• Transactions of the Korean Society of Mechanical Engineers A
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• v.38 no.6
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• pp.637-647
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• 2014

In pressurized water reactors, the upper head of the reactor pressure vessel (RPV) contains numerous control rod drive mechanism (CRDM) nozzles. These nozzles are fabricated by welding after being inserted into the RPV head with a room temperature shrink fit. The tensile residual stresses caused by this welding are a major factor in primary water stress corrosion cracking (PWSCC). Over the last 15 years, the incidences of cracking in alloy 600 CRDM nozzles have increased significantly. These cracks are caused by PWSCC and have been shown to be driven by the welding residual stresses and operational stresses in the weld region. Various measures are being sought to overcome these problems. The defects resulting from the welding process are often the cause of PWSCC acceleration. Therefore, any weld defects found in the RPV manufacturing process are immediately repaired by repair welding. Detailed finite-element simulations for the Korea Nuclear Reactor Pressure Vessel were conducted in order to predict the magnitudes of the repair weld residual stresses in the tube materials.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.25 no.4
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• pp.756-763
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• 2001

In the process of integrity evaluation for nuclear power plant components, a series of fracture mechanics evaluation on surface cracks in reactor pressure vessel(RPV) must be conducted. These fracture mechanics evaluation are based on stress intensity factor, K. However, under pressurized thermal shock(PTS) conditions, the combination of thermal and mechanical stress by steep temperature gradient and internal pressure causes considerably high tensile stress at the inside of RPV wall. Besides, the internal pressure during the normal operation produces high tensile stress at the RPV wall. As a result, cracks on inner surface of RPVs may experience elastic-plastic behavior which can be explained with J-integral. In such a case, however, J-integral may possibly lose its validity due to constraint effect. In this paper, in order to verify the suitability of J-integral, tow dimensional finite element analyses were applied for various surface cracks. A total of 18 crack geometries were analyzed, and $\Omega$ stresses were obtained by comparing resulting HRR stress distribution with corresponding actual stress distributions. In conclusion, HRR stress fields were found to overestimate the actual crack-tip stress field due to constraint effect.

• Analytical Science and Technology
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• v.5 no.3
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• pp.277-283
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• 1992

Acid effects on the ICP-AES signals are studied as the concentrations of nitric acid, hydrochloric acid, sulfuric acid, and 1:1 mixture of nitric acid and hydrochloric acid are changed. Almost all analyte signals are depressed. The extent of the depression due to the pressence of the acids became to be pecular when the acid concentration was over 1%. Among the acids used, the suppression due to sulfuric acid is most severe and unexpectable. The ratios of the analyte signal to Ar signal and the Mg II signal to Mg I signal are measured as the concentration of the acids changed. In this study, it is proved that the main reason of the signal reduction is the change in the nebulization efficiency, for example, droplet size distribution, viscosity and surface tension variation, not the alteration of plasma excitation characteristics. There was no relationship found between ionization potential and analyte signal reduction in ICP-MS.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.36 no.10
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• pp.1235-1239
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• 2012

Primary water stress corrosion cracking (PWSCC) has been observed around the weld region of control rod drive mechanism (CRDM) nozzles in nuclear power plants overseas. The weld has a J-shaped groove and it connects the CRDM nozzle with the reactor vessel upper head (RVUH). It is a dissimilar metal weld (DMW), because the CRDM is made of alloy 600 and the RVUH is made of carbon steel. In this study, finite element analysis (FEA) was performed to estimate the stress condition around the weld region. Generally, it is known that a high tensile region is more susceptible to PWSCC. FEA was performed as for the condition of welding, hydrostatic test and normal operation successively to observe how the residual stress changes due to plant condition. The FEA results show that a high tensile stress region is formed around the weld starting point on the inner surface and around the weld stop point on the outer surface.

• Journal of the Korean Magnetics Society
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• v.16 no.5
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• pp.229-233
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• 2006

We investigated the magnetic properties of Fe nanostripes by using the all electron full-potenial linearized augmented plane-wave (FLAPW) energy band method within the generalized gradient approximation (GGA). The magnetic moments of the Fe atoms in the edge Fe chains of the stripes composed of three, five, and seven chains have saturated values of 2.97 or 2.98 ${\mu}_B$, and the values of the center chains are 2.82 ${\mu}_B$ which is similar to that of 2D square lattice. The charge and spin density contour plots showed that the flat distribution in the edge region of the stripes, and it is due to the spilled out p-electrons from the atoms in the edge line. The calculated density of states for the edge atoms in the stripes with seven Fe chains showed that the narrowed width compared to that of center atoms due to the band narrowing effect at the edge.

• Analytical Science and Technology
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• v.7 no.3
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• pp.301-313
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• 1994

X-Ray Photoelectron Spectroscopy(XPS) and Rutherford Backscattering Spectroscopy(RBS) are used for the analysis of additives, examination of chemical structure and determination of identity with qualitative and quantitative analysis of surface elements, binding energy level and depth profiling in the surface. We analyzed surface of polyethylene, acrylonitrile butadien rubber, polypropylene, glass, fiber and paper treated with $XeF_2$ or C-F plasma by XPS and RBS. It was found that fluoro element was penetrated to sample surface and the distribution of surface elements are different than untreated samples.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.37 no.5
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• pp.649-655
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• 2013

During welding, the dissimilar metal butt welds of nuclear piping are typically subjected to repair welding in order to eliminate defects that are found during post-weld inspection. It has been found that the repair weld can significantly increase the tensile residual stress in the weldment, and therefore, accurate estimation of the weld residual stress due to repair weld, especially for dissimilar metal welds using Ni-based alloy 82/182 in nuclear components, is of great importance in order to assess susceptibility to primary water stress corrosion cracking. In the present study, the stress distributions of dissimilar metal butt welds in nuclear reactor piping subjected to repair weld were investigated based on detailed nonlinear finite element analyses. Particular emphasis was placed on the variation of the stress distribution in the dissimilar metal butt weld according to the finite element welding analysis sequence for the repair welding process.

