Young-Hwan Choi

A COUPLED CFD-FEM ANALYSIS ON THE SAFETY INJECTION PIPING SUBJECTED TO THERMAL STRATIFICATION

Thermal stratification has continuously caused several piping failures in nuclear power plants since the early 1980s. However, this critical thermal effect was not considered when the old nuclear power plants were designed. Therefore, it is urgent to evaluate this unexpected thermal effect on the structural integrity of piping systems. In this paper, the thermal effects of stratified flow in two different safety injection piping systems were investigated by using a coupled CFD-FE method. Since stratified flow is generally generated by turbulent penetration and/or valve leakage, thermal stress analyses as well as CFD analyses were carried out considering these two primary causes. Numerical results show that the most critical factor governing thermal stratification is valve leakage and that temperature distribution significantly changes according to the leakage path. In particular, in-leakage has a high possibility of causing considerable structural problems in RCS piping.

On Relevant Ramberg-Osgood Fit to Engineering Non-Linear Fracture Mechanics Analysis

This paper proposes a robust method for the Ramberg-Osgood(R-O)fit to accurately estimate elastic-plastic J from engineering fracture mechanics analysis based on deformation plasticity. The proposal is based on engineering stress-strain data to determine the R-O parameters, instead of true stress-strain data. Moreover, for practical applications, the method is given not only for the case when full stress-strain data are available but also for the case when only yield and tensile strengths are available. Reliability of the proposed method for the R-O fit is validated against detailed 3-D Finite Element (FE) analyses for circumferential through-wall cracked pipes under global bending using five different materials, three stainless steels and two ferritic steels. Taking the FE J results based on incremental plasticity using actual stress-strain data as reference, the FE J results based on deformation plasticity using various R-O fits are compared with reference J values. Comparisons show that the proposed R-O fit provides more accurate J values for all cases, compared to existing methods for the R-O fit. Advantages of the proposed R-O fit in practical applications are discussed, together with its accuracy.

FATIGUE LIFE ASSESSMENT OF REACTOR COOLANT SYSTEM COMPONENTS BY USING TRANSFER FUNCTIONS OF INTEGRATED FE MODEL

Recently, efficient operation and practical management of power plants have become important issues in the nuclear industry. In particular, typical aging parameters such as stress and cumulative usage factor should be determined accurately for continued operation of a nuclear power plant beyond design life. However, most of the major components have been designed via conservative codes based on a 2-D concept, which do not take into account exact boundary conditions and asymmetric geometries. The present paper aims to suggest an effective fatigue evaluation methodology that uses a prototype of the integrated model and its transfer functions. The validity of the integrated 3-D Finite Element (FE) model was proven by comparing the analysis results of individual FE models. Also, mechanical and thermal transfer functions, known as Green’s functions, were developed for the integrated model with the standard step input. Finally, the stresses estimated from the transfer functions were compared with those obtained from detailed 3-D FE analyses results at critical locations of the major components. The usefulness of the proposed fatigue evaluation methodology can be maximized by combining it with an on-line monitoring system, and this combination, will enhance the continued operations of old nuclear power plants.

Model Predictions to the 2004 Ultrasonic Benchmark Problems

The 2004 ultrasonic benchmark problems addressed by World Federation of NDE Centers (WFNDEC) are solved by using ultrasonic measurement models. To predict the beam fields generated by ultrasonic transducers, we adopt two beam models (the multi‐Gaussian beam model (MGB) and the generalized Rayleigh‐Sommerfeld integral (GRSI)) and compare their predictions to the given problems. And, the scattering fields for three types of flaws (side‐drilled holes, flat‐bottom holes, and spherical voids) are calculated by using the Kirchhoff approximation. In this paper, we summarize the model predictions obtained by Sungkyunkwan University, Suwon, Korea.

Creep crack initiation and propagation in type 304 stainless steel at 873 K

Abstract Creep fracture tests were performed on type 304 stainless steel using compact tension type specimens at 873 K in an air environment. Both the creep crack initiation time and the creep crack growth rate were measured as functions of various fracture parameters such as the stress intensity factor, the net section stress, the C ∗ integral and the crack-tip-opening displacement. It is shown that it is possible to correlate the creep crack initiation time with the C ∗ integral; however, the results obtained from experiments are not in agreement with those predicted from the creep crack initiation model based on the critical strain failure criterion. The reasons for this discrepancy are discussed. It is also shown that the creep crack growth rate can be correlated with the C ∗ integral.

Plastic Limit Loads for Slanted Through-Wall Cracks in Cylinder and Plate Based on Finite Element Limit Analyses

The plastic limit load solutions for cylinder and plate with slanted through-wall cracks (TWCs) are developed based on the systematic three-dimensional (3D) finite element (FE) limit analyses. As for loading conditions, axial tension, global bending, and internal pressure are considered for a cylinder with slanted circumferential TWC, whereas, axial tension and internal pressure are considered for a plate and a cylinder with slanted axial TWC. Then, the verification of FE model and analysis procedure employed in the present numerical work was confirmed by employing the existing solutions for both cylinder and plate with idealized TWC. Also, the geometric variables of slanted TWC which can affect plastic limit loads were considered. Based on the systematic FE limit analysis results, the slant correction factors which represent the effect of slanted TWC on plastic limit load were provided as tabulated solutions. By adopting these slant correction factors, the plastic limit loads of slanted TWC can be directly estimated from existing solutions for idealized TWC. Furthermore, the modified engineering estimations of plastic limit loads for slanted TWC are proposed based on equilibrium equation and von Mises yield criterion. The present results can be applied either to diverse structural integrity assessments or for accurate estimation of fracture mechanics parameters such as J-integral, plastic crack opening displacement (COD) and C*-integral for slanted TWC based on the reference stress concept (Kim, et al., 2002, “Plastic Limit Pressure for Cracked Pipes Using Finite Element Limit Analyse,” Int. J. Pressure Vessels Piping, 79, pp. 321–330; Kim, et al., 2001, “Enhanced Reference Stress-Based J and Crack Opening Displacement Estimation Method for Leak-Before-Break Analysis and Comparison With GE/EPRI Method,” Fatigue Fract. Eng. Mater. Struct., 24, pp. 243–254; Kim, et al., 2002, “Non-Linear Fracture Mechanics Analyses of Part Circumferential Surface Cracked Pipes,” Int. J. Fract., 116, pp. 347–375.)

Prediction of Angle Beam Ultrasonic Testing Signals from a Surface Breaking Crack in a Plate Using Multi‐Gaussian Beams and Ray Methods

This paper presents a modeling approach to predict the angle beam ultrasonic pulse‐echo signals that can be captured from a surface breaking, vertical crack in a plate specimen in a computationally efficient manner. For this purpose, the 3‐D multi‐Gaussian beam models are adopted to describe the reflected beam fields from the surfaces of the crack and the specimen as well as the radiating beam field from the transducer, and the 2‐D ray methods to calculate the diffracted beam field from the crack tip. In addition, the characteristics of the ultrasonic testing system are