• 한국해양공학회지
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• 제10권1호
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• pp.65-74
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• 1996

Up to now, the existing crack growth modelling has used a mathematical approximation but an assumed function have a great influence on this method. Especially, crack growth behavior that shows very strong nonlinearity needed complicated function which has difficulty in setting parameter of it. The main characteristics of neural network modelling to engineering field are simple calculations and absence of assumed function. In this paper, after discussing learning and generalization of neural networks, we performed crack growth modelling on the basis of above learning algorithms. J'-da/dt relation predicted by neural networks shows that test condition with unlearned data is simulated well within estimated mean error(5%).

• 대한기계학회논문집A
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• 제20권4호
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• pp.2752-2759
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• 1996

This paper presents crack growth analysis approach on the basis of neural networks, a branch of cognitive science to high temperature low cycle fatigue that shows strong nonlinearity in material behavior. As the number of data patterns on crack growth increase, pattern classification occurs well and two point representation scheme with gradient of crack growth curve simulates crack growth rate better than one point representation scheme. Optimal number of learning data exists and excessive number of learning data increases estimated mean error with remarkable learning time J-da/dt relation predicted by neural networks shows that test condition with unlearned data is simulated well within estimated mean error(5%).

• Journal of Electrical Engineering and Technology
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• 제3권4호
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• pp.499-508
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• 2008

Frequency regulation in off-normal conditions has been an important problem in electric power system design/operation and is becoming much more significant today due to the increasing size, changing structure and complexity of interconnected power systems. Increasing economic pressures for power system efficiency and reliability have led to a requirement for maintaining power system frequency closer to nominal value. This paper presents a decentralized frequency control framework using a modified low-order frequency response model containing a proportional-integral(PI) controller. The proposed framework is suitable for near-normal and emergency operating conditions. An $H_{\infty}$ control technique is applied to achieve optimal PI parameters, and an analytic approach is used to analyse the system frequency response for wide area operating conditions. Time-domain simulations with a multi-area power system example show that the simulated results agree with those predicted analytically.

• 대한기계학회논문집A
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• 제29권10호
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• pp.1361-1368
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• 2005

The structural and leakage integrity of steam generator tubes should be sustained all postulated loads with appropriate margin even if a crack is present. During the past three decades, for effective integrity evaluation, several limit load solutions have been used world-widely. However, to predict accurately load carrying capacities of specific components under different conditions, the solutions have to be modified by using lots of experimental data. The purpose of this paper is to propose a new burst pressure estimation scheme based on fracture mechanics analyses for steam generator tube with an axial or circumferential through-wall crack. A series of three dimensional elastic-plastic finite element analyses were carried out and, then, closed-form estimation equations with respect to both J-integral and crack opening displacement were derived through reference stress method. The developed engineering equations were utilized for structural integrity evaluation and the resulting data were compared to the corresponding ones fiom experiments as well as limit load solutions. Thereafter, since the effectiveness was proven by promising results, it is believed that the proposed estimation scheme can be used as an efficient tool for integrity evaluation of cracked steam generator tubes.

• Nuclear Engineering and Technology
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• 제54권3호
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• pp.781-789
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• 2022

KHNP had licensed Dynamic Control rod Reactivity Measurement (DCRM) method using detector current signals of PWRs in 2006. The method has been applied to all PWRs in Korea for about 15 years successfully. However, the original method was inapplicable to PWRs using low-sensitivity integral fission chamber as ex-core detectors because of their pulse pile-up and the nonlinearity of the mean-square voltage at low power region. Therefore, to overcome this disadvantage, a modified method, DCRM-EK, was developed using kinetics behavior after equilibrium condition where the pulse counts maintain the maximum value before pulse pile-up. Overall measurement, analysis procedure, and related computer codes were changed slightly to reflect the site test condition. The new method was applied to a total of 15 control rods of 1000 MWe and 1400 MWe PWRs in Korea with worths in the range of 200 pcm -1200 pcm. The results show the average difference of -0.4% and the maximum difference of 7.1% compared to the design values. Therefore, the new DCRM-EK will be applied to PWRs using low sensitivity integral fission chambers, and also can replace the original DCRM when the evaluation fails by big noises present in current or voltage signals of uncompensated/compensated ion chambers.

• Nuclear Engineering and Technology
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• 제38권7호
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• pp.639-656
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• 2006

In order to apply the leak before break (LBB) concept to nuclear piping systems, the dynamic strain aging effect of low carbon steel materials has to be taken into account, in compliance with the requirements of the Korean Standard Review Guide (KSRG) 3.6.3-1. For this goal, J-R tests are needed for a range of various temperatures and loading rates, including dynamic loading conditions. In the dynamic loading J-R test, the unloading compliance method can not be applied to measure the crack growth and direct current potential drop (DCPD) method; this method also has a problem defining the crack initiation point. The normalization method is known as a very useful method to determine the J-R curve under dynamic loading because it does not need additional equipment or complicated loading sequences such as electric current or unloading. This method was accepted by the American Society for Testing and Materials (ASTM) as a standard test method E1820 A15 in 2001. However, it has not yet been clearly verified yet if the normalization method is sufficiently reliable to be applied to LBB. In this study, the basic background of the J-integral, LBB and dynamic loading J-R test are explained, and the current status for dynamic loading J-R test methods are reviewed from the view point of LBB for nuclear piping. In particular, the theoretical and historical background of the normalization method which has received attention recently, is summarized. Recent studies for this method are introduced and future works are suggested that may improve the reliability of LBB for nuclear piping.

