Papers by Author: Sang Min Lee

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: 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: 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.

Abstract: Risk-based inspection (RBI) guideline based on API 581 provides a methodology for calculating the risks of equipment in refinery or petrochemical plant. However, there is a major limitation of its application to the petrochemical plant directly since only a representative material is considered in calculating the risk, especially in part of the consequence of failure, even though the equipment is composed of numerous materials. The objectives of this paper are to develop an enhanced RBI program to resolve shortcoming inclusive of the above issue and to evaluate the risks of equipment in petrochemical plant using the program. In this respect, the mole fractions of materials were used to fully incorporate the characteristics of different materials. The proposed RBI program consists of qualitative, semi-quantitative and quantitative risk evaluation modules in which toxic materials as well as representative materials were selected automatically for comparison with those in the current guideline. The RBI program has been applied to evaluate the risks of equipment in Naphtha Cracking Center (NCC) which is a typical facility of petrochemical plant. Thereby, promising evaluation results were obtained and applicability of the proposed RBI program was proven.

Abstract: The integrity of major components in nuclear power plant should be maintained during operation. In order to maintain the integrity of these components, complicated assessment procedures are required including fracture mechanics analysis, etc. The integrity assessment of components has been performed by using conventional deterministic approaches whilst there are lots of uncertainties to carry out a rational evaluation. In this respect, probabilistic integrity assessment is considered as an alternative method for nuclear component evaluation. The objectives of this paper are to develop an integrity assessment system based on probabilistic fracture mechanics and to estimate the failure probability of major nuclear components containing a defect. The integrity assessment system consists of three evaluation modules which are first order reliability method, second order reliability method and crude Monte Carlo simulation method. The developed system has been applied to evaluate failure probabilities of nuclear structural components such as steam generator tube and piping. The evaluation results showed a promising applicability of the probabilistic integrity assessment system.