• Nuclear Engineering and Technology
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• v.23 no.3
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• pp.352-362
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• 1991

Recent popular trends in pressurized water reactor(PWR) fuel management are to extend the cycle length and to employ the low-leakage core designs for the optimal utilization of the uranium resources. In control strategy incorporated with the fuel management, turnable absorbers are required to control the power peaking and to ensure a negative moderator temperature coefficient during reactor operation. In this study, the nuclear characteristics and the optimal allocation of gadolinium-poisoned rods within the fuel assembly are considered using KWU SAV 79 A Code Package. First, analyses are carried out to compare the nuclear characteristics of the fuel assemblies contain-ing WABA(Wet Annular Burnable Absorber) and Gadolinium burnable absorbers respectively. The analyses show that the gadolinium-bearing fuel assembly has peculiar depletion characteristics ensuing from the very large thermal neutron absorption cross section. Peculiar characteristics of gadolinium provide basis for the optimal allocation of Gd rods in fuel assembly. Second, the methodology of an optimal allocation of gadolinium-poisoned rods within the fuel assembly is developed and applied to some nuclear designs.

• Journal of IKEEE
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• v.21 no.1
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• pp.96-99
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• 2017

In this study the influence of the random dopant fluctuation (RDF) depending on the halo and LDD implantations for the metal-oxide-semiconductor field effect transistor is investigated through the 3D atomistic device simulation. For accuracy in calculation, the kinetic monte carlo method that models individual impurity atoms and defects in the device was applied to the atomistic simulation. It is found that halo implantation has the greater influence on RDF effects than LDD implantation; three-standard deviation of $V_{TH}$ and $I_{ON}$ induced by halo implantation is about 6.45 times and 2.46 times those of LDD implantation. The distributions of $V_{TH}$ and $I_{ON}$ are also displayed in the histograms with normal distribution curves.

• Proceedings of the Korean Powder Metallurgy Institute Conference
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• 2001.04a
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• pp.9-10
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• 2001

용융공정 $YBa_2Cu_3O_{7-x}$(123) 초전도체는 고자장 하에서도 통전특성이 우수하다 그러나 123 초전도체에는 미세균열이나 기공과 같이 초전도체의 통전특성에 유해 한 요인들도 다수 포함된다. 미세균열은 고온 정방정 상이 저온 사방정상으로 상변 태 시 발생하는 웅력에 의해 생성된다. 반면, 기공은 123 성형체를 녹이는 과정에서 123 상에 포함된 산소원자들이 격자로부터 이탈되고, 이 산소원자들이 모여 액상에서 기공을 형성한다. 제조공정에 따라 기공의 크기와 밀도가 다르지만 대략 수십 이크론 정도로 대단히 크다 생성된 기공 중 일부는 열처리 중에 소멸되나, 어떤 것들은 그대로 남아 초전도체의 치밀화를 방해한다. 본 연구에서는 123의 용융 및 $YBa_2Cu_3O_{7-x}$(211)과 액상으로의 분해 과정 및 포정반응과 관련된 미세조직을 조사하여 기공생성과 소멸과정을 조사하였고, 123의 최종 미세조직에 대한 기공의 영향에 대 하여 연구하였다. 열처리 스케쥴은 123-211-액상의 그림 l의 2원 상태도를 기초로 하여 결정하였다. 먼저 부분 용융상태에서의 기공의 분포를 알고자 시편을 105$0^{\circ}C$에서 0.5-1 시 간 유지한 후, 액체 질소통에 넣어 냉각하였다 (그림 2의 열처리 경로 CD)$\circled1$부분 용 융상태에서 급랭할 경우 211과 액상 상태가 그대로 유지되므로 액상에서의 기공분 포를 관찰할 수 있다. 또 다른 시편들은 그림 2의 @$\circled2$경로로 열처리하였다. 이 시편에서는 고온에서 생성된 211과 액상이 반웅하여 123 결정이 생성, 성장하므로 123 결정립 내의 기공분포를 알 수 있다. 그림 3은 시편에서의 기공과 액상포켓의 분포를 모식도와 각 부위의 미세조직 사진이다. 시편에는 산소가스 발생으로 인해 생성된 수형의 기공이 관찰된다. 기공은 시편의 중앙에 집중되며, 시편 바깥부분은 기공에 액상이 채워진 액상포켓이 관찰된다. 기공의 생성과 소멸과정은 다음과 같다. 출발물질인 123 분말이 211과 액상으로 분해될 때 산소가스가 배출되며, 이로 인해 액상에서 구형의 기공이 생성된다. 이들 중 일부는 액상으로 채워져 소멸되나, 나머지는 그대로 남는다. 특히, 시편 중앙에 서는 수십-수백 마이크론 크기의 커다란 기공이 다수 관찰된는데, 이는 기공의 합체로 만들어진 것이다. 포정반응 열처리 시 기공 소멸로 만들어진 액상포켓들은 주변 211 입자와 반응하여 123 영역으로 변한다. 이곳은 다른 지역과 비교하여 211 밀도 가 낮기 때문에, 미반응 액상이 남거나 211 밀도가 낮은 123 영역이 된다. 액상으로 채워지지 못한 구형의 기공들 중 다수가 123 결정 내로 포획되며, 그 형상은 액상/ 기공/고상 계면에너지에 의해 결정된다.