등전위 교번식 배열형 탐촉자 직류전위차법을 이용한 실시간 균열 감시기술 개발

학위논문 (박사)-- 서울대학교 대학원 : 에너지시스템공학부, 2016. 8. 황일순.Unacceptable cracks exceeding the limits of ASME Boiler Pressure Vessel Code Sec XI at dissimilar metal weld of piping during non-destructive testing using ultrasonic testing (UT), under the conventional periodic inspection approach. As many nuclear power plants (NPP) renter into long-term operation beyond their design life more sensitive on-line cracking monitoring approach became necessary. In this thesis, a direct current potential drop (DCPD) method utilizing array probes with measurement ends maintaining an equalized potential designated as equi-potential switching array probe direct current potential drop (ESAP-DCPD) technique has been developed for on-line crcak monitoring. The developed ESAP-DCPD method has been validated by showing consistency among experimental measurements, analytical solution and numerical predictions using finite element analysis (FEA) with crack growth in weld metals. The FEA-based numerical prediction was verified by a good agreement with an analytic solution for center-cracked tension (CCT) specimen in accordance with ASTM Method E647. The optimal configuration of array probes was obtained from sensitivity study of verified FEA prediction method. In order to examine the viability of ESAP-DCPD for an in-service inspection (ISI), artificial inner surface cracks were introduced in a full-scale dissimilar metal weld (DMW) mockup pipe with circumferential weldment between low alloy steel and stainless steel. A round-robin measurement has been made by participation of three independent laboratories. It has been found that the developed ESAP-DCPD can detect circumferential cracks with a depth of 40% or greater of wall thickness in stainless steel. In contrast, the ESAP-DCPD system is shown to have very good viability and sensitivity under on-line monitoring condition. In order to examine its sensitivity under on-line monitoring of crack growth at the DMW, fatigue tests were performed by using modified CCT specimens manufactured from a full-scale mock-up welded pipe. The depth of initial crack was 20% of pipe wall thickness (a/t) and crack growth under fatigue testing was measured by both ESAP-DCPD and direct optical inspection. ESAP-DCPD detected the crack growth from the beginning of the test with high signal to noise ratio, demonstrating its suitability for on-line monitoring. Using the experimental results, the probability of detection (POD) curve was obtained for use in In-Service Inspection (ISI) mode and on-line monitoring mode of ESAP-DCPD through the signal sensitivity and noise analysis. Results were compared with POD of conventional ISI using UT. A threshold crack size of detection was defined by measured ESAP-DCPD value exceeded four times standard deviation of noise distribution measured at the initial state prior to the crack growth. The POD of ESAP-DCPD under on-line monitoring mode reaches almost 100 percent when size of crack is only about 10% of wall in contrast to the conventional ISI using UT where excessive cracks including even through-wall cracks are often undetected in DMW region. However ESAP-DCPD used under ISI mode is shown to suffer from POD decrease due to increased noise, like ISI using UT. ESAP-DCPD is, therefore, found to be one of the best method for on-line monitoring of piping weld crack growth during long-term operation of nuclear power plants.1. INTRODUCTION 1 1.1 Background 1 1.2 Cracking of dissimilar metal welds 3 1.3 Management of structural integrity at nuclear power plants 5 1.3.1 In-service inspection 5 1.3.2 On-line monitoring of EAC 6 2. LITERATURE REVIEW 14 2.1 Non-destructive testing 14 2.1.1 In-service inspection 14 2.1.2 On-line monitoring 16 2.2 Application of direct current potential drop 21 2.2.1 TÜV Rheinland 21 2.2.2 Equi-potential Switching DCPD (ES-DCPD) 22 2.2.3 Other related studies 24 2.3 Probability of detection (POD) 27 2.3.1 Empirical probability of detection (POD) 27 2.3.2 Model assisted POD (MAPOD) 31 3. RATIONALE AND APPROACH 48 3.1 Problem statement 48 3.2 Goals of thesis 51 3.3 Rationale and approach 52 4. DEVELOPMENT OF EQUIPOTENTIAL SWITCHING ARRAY PROBE DIRECT CURRENT POTENTIAL DROP (ESAP-DCPD) FOR CRACK MONITORING 58 4.1 Development of ESAP-DCPD 58 4.2 Reliability of finite element analysis (FEA) for DC electric field in metal 61 4.3 Optimal configuration of array probes 64 4.3.1 Sensitivity study 64 4.3.2 Theoretical analysis 67 4.4 Effect of temperature and probe rigidity 71 4.5 Noise reduction methods 73 4.6 Development of axial crack detection method 77 5. VERIFICATION OF EQUIPOTENTIAL SWITCHING ARRAY PROBE DIRECT CURRENT POTENTIAL DROP 105 5.1 Fixed crack monitoring 105 5.1.1 Fixed crack monitoring of pipe with dissimilar metal weld 105 5.1.2 Fixed crack monitoring of stainless steel pipe 106 5.1.3 Round robin test 108 5.2 On-line monitoring 109 5.2.1 Monitoring of through-wall crack at the center cracked tension specimen 109 5.2.2 Monitoring of part-through-wall crack at the modified center cracked tension specimen 111 6. DEVELOPMENT OF PROBABILITY OF DETECTION FOR ON-LINE MONITORING 133 6.1 Uncertainties of POD 133 6.2 Development of POD 135 6.2.1 ISI POD 135 6.2.2 On-line monitoring POD 136 7. CONCLUSIONS AND FUTURE WORK 146 7.1 Summary and Conclusions 146 7.2 Future Work 149 REFERENCES 153 국문 요약서 162Docto

