Yoon Suk Chang

Assessment of Round Robin Analyses Results on Welding Residual Stress Prediction in a Nuclear Power Plant Nozzle

This paper provides simulational round robin test results for welding residual stress prediction of safety/relief nozzle. To quantify the welding variables and define the recommendation for prediction and determination of welding residual stress, 6 partners in 5 institutes participated in round robin test. It is concluded that compressive axial and hoop residual stress occurs in dissimilar metal weld and pre-existing residual stress distribution in dissimilar metal weld was affected by similar metal weld due to short length of safe end. Although the reason for the deviation among the results was not pursued further, the effect of several key elements of FE analyses on welding residual stress was investigated in this paper.

Resonant behavior and microfluidic manipulation of silicone cilia due to an added mass effect

In a microfluidic device, cilia are good candidates to generate complex flow in solution by stirring the microscale fluid. To realize the potential as a novel microfluidic system, it is essential to understand the resonant behavior of cilia in solution and optimize the performance of a cilia-based device. In this paper, the resonant behavior of polydimethylsiloxane (PDMS) cilia in water is investigated by a novel computational method in order to understand the underlying physics and to suggest the optimal design of a cilia-based device. The resonant frequency of a single cilium in water is quantitatively compared to both analytical solutions and experimental results considering an added mass effect. Also, to propose the optimal design of cilia-based device as a microfluidic mixer, the fluid velocity and the pressure in multiple cilia are analysed according to the spacing between neighboring cilia. When the spacing increases from 100 µm to 600 µm, the flow pattern generated by the tips of cilia is changed from a large vortex to multiple small vortexes, which can be utilized for transport, separation and reaction of biomolecules. The particle dispersion with the mixing efficiency in multiple cilia is analysed to predict the performance as a microfluidic device.

Burst Pressure Estimation of Steam Generator Tubes Based on Fracture Mechanics Analyses

During the past couple of decades, several limit load solutions have been proposed to resolve steam generator (SG) tube integrity issue. However, for estimation of specific load carrying capacity under different conditions, these solutions have to be modified by using lots of experimental data. The objective of this paper is to introduce a new burst pressure estimation scheme based on fracture mechanics analyses for SG tubes with a crack. In this context, closed-form equations were derived to get relevant parameters from three dimensional elastic-plastic finite element analyses and, then, a series of structural integrity analyses were carried out using the predicted J-integrals from the equations. Finally, in comparison with the experimental data as well as corresponding estimation results from conventional limit load solutions, it was proven that the proposed estimation scheme can be used as an efficient tool for integrity evaluation of cracked SG tubes.

THE EFFECT OF POSTULATED FLAWS ON THE STRUCTURAL INTEGRITY OF RPV DURING PTS

Postulation of flaws, one of the most important areas in RPV integrity assessment, significantly affects the results. In the present work, several parameters, such as orientation, underclad vs. surface cracking, crack depth and shape, etc., are postulated and parametric studies are performed to investigate the influence of the flaw parameters on the structural integrity assessment of the reactor pressure vessel during pressurized thermal shock. The influence of individual parameters describing the crack is evaluated based on sensitivity study results.

Fluid Structure Interaction Analysis on Wall Thinned Pipes

The wall thinning due to erosion, corrosion and flow accelerated corrosion is one of critical issues in nuclear industry. To secure against loss of integrity of pipes with a flaw, ASME Code Section III and Code Case N-597 etc have been used in design and operating stages, respectively. However, despite of their inherent conservatisms, it may reach unanticipated accidents due to degradation at local region. In this paper, a new evaluation scheme is suggested to estimate load-carrying capacities of wall thinned pipes. At first, computational fluid dynamics analyses employing steady-state and incompressible flow are carried out to determine pressure distributions in accordance with conveying fluid. Then, the discriminate pressures are applied as input condition of structural finite element analyses to calculate local stresses at the deepest point. A series of combined analyses were performed for different fluid flow velocities as well as d/t, Rm/t and l/t ratios. The efficiency of proposed scheme was proven from comparison with conventional analyses results and it is recommended to consider the fluid structure interaction effect for exact integrity evaluation.

