixing studies. It is equipped with advanced instrumentation, which delivers high-resolution information for temperature or boron concentration fields. This paper presents a ROCOM experiment in which water with higher density was injected into a cold leg of the reactor model. Wire-mesh sensors measuring the tracer concentration were installed in the cold leg and upper and lower part of the downcomer. The experiment was run with 5% of the design flow rate in one loop and 10% density difference between the ECC and loop water especially for the validation of the Computational Fluid Dynamics (CFD) software ANSYS CFX. A mesh with two million control volumes was used for the calculations. The effects of turbulence on the mean flow were modelled with a Reynolds stress turbulence model. The results of the experiment and of the numerical calculations show that mixing is dominated by buoyancy effects: At higher mass flow rates (close to nominal conditions) the injected slug propagates in the circumferential direction around the core barrel. Buoyancy effects reduce this circumferential propagation. Therefore, density effects play an important role during natural convection with ECC injection in PWRs. ANSYS CFX was able to predict the observed flow patterns and mixing phenomena quite well.

• Progress in Superconductivity and Cryogenics
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• v.20 no.4
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• pp.65-69
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• 2018

Due to large in-field current carrying capacity and strong mechanical strength, a REBCO wire has been regarded as a viable high temperature superconductor (HTS) option for high field MRI and > 1 GHz (>23.5 T) NMR magnets. However, a REBCO magnet is well known to have an inherent problem of field inhomogeneity, so-called 'Screening Current induced magnetic Field (SCF)'. Recently, 'field shaking' and 'current overshoot operation' techniques have been successfully demonstrated to mitigate the SCF and enhance the field homogeneity by experiments. To investigate the effectiveness of current overshooting operation technique, a numerical simulation is conducted for a test REBCO magnet composed of a stack of double pancake coils using '2D edge-element magnetic field formulation' combined with 'domain homogenization' scheme. The simulation result demonstrates that an appropriate amount of current overshoot can negate the SCF. To verify the simulation results, current overshoot experiments are conducted for the REBCO magnet in liquid nitrogen. Experimental results also demonstrate the possible application of current overshoot technique to mitigate the SCF and enhance the field homogeneity.

• Korean Journal of Metals and Materials
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• v.49 no.11
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• pp.893-899
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• 2011

Fabrication of a complex aluminum alloy by the ARB process using dissimilar aluminum alloys has been carried out. Two-layer stack ARB was performed for up to six cycles at ambient temperature without a lubricant according to the conventional procedure. Dissimilar aluminum sheets of AA1050 and AA5052 with thickness of 1 mm were degreased and wire-brushed for the ARB process. The sheets were then stacked together and rolled to 50% reduction such that the thickness became 1 mm again. The sheet was then cut into two pieces of identical length and the same procedure was repeated for up to six cycles. A sound complex aluminum alloy sheet was successfully fabricated by the ARB process. The tensile strength increased as the number of ARB cycles was increased, reaching 298 MPa after 5 cycles, which is about 2.2 times that of the initial material. The average grain size was $24{\mu}m$ after 1 cycle, and became $1.8{\mu}m$ after 6 cycles.

• Journal of Advanced Engineering and Technology
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• v.6 no.2
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• pp.121-124
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• 2013

This paper proposed a new concept of estimating method for LED(light-emitting diode) delamination with high accuracy. Usually, The LED is composed several materials which are LED chips, gold wire, phosphor, epoxy resin, adhesive, reflector and lead frame. These different materials are usually delaminated in a trouble conditions which are huge temperature difference, hot and humid or mechanical shocked. When the components are delaminated, a luminance will be lost, moisture be absorbed easily and a thermal resistance be increased attendantly. As a conventional method to estimate a delamination of LEDs, a solution immersing method is usually used in a field of LED manufacturing companies or researching institutes. This method has an advantage of simplicity but it is only shown that the existence of delamination or not. In this paper, we have proposed an estimating method for LEDs delamination using the polishing and the electron microscope. New proposed method has shown the result of confirming delamination without destruction and enabled quantitative analysis for LED delamination.

• Progress in Superconductivity and Cryogenics
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• v.8 no.3
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• pp.23-26
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• 2006

For mechanical and electrical stability and environment protection. Cu and stainless steel stabilizers are laminated to a Ag layer to produce a composite neutral-axis(N-A) architecture in which the YBCO layer is centered between the oxide buffered metallic substrate and stabilizer strip lamination. This architecture allows the wire to meet operational requirements including stresses at cryogenic temperature. winding tensions as well as mechanical bending requirements including thermal and electrical stability under fault current conditions. We have experimentally studied mechanical properties of the laminated stainless steel and Cu stabilizers on YBCO coated conductors. We have laminated YBCO coated conductors by continuous dipping soldering process. We have investigated lamination interface between solder and stabilizer of the YBCO coated conductor. We evaluated bonding properties. tensile / shear bonding strength. and peeling strength laminated YBCO coated conductors.