Fast-neutron에 의해 유도된 애기장대 조기개화 돌연변이체들의 분리 및 동정

Thesis (master`s)--서울대학교 대학원 :생명과학부,2002.Maste

니켈 합금 X-750의 환경피로균열 성장속도모형에 대한 연구

학위논문 (석사)-- 서울대학교 대학원 : 에너지시스템공학부, 2012. 2. 황일순.Nickel base Alloy X-750, which is used as fastener parts such as beams, bolts and springs in LWRs, has experienced many failures by environmentally assisted cracking (EAC). In order to improve the reliability of passive components for nuclear power plants (NPPs), it is necessary to study the failure mechanism of Alloy X-750 and to predict crack growth behavior by developing a probabilistic failure model. The probabilistic safety analysis has been performed in order to enhance the safe of NPPs, because traditional deterministic approach has limitations to predict the risk of failure from behavior of cracks growth in NPPs. In this thesis, the Bayesian inference was employed to reduce the uncertainties contained in EAC modeling parameters that have been established from experiments with Alloy X-750. Hydrogen embrittlement is known as the intergranular crack growth mechanism of Alloy X-750. To study the corrosion fatigue cracking mechanisms, two different heat treatments were employed and corrosion fatigue crack growth rate modeling was developed at the different environments. AH heat treatment (heating at 885°C for 24hr and 704°C for 20hr, then air cooling) and HTH heat treatment (heating at 1093°C for 1hr and 704°C for 20hr, then air cooling) were used. From the microstructure observations of two different types of heat treated specimens, it was found that gamma prime (γ) phase precipitates are generally formed at the grain boundary of AH with discontinuous intergranular carbides, while HTH has continuous intergranular carbides. The hydrogen embrittlement is explained by the generation and absorption of dissolved hydrogen by selection corrosion of the gamma prime (γ) phase precipitates at grain boundary. Corrosion fatigue crack growth rate model was developed by fitting into Paris Law of measured data from the several fatigue tests with varying environments. At the same mechanical load condition, Alloy X-750 was tested using single edge notched (SEN) specimens in different pure water, Na2SO4 solution, as well as air environments respectively. The cathodic potential was applied to change the dissolved hydrogen concentration at the crack tip by the cathodic polarization of X-750 specimen. All equipment and procedures are prepared and conducted in accordance with the ASTM method E647-08. Corrosion fatigue crack growth rate measurements were conducted either in constant load mode or constant ΔK mode for a given environments for fatigue crack growth rate (FCGR) modeling. Paris law relates the stress intensity