Cleavage Fracture Evaluation Using Local Approach for SA508 Carbon Steel at -60°C

The scatter of measured fracture toughness data and transferability problems for specimens with different crack configurations and loading conditions are major obstacles for integrity assessment of ferritic steels in ductile-brittle transition region. To address these issues, recently, concerns for local approach adopting micro-mechanical damage models are being increased again in connection with a progress of computational technology. In this paper, cleavage fracture evaluation based on Weibull statistics was carried out for SA508 carbon steel. A series of three dimensional finite element analyses as well as corresponding fracture toughness tests were performed for 1T-CT and PCVN specimens at -60°C. Also, failure probability analyses for different configurations and sensitivity analyses for Weibull parameters were conducted. Thereby, promising results have been derived through comparison between measured and estimated fracture toughness data, which can be utilized to make the basis for demonstrating real safety margins of components containing defect.

Probabilistic Fracture Mechanics Round Robin Analysis of Reactor Pressure Vessels during Pressurized Thermal Shock

In this study, round robin problems for the failure probabilities of a reactor pressure vessel are solved using the probabilistic fracture mechanics code. The flaw distribution and flaw density were modified to incorporate the effects of inspection quality. Then, the impact of the inspection quality and other key parameters on the failure probability was quantitatively evaluated. The results showed that the effect of inspection quality on the failure probability has the same characteristics irrespective of the two quite different transients and the wide range of fluence level. Overall, the various inspection qualities considered in this study resulted in about an order of magnitude difference in failure probability. Additionally, it was found that the effect of warm prestressing on the failure probability depends on the characteristics of the transients.

Development of Web-Based Fatigue Life Evaluation System for Reactor Pressure Vessel

While the demand on electric power is consistently increasing, public concerns and regulations for the construction of new nuclear power plants are getting restrict, and also operating nuclear power plants are gradually ageing. For this reason, the interest on lifetime extension for operating nuclear power plants by applying lifetime management system is increasing. The 40-year design life concept was originally introduced on the basis of economic and safety considerations. In other words, it was not determined by technological evaluations. Also, the transient design data which were applied for fatigue damage evaluation were overly conservative in comparison with actual transient data. Therefore, the accumulation of fatigue damage may result in a big difference between the actual data and the design data. The lifetime of nuclear power plants is mostly dependent on the fatigue life of a reactor pressure vessel, and thus, the exact evaluation of fatigue life on a reactor pressure vessel is a crucial factor in determining the extension of operating life. The purpose of this paper is to introduce a real-time fatigue monitoring system for an operating reactor pressure vessel which can be used for the lifetime extension. In order to satisfy the objectives, a web-based transient acquisition system was developed, thereby, real-time thermal-hydraulic data were reserved for 18 operating reactor pressure vessels. A series of finite element analyses was carried out to obtain the stress data due to actual transient. The fatigue life evaluation has been performed based on the stress analysis results and, finally, a web-based fatigue life evaluation system was introduced by combining analysis results and on-line monitoring system. Comparison of the stress analysis results between operating transients and design transients showed a considerable amount of benefits in terms of fatigue life. Therefore, it is anticipated that the developed web-based system can be utilized as an efficient tool for fatigue life estimation of reactor pressure vessel.

Weibull Statistics as a Basis for Assessment of Ductile-Brittle Transition Behavior

The objective of this paper is to investigate failure characteristics of SA508 carbon steel in ductile-brittle transition temperature region. To achieve this goal, a series of finite element analyses as well as fracture toughness tests are performed for pre-cracked V-notch specimens. An assessment of failure probabilities is, then, carried out employing Weibull stress models with different rank probability options. Finally, a prototype of toughness scale diagram is derived through comparison of estimated fracture toughness data with those for compact tension specimens. The present results can be utilized to found a basis of realistic integrity evaluation on major nuclear components containing defect.

Assessment of Geometry Independent Fracture Resistance Characteristics Based on Local Approach

The influences of stress triaxiality on ductile fracture have been emphasized to explain the geometry independent fracture resistance characteristics of specimens and structures during past two decades. For the estimation of this material behavior, two-parameter global approach and local approach can be used as case by case manner. Recently, the interests for the local approach and micro-mechanical damage model are increased again due to progress of computational environments. In this paper, the applicability of the local approach has been assessed through a series of finite element analyses incorporating both modified GTN model and Rousselier model. The ductile crack growth behaviors are examined to guarantee the transferability on different sizes and geometries of C(T) specimens and SE(T) specimens. The material fitting constants are determined from calibration of tensile tests and numerical analyses results, and used to simulate the fracture behaviors of typical specimens. Then, a comparison is drawn between the numerically estimated crack resistance curves and experimentally determined ones. The comparison results show a good agreement and the two damage models are regarded as promising solutions for ductile crack growth simulation.