Juyun Choi

Development of limit load solutions for corroded gas pipelines

Abstract Pipelines have the highest capacity and are the safest and the least environmentally disruptive means for gas or oil transmission. Recently, failures due to corrosion defects have become of major concern in maintaining pipeline integrity. A number of solutions have been developed for the assessment of remaining strength of corroded pipelines. However, these solutions are known to be dependent on material properties and pipeline geometries. In this paper, a fitness-for-purpose (FFP) type limit load solution for corroded gas pipelines made of X65 steel is proposed. For this purpose, a series of burst tests with various types of machined pits are performed. Finite element simulations are carried out to derive an appropriate failure criterion. Then, further, extensive finite element analyses are performed to obtain the FFP type limit load solution for corroded X65 gas pipelines as a function of defect depth, length and pipeline geometry.

Change of the trap energy levels of the atomic layer deposited HfLaOx films with different La concentration

Ultrathin HfO2 and HfLaOx films with La/(Hf+La) ratios of 42%, 57%, and 64% were synthesized with an atomic layer deposition process. By measuring the leakage current at different temperatures, the conduction mechanism of HfO2 and HfLaOx films was shown to follow the Poole–Frenkel emission model under a gate injection condition. Based on the temperature and field-dependence measurements, the intrinsic trap energy levels were found to be 1.42, 1.34, 1.03, and 0.98 eV for the HfLaOx samples with La/(Hf+La) ratios of 0%, 42%, 57%, and 64%, respectively, showing a decreasing behavior as the La content increased.

Effect of Y, Gd, Dy, and Ce Doping on the Microstructural and Electrical Properties of Sol-Gel-Deposited ZrO2 Film

ZrO 2 films doped with Y, Gd, Dy, and Ce at a concentration of ∼13 cation atom % were deposited by a sol-gel technique, and the early-stage dopant effect on the stabilization of the high temperature phase and dielectric properties was systematically compared after a low-temperature annealing process (400°C). The high temperature phase formation of the ZrO 2 films was hindered by the Y, Gd, and Dy doping, thereby reducing the dielectric constant. In addition, the hysteresis was significantly increased via electron trapping, which was attributed to the increase in the number of oxygen vacancies possibly due to the difference in the valence number of the dopants with that of the substituted Zr atoms. However, the dielectric constant of the ZrO 2 film doped with tetravalent Ce atoms increased without retarding the stabilization of the high temperature phase and degrading the hysteresis characteristics.

A parametric study on pressure–temperature limit curve using 3-D finite element analyses

Abstract In order to operate a reactor pressure vessel (RPV) safely, it is necessary to keep the pressure–temperature ( P – T ) limit during the heatup and cooldown process. While the ASME Code provides the P – T limit curve for safe operation, this limit curve has been prepared under conservative assumptions. In this paper, the effects of conservative assumptions involved in the P – T limit curve specified in the ASME Code Sec. XI were investigated. Three different parameters, the crack depth, the cladding thickness and the cooling rate, were reviewed based on 3-D finite element analyses. Also, the constraint effect on P – T limit curve generation was investigated based on J – T approach. It was shown that the crack depth and constraint effect change the safety region in P – T limit curve dramatically, and thus it is recommended to prepare a more precise P – T limit curve based on finite element analysis to obtain P – T limit for safe operation of a RPV.

Effect of cladding on the stress intensity factors in the reactor pressure vessel

Abstract In general, reactor pressure vessels (RPV) are cladded with stainless steel to prevent corrosion and radiation embrittlement. The ASME Sec. XI specifies that a subclad crack which may be found during the in-service inspection must be considered as a semi-elliptical surface crack when the thickness of cladding is less than 40% of the crack depth. In order to refine the fracture assessment procedures for such subclad cracks, three-dimensional finite element analyses were applied for various subclad cracks embedded in the base metal. A total of 18 crack geometries were analyzed, and the results were compared with those for idealized semi-elliptical surface cracks for two different loading conditions, i.e. internal pressure and pressurized thermal shock. The resulting stress intensity factors for subclad cracks were 50–70% less than those for idealized surface cracks. It has been proven that the condition specified on the ASME Sec. XI is overly conservative for subclad cracks which are assumed to be surface cracks.

Defect-free erbium silicide formation using an ultrathin Ni interlayer.