factor range to sub-critical crack growth under a fatigue stress regime (generally expressed as stage2). In this regime, the in-service inspections (ISIs) and the crack growth prediction for structural materials can be used for the assessment of remaining life of components. In this thesis Bayesian inference method has been treated to confirm that the prediction of uncertainty can be reduced by updating model parameters using the most recent inspection data. The parameters C and m of Paris Law model were assumed to obey the Gaussian distribution. In this study, these parameters characterizing the corrosion fatigue crack growth behavior of X-750 were updated to reduce the uncertainty in the model by using the Bayesian inference method. It was successfully confirmed the decrease of standard deviation of C and m by employing the Bayesian updating procedure. Results of this thesis have demonstrated that probabilistic models can be developed by updating a laboratory model with in-service inspection data based on Bayesian inference method. The probabilistic environmentally assisted cracking model will prove to be instrumented in probability safety assessment (PSA) to analyze the failure probability of passive components.니켈계 합금 X-750은 경수로 내에서 빔, 볼트, 스프링으로 쓰이며 여기에는 여러 사고가 일어났다. 원자력발전소의 피동형 기기의 신뢰성을 증진시키기 위해 합금 X-750의 균열기구를 연구하고 확률론적 사고 예측 모델을 개발하여 균열 성장 거동을 예측하는 것이 필요하다. 결정론적 방법은 균열 성장 거동과 발전소 잔존수명을 예측하기 어렵기 때문에 발전소 수명을 늘리기 위해 확률론적 안정성 분석이 사용된다. 본 논문에서는 변수나 식 안에 포함되어 있는 불확실도를 줄이기 위해 베이지안 추론법이 사용되었다. 합금 X-750의 취성 기구는 수소 취성으로 알려져 있다. 환경 피로 균열 기구를 이해하기 위해서 서로 다른 두 가지 열처리가 이루어졌으며 환경 피로 균열 성장 속도 모델링이 환경을 달리하여 시행되었다. 열처리 방법으로는 AH 열처리 (24시간 동안 885℃, 20시간 동안 704℃ 그리고 공기 중 냉각)와 HTH 열처리 (1시간 동안 1093℃, 20시간 동안 704℃ 그리고 공기 중 냉각)가 사용되었다. 이 두 열처리를 한 시료의 미세구조 관측을 통해, 입계에서 감마 프라임상 석출물이 생성되고 AH 열처리한 재료에서는 불연속 입계 탄화물을, HTH 열처리한 재료에서는 연속 입계 탄화물이 생성된다는 것을 발견하였다. 수소취성은 입계에서의 감마 프라임상 석출물과 용존 수소의 발생 반응 및 흡수에 의해 설명된다. 환경 피로 균열 성장 속도 모델이 환경을 달리하며 시행한 피로 시험을 통해 개발되었다. 일정한 기계적 하중 조건하에 합금 X-750으로 만든 SEN 시편을 사용해 순수, 황산나트륨 용액, 그리고 공기 중에서 피로 시험이 진행되었다. 전기화학 부식 전위를 걸어 시편에 음극분극을 형성시켜 균열선단에 수소 농도를 바꾸어 주었다. 모든 장비와 시험 절차는 ASTM 표준에 따라 준비되었다. 피로 균열 성장 속도 모델을 만들기 위해 일정 하중과 일정 응력확대 계수로 몇몇 실험이 시행되었다. 균열 성장 속도는 가장 널리 알려진 Paris 법칙을 따른다고 가정하였다. Paris 법칙은 피로 응력이 걸릴 때 응력 확대 계수와 미임계 균열 성장의 관계를 보여주며 제 2 영역으로 불리는 이 구역에서는 가동 중 검사와 균열이 생긴 구조 재료의 보수가 가능하다. Paris 법칙을 만족하는 관계식을 결정하기 위해 Paris 변수 C와 m이 사용되었고 이들은 가우시안 분포를 따르는 분포로 가정되었다. 본 연구에서는 X-750의 환경 피로 균열 성장 거동을 결정짓는 변수들을 베이지안 추론법을 사용해 업데이트하여 모델 내 존재하는 불확실도를 줄였다. 이는 업데이트를 계속함으로써 C와 m의 표준 편차가 줄어드는 것으로 확인하였다. 업데이트된 C와 m의 분포로부터 최종 피로균열성장속도가 결정되었다. 이를 통해 현장 데이터를 사용해 실험실 모델을 업데이트 하여 확률론적 사고 예측 모델이 개발할 수 있음을 보였다. 이 모델은 확률론적 안전성 평가 툴과 결합시켜 다른 경연열화 현상을 갖는 피동형 기기의 사고도 분석할 수 있게 해준다.Maste