• 한국전산구조공학회논문집
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• 제15권3호
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• pp.389-398
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• 2002

동전모양의 균열이 이상복합 실린더 계면에 존재하는 혼합모드 조건(I, II)에 대해 유한요소법을 사용하여 에너지해방율을 구하였다. 두재료의 탄성비와 노치율을 변화시켜 상업용 FEM 프로그램인 ABAQUS로부터 얻은 결과를 가상 균열법과 J 적분법에 적용하였으며 에너지해방율을 구하여 무차원함수로 표현하였다. 모드 II의 무차원 에너지해방율($\sqrt{G_{II}E^*}/\sqrt[p]{\pi a}$)은 균열길이와 탄성비가 증가되면서 그 값이 증가됨을 알수 있었다. 반면, 모드 I의 무차원 에너지해방율($\sqrt{G_{I}E^*}/\sqrt[p]{\pi a}$)은 탄성비가 증가하면서 그 값이 감소하며, 두재료의 탄성비가 3 이상인 경우에 균열길이가 증가되면서 무차원 에너지해방율이 감소하다가 다시 증가하게 나타났다. 또한 수치해석된 결과치를 무한판 실린더의 응력확대계수에 대한 정해와 비교하여 본 해석의 신뢰성을 확보하였다.

• Nuclear Engineering and Technology
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• 제47권7호
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• pp.791-803
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• 2015

The effective cross sections (XSs) in the direct whole core calculation code nTRACER are evaluated by the equivalence theory-based resonance-integral-table method using the WIMS-based library as an alternative to the subgroup method. The background XSs, as well as the Dancoff correction factors, were evaluated by the enhanced neutron-current method. A method, with pointwise microscopic XSs on a union-lethargy grid, was used for the generation of resonance-interference factors (RIFs) for mixed resonant absorbers. This method was modified by the intermediate-resonance approximation by replacing the potential XSs for the non-absorbing moderator nuclides with the background XSs and neglecting the resonance-elastic scattering. The resonance-escape probability was implemented to incorporate the energy self-shielding effect in the spectrum. The XSs were improved using the proposed method as compared to the narrow resonance infinite massbased method. The RIFs were improved by 1% in $^{235}U$, 7% in $^{239}Pu$, and >2% in $^{240}Pu$. To account for thermal feedback, a new feature was incorporated with the interpolation of pre-generated RIFs at the multigroup level and the results compared with the conventional resonance-interference model. This method provided adequate results in terms of XSs and k-eff. The results were verified first by the comparison of RIFs with the exact RIFs, and then comparing the XSs with the McCARD calculations for the homogeneous configurations, with burned fuel containing a mixture of resonant nuclides at different burnups and temperatures. The RIFs and XSs for the mixture showed good agreement, which verified the accuracy of the RIF evaluation using the proposed method. The method was then verified by comparing the XSs for the virtual environment for reactor applicationbenchmark pin-cell problem, as well as the heterogeneous pin cell containing burned fuel with McCARD. The method works well for homogeneous, as well as heterogeneous configurations.

• Journal of Mechanical Science and Technology
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• 제18권12호
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• pp.2158-2173
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• 2004

In this paper, the higher order terms in the crack tip stress fields are investigated macroscopically for more realistic assessment of structural material behaviors. For reactor pressure vessel material of A533B ferritic steel, effects of crack size and temperature have been evaluated using 3-point SENB specimens through a series of finite element analyses, tensile tests and fracture toughness tests. The T-stress, Q-parameter and q-parameter as well as the K and J-integral are calculated and mutual relationships are investigated also. Based on the evaluation, it has proven that the effect of crack size from standard length (a/W=0.53) to shallow length (a/W=0.11) is remarkable whilst the effect of temperature from -20$^{\circ}C$ to -60$^{\circ}C$ is negligible. Finally, the cleavage fracture toughness loci as a function of the promising Q-parameter or q-parameter are developed using specific test results as well as finite element analysis results, which can be applicable for structural integrity evaluation considering constraint effects.

• 한국광학회지
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• 제5권3호
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• pp.349-355
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• 1994

Rayleigh의 분해능 한계 $\epsilon_R$보다 작은 선폭 $\epsilon_0$을 분해할 수 있는 광결상계사 이론적으로 취급되었고, 이어서 초분해능을 지니는 광학계가 제시되었다. 먼저 초분해능을 역변환 문제로 취급하여 초분해능 회절상의 진폭 임펄스 함수를 주는 적분방정식을 도입하고 매개변수 $\beta$(양정수)에 따라서 진폭 임펄스 함수의 반치폭이 조사되었다. 이 초분해능 광학계의 구경함수의 진폭 분포가 변조되었고 또 구경에서 나가는 Huygens 파동의 위상도 Holography로 변조되었다. 이들의 변조를 위하여 이용된 광변조 소자가 제시되었다. 초미세 광리소그라피에 유용한 결론을 얻었다.