• Progress in Superconductivity and Cryogenics
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• v.9 no.3
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• pp.9-12
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• 2007

In this study. we searched for the mechanical and electrical properties of laminated coated conductors with stabilizer tape. Stabilizer tape plays a role for mechanical and electrical stability and environmental protection. Cu material stabilizer was laminated to Ag capping layer on SmBCO conductor layer. This architecture allows the wire to meet operational requirements including the stressless at cryogenic temperature and winding tension as well as mechanical bending requirements including thermal and electrical stability under fault current conditions. First, we have experimentally studied mechanical bonding properties of the laminated Cu stabilizers on SmBCO coated conductors. We have laminated SmBCO coated conductors by continuous dipping soldering process, Second, we have investigated electrical properties of the SmBCO coated conductors with stabilizer lamination. We evaluated bonding properties, peeling strength and critical current for laminated SmBCO coated conductors with Cu stabilizers.

• Journal of Ceramic Processing Research
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• v.13 no.spc2
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• pp.428-432
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• 2012

Polyamidimide (PAI)-colloidal silica (CS) nanohybrid films were synthesized by an advanced sol-gel process. The synthesized PAI-CS hybrid films have a uniform and stable chemical bonding and there is no interfacial defects observed by TEM. The thermal degradation ratio of PAI-CS (10 wt%) hybrid films is delayed by 100 ℃ compared with pure PAI sample determined by on set temperature range in TGA. The dielectric constant of PAI-CS (10 wt%) hybrid films decreases with increasing CS content up to about 5 wt% but increases at higher CS content, which is not explained simply by effective medium therories (EMT). The duration time of PAI-CS (10 wt%) hybrid coil is 38 sec, which is very longer than that of pure PAI coil sample. The PAI-CS (10 wt%) hybrid film has a higher breakdown voltage resistance than the pure PAI film at surge environment and exhibits superior heat resistance. The PAI-CS (10 wt%) sample shows the advanced and stable thermal emission properties in transformer module compared with the pure PAI sample. This result illustrates that the advanced thermal conductivity and expansion properties of PAI-CS sample in the case of appropriate sol-gel processes brings the stable thermal emission in transformer system. Therefore, new PAI-CS hybrid samples with such stable thermal emission properties are expected to be used as a high functional coating application in ET, IT and electric power products.

• Journal of the Korean GEO-environmental Society
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• v.15 no.1
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• pp.5-12
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• 2014

In this study, we developed the electric heating equipment and applied for soft ground improvement. The developed heat pipe is 4 m-length and consumes 1 kW/m, which is consisted of Ni-Cr wire. It was installed in 3.5~4.5 m below ground surface and heated for 96 hours (48 hours, 2 times). The temperature variation and vapor pressure caused by electric heating was measured by the thermometer and pressure gauge which were installed in the ground (5.0 m), and the tip resistances were measured by static electronic piezo-cone penetration test (CPT). As the results of experiments, 2-order polynomial curve was shown to adjust the variation of tip resistance and the temperature distribution with the horizontal distance from electric heater, whose R2 value is close to 1. In addition, in-situ pore-water pressure and water content was decreased.

• Journal of Powder Materials
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• v.12 no.6 s.53
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• pp.422-427
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• 2005

The $Al_2O_3$ with various phases were prepared by simple ex-situ hydrolysis and spark plasma sintering (SPS) process of Al powder. The nano bayerite $(\beta-Al(OH)_3)$ phase was derived by hydrolysis of commercial powder of Al with micrometer size, whereas the bohemite (AlO(OH)) phase was obtained by hydrolysis of nano Al powder synthesized by pulsed wire evaporation (PWE) method. Compaction as well as dehydration of both nano bayerite and bohemite was carried out simultaneously by SPS method, which is used to fabricate dense powder compacts with a rapid heating rate of $100^{\circ}C$ per min. under the pressure of 50MPa. After compaction treatment in the temperature ranges from $100^{\circ}C\;to\; 1100^{\circ}C$, the bayerite and bohemite phases change into various alumina phases depending on the compaction temperatures. The bayerite shows phase transition of $Al(OH)_3{\to}{\eta}-Al_2O_3{\to}{\theta}-Al_2O_3{\to}\alpha-Al_2O_3$ sequences. On the other hand, the bohemite experiences the phase transition from AlO(OH) to ${\gamma}-Al_2O_3\;at\;350^{\circ}C.$ It shows AlO(OH) ${\gamma}-Al_2O_3{\to}{\delta}-Al_2O_3{\to}{\alpha}-Al_2O_3$ sequences. The ${\gamma}-Al_2O_3$ compacted at $550^{\circ}C$ shows a high surface area $(138m^2/g)$.