Short-Term Synaptic Plasticity and Persistent Activity in the Prefrontal Cortex

학위논문 (박사)-- 서울대학교 대학원 : 자연과학대학 생명과학부, 2019. 2. 이석호최석우.Persistent activity of cue-representing neurons in the prefrontal cortex (PFC) is regarded as a neural basis for working memory. Contribution of short-term synaptic plasticity (STP) at different types of synapses comprising the cortical network to persistent activity, however, remains unclear. Upon characterization of STP at synapses of the PFC network, PFC synapses were found to exhibit distinct STP patterns according to presynaptic and postsynaptic identities. Excitatory synapses from corticopontine (Cpn) neurons were well sustained throughout continued activity, with stronger depression at synapses onto fast-spiking interneurons than those onto pyramidal cells. Inhibitory postsynaptic currents were sustained at a weaker level compared to excitatory postsynaptic currents from Cpn synapses. Computational modeling of a balanced network incorporating empirically observed STP revealed that little depression at recurrent excitatory synapses, combined with stronger depression at other synapses, could provide the PFC with a unique synaptic mechanism for the generation and maintenance of persistent activity.Abstract - 1 Table of Contents - 3 Introduction - 8 Methods - 14 Virus injection - 14 Slice preparation - 15 Whole-cell patch clamp - 15 Synaptic stimulation - 17 Quantal analysis - 19 Data analysis - 21 Synaptic vesicle dynamics model - 22 Determining the initial parameters for model fits to STP data at depressing synapses - 24 Determining the initial parameters for model fits to STP data at facilitating synapses - 27 Network model based on conductance synaptic inputs - 30 Network model based on voltage synaptic inputs - 35 Results - 39 EPSC at Cpn synapses are well sustained throughout prolonged activity - 39 Com and thalamocortical synapses are largely depressive - 41 Inhibitory synapses maintain steady-state activity comparable to Cpn synapses - 43 Individual PFC neurons receive proportional levels of excitation and inhibition - 44 Estimation of quantal parameters at PFC synapses - 46 Contribution of STP in the PFC network to persistent activity - 49 Numerical analysis of the spiking network model - 51 Discussion - 59 STP and bistability as alternative or complementary mechanisms for persistent activity in a balanced network - 59 Network organization and persistent activity - 62 Effects of neuromodulation on STP and persistent activity - 63 Figures 1-20 and Tables 1-5 - 65 Figure 1. Experimental design - 66 Figure 2. Intrinsic membrane properties of PFC neurons - 68 Figure 3. Consistency of ChIEF activation - 70 Figure 4. PSC kinetics - 72 Figure 5. STP at Cpn synapses - 74 Figure 6. STP at Com synapses - 76 Figure 7. STP at MDT synapses - 78 Figure 8. STP at somatosensory cortex synapses - 80 Figure 9. STP at IN synapses - 82 Figure 10. Excitation-inhibition ratio at PFC neurons - 84 Figure 11. Quantal parameters at Cpn and IN synapses - 86 Figure 12. Quantal parameters at Com synapses - 88 Figure 13. Quantal current measurements from Sr2+-induced asynchronous release - 90 Figure 14. Simple vesicle dynamics model - 92 Figure 15. Network model of spiking neurons - 94 Figure 16. Extended simulation of the network model - 96 Figure 17. Network model analysis - 98 Figure 18. Network model based on voltage synaptic inputs - 100 Figure 19. Network model behaviors with different forms of STP - 102 Figure 20. Determination of STP model parameters - 104 Table 1. Parameters for the synaptic vesicle dynamics model - 106 Table 2. Parameters for the network model of spiking neurons - 107 Table 3. Additional Parameters for the network model of spiking neurons based on voltage synaptic inputs - 108 Table 4. Notations used for the synaptic vesicle dynamics model - 109 Table 5. Notations used for the network model - 110 References - 111Docto