Papers by Author: Yoon Suk Chang

Abstract: A piping system including straight pipes, elbows and tee branches in a nuclear power plant is mostly subjected to severe loading conditions with high temperature and pressure. In particular, the wall-thinning of an elbow due to flow accelerated corrosion is one of safety issues in the nuclear industry. In this respect, it is necessary to investigate the limit loads of an elbow with a wall-thinned part for evaluating integrity. In this paper, three dimensional plastic limit analyses are performed to obtain limit loads of an elbow with different bend angles as well as defect geometries under internal pressure and in-plane/out-of-plane bending moment. The limit loads are also compared with the results from limit load solutions of an uninjured elbow based on the von Mises yield criteria. Finally, the effects of significant factors, bend angle and defect shape, are quantified to estimate the exact load carrying capacity of an elbow during operation.

Abstract: The purpose of this paper is to investigate fluid characteristics and flat panel display behaviours on contact-free flat panel display handler nozzles. Since air force distributions streaming bottom surface of a flat panel display highly dependent on operating as well as design conditions and configuration of contact-free handler nozzle, influences of various parameters such as flow rate, supply air pressure, floating height are examined through a series of computational fluid dynamics analyses. Moreover, dynamic finite element analyses of the flat panel display are carried out to assure that an oscillation effect caused by disturbances is not significant. Key findings from the both computational fluid dynamics and structural integrity analysis results are presented and discussed, which can be figured out an optimized operating condition of contact-free handler nozzles.

Abstract: In the ASME Code Section III ‘design by analysis’ approach, stresses are determined by numerical method and compared with corresponding stress limits. This approach provides several stress criteria for fatigue life assessment and procedures for categorizing the representative stress components. Since the stress criteria were derived from two-dimensional basis, however, it may inappropriate to delineate structural components with complex geometry. In this paper, detailed transient analyses are performed for modular pressurizer with an asymmetric geometry, which includes perforated parts to mount various piping and equipments. Also, the applicability of an effective elastic modulus to consider the perforation and the appropriateness of stress linearization method using stress classification line are assessed. Then, the cumulative usage factor as well as stress intensities at critical locations of the pressurizer are calculated and compared with corresponding allowable design stress limits. The key findings of this work can be used to make regulatory guides for evaluation and confirmation of structural intensity of components with asymmetric perforated parts.

Abstract: Goal setting safety legislation for high pressure systems was first introduced in 1973 and retained in the High Pressure Safety Management Law in Korea. This has enabled a move towards inspection strategies based on the risk of failure. This trend towards a risk based approach is being supported by extensive plant operating experience, improved understanding of material degradation mechanisms, and the availability of fitness-for-service assessment procedures. RBI (Risk-Based Inspection) is very effective and efficient of the inspection and maintenance techniques. In particular, RBI will be found how to apply both inspection interval as a result of RBI implementation and re-inspection interval of pressurized equipment defined by the High Pressure Safety Management Law in Korea. The objectives of this paper are to develop an enhanced KGS-RBITM program to resolve shortcoming inclusive of the above issue and to evaluate the risks of equipments in Naphtha Cracking Center (NCC) which is a typical facility of petrochemical plant using the KGS-RBITM program. The results of the risk assessment and re-inspection interval of internal opening using KGS-RBITM program are useful in determining the inspection planning.

Abstract: The purpose of this paper is to investigate principles of levitation and restitution of blowing nozzle prior to fabricating a prototype of air handling system. Since air force distributions streaming bottom surface of a flat panel display (FPD) highly dependent on operating as well as design condition and configuration of air handling system, influences of various parameters such as flow rate, supply air pressure, floating height and tilted angle are examined through a series of computational fluid dynamics (CFD) analyses. Moreover, dynamic finite element analyses of the FPD are carried out to assure that an oscillation effect caused by disturbances is not significant. Key findings from the both CFD and structural analysis results are presented and discussed, which can be utilized as technical bases for development of the practical air handling system.

Abstract: The present work deals with an applicability of the local approach to assess in-plane size effects among different sized compact tension (CT) specimens. To characterize ductile crack growth of typical nuclear materials, SA515 Gr.60 and SA516 Gr.70 carbon steels, finite element analyses employing modified GTN and Rousselier models as well as fracture toughness tests were carried out. Material damage parameters were calibrated using standard CT specimens and reflected to predict fracture resistance (J-R) curves of larger CT specimens. Since comparison results between numerically estimated J-R curves and experimentally determined ones corresponded well, it is anticipated that the local approach might be used as a promising tool for ductile fracture evaluation incorporating the in-plane size effect.

Abstract: Steam generator in a nuclear power plant is huge heat exchanger that transfers heat from reactor to make steam to drive turbine-generator. Failure of the steam generator tubes can result in the release of fission products to the secondary side. Therefore, accurate integrity assessment of the cracked steam generator tubes is of great importance for maintaining the safety of the nuclear power plant. This paper provides limit loads for circumferential through-wall cracks in steam generator tubes under combined internal pressure and bending loads. Such limit loads are developed on the basis of three dimensional finite element analyses assuming elastic-perfectly plastic material behavior. As for the crack location, both the top of the tubesheet and U-bend regions are considered. The analysis results can be directly applied to the practical integrity assessment of cracked steam generator tubes, because the comparison between experimental data and FE results shows a very good agreement.

Abstract: Fracture toughness data from the cleavage resistance test of structural steels often show a large scatter. Geometry dependency as well as the scatter makes it difficult to evaluate appropriate fracture integrity of cracked components. To address these restrictions, several stochastic models have been proposed by Beremin group, Mudry and other researchers while each of them employs specific estimation scheme and micro-mechanical parameters. The purpose of this paper is to investigate applicability of the Weibull stress model in transition temperature regime and to quantify constraint effect among different-sized CT and PCVN specimens. The constituting parameters m and σu are determined at three temperatures by maximum likelihood estimate (MLE) technique in use of FE analysis results and experimental data of PCVN specimens. Also, failure probabilities of PCVN and CT specimens are calculated from the Weibull parameters, which are used for derivation of a prototype of toughness scale diagram. The diagram provides a technical basis to resolve transferability issue in the same material under different temperatures and constraint conditions.

Abstract: Steam generators working in nuclear power plants convert water into steam from heat produced in the reactor core and each of them contains from 3,000 to 16,000 tubes. Since these tubes constitute one of primary barriers under radioactive and high pressure condition, the integrity should be maintained carefully during the operation. The objective of this research is to introduce an integrity evaluation system for steam generator tubes as a substitute of well-trained engineers or experts. For this purpose, a couplet examination has been carried out on the complicated evaluation procedure and an efficient system named as STiES was developed employing three representative integrity evaluation methods: fracture mechanics analysis (crack driving force diagram and J-integral/Tearing modulus method) and limit load method. Exemplary analyses for steam generator tubes with various types of flaws showed good applicability of the proposed integrity evaluation system. So, it is anticipated that the system can be used for the calculation of reference pressure to decide either the continued operation or repair until next outage.

Abstract: In this paper, conservatisms of current plugging criteria on steam generator tubes are reviewed and six new failure prediction models for dual through-wall cracks are proposed. In order to determine the optimum ones among these local or global failure prediction models, a series of plastic collapse tests and corresponding finite element analyses are carried out with respect to two adjacent axial through-wall cracks in thin plates. Then, reaction force model, plastic zone contact model and COD (Crack Opening Displacement) base model were selected as the optimum ones for integrity assessment of steam generator tubes with dual cracks.