An ultrathin Ni interlayer (∼1 nm) was introduced between a TaN-capped Er film and a Si substrate to prevent the formation of surface defects during thermal Er silicidation. A nickel silicide interfacial layer formed at low temperatures and incurred uniform nucleation and the growth of a subsequently formed erbium silicide film, effectively inhibiting the generation of recessed-type surface defects and improving the surface roughness. As a side effect, the complete transformation of Er to erbium silicide was somewhat delayed, and the electrical contact property at low annealing temperatures was dominated by the nickel silicide phase with a high Schottky barrier height. After high-temperature annealing, the early-formed interfacial layer interacted with the growing erbium silicide, presumably forming an erbium silicide-rich Er-Si-Ni mixture. As a result, the electrical contact property reverted to that of the low-resistive erbium silicide/Si contact case, which warrants a promising source/drain contact application for future high-performance metal-oxide-semiconductor field-effect transistors.

Silicide formation process of Er films with Ta and TaN capping layers.

The phase development and defect formation during the silicidation reaction of sputter-deposited Er films on Si with ∼20-nm-thick Ta and TaN capping layers were examined. TaN capping effectively prevented the oxygen incorporation from the annealing atmosphere, which resulted in complete conversion to the ErSi2-x phase. However, significant oxygen penetration through the Ta capping layer inhibited the ErSi2-x formation, and incurred the growth of several Er-Si-O phases, even consuming the ErSi2-x layer formed earlier. Both samples produced a number of small recessed defects at an early silicidation stage. However, large rectangular or square-shaped surface defects, which were either pitlike or pyramidal depending on the capping layer identity, were developed as the annealing temperature increased. The origin of different defect generation mechanisms was suggested based on the capping layer-dependent silicidation kinetics.

Assessment of Local Wall Thinned Pipeline Under Combined Bending and Pressure

Failure of a pipeline due to local wall thinning is getting more attention in the nuclear power plant industry. Although guidelines such as ANSI/ASME B31G are still useful for assessing the integrity of a wall thinned pipeline, there are some limitations in these guidelines. For instance, these guidelines consider only pressure loading and thus neglect bending loading. However, most pipelines in nuclear power plants are subjected to internal pressure and bending moment due to dead-weight loads and seismic loads. Therefore, an assessment procedure for locally wall thinned pipeline subjected to combined loading is needed. In this paper, three-dimensional finite element (FE) analyses were performed to simulate full-scale pipe tests conducted for various shapes of wall thinned area under internal pressure and bending moment. Maximum moments based on ultimate stress (σu ) were obtained from FE results to predict the failure of the pipe. These results were compared with test results, which showed good agreement. Additional finite element analyses were performed to investigate the effect of key parameters, such as wall thinned depth, wall thinned angle and wall thinned length, on maximum moment. Also, the effect of internal pressure on maximum moment was investigated. Change of internal pressure did not show significant effect oll the maximum moment.Copyright

Development of a Three-Dimensional Green’s Function and Its Application to the Fatigue Evaluation of Reactor Pressure Vessel

The design of major nuclear components for the prevention of fatigue failure has been achieved on the basis of ASME codes, which are usually very conservative. However, it is necessary to make it more accurate for the continued operation beyond the design life. In this paper, 3-dimensional stress and fatigue analyses reflecting entire geometry have been carried out. The number of operating transient data obtained from a monitoring system were filtered and analyzed. Then, Green’s function which transfers temperature gradient into the corresponding thermal stress is proposed and applied to critical locations of a reactor pressure vessel. The validity of proposed Green’s function is approved by comparing the result with corresponding 3-D finite element analysis results. Also, the amount of conservatism included in design transients in comparison with real transients is analyzed. The results for 3-D finite element analysis are also compared with corresponding 2-D finite element analysis results, and a considerable amount of difference was observed in terms of fatigue life. Therefore, it is expected that the proposed evaluation scheme adopting real operating data and Green’s function can provide more accurate fatigue life evaluation for a reactor pressure vessel.Copyright

Application of a Knowledge-Based Information System for Life Extension of Steel Making Plant

In this paper, a knowledge-based information system for the plant operation of steel making company has been proposed. This system, which is named as K-VRS (Knowledge-based Virtual Reality System), provides a connection between ERP PM module and knowledge-based engineering methodologies, and thus, enables network-based highly effective PM process. While the virtual plant is used for the master of K-VRS, there are four expert modules attached; engineering document management module, real time lifetime estimation module, fitness for service module and risk-based inspection module. Each module provides engineering knowledge based evaluation and judgment for more effective PM. K-VRS provides collaborative and concurrent working environment to workers and relevant experts. The developed system is expected to play a great role for plant life extension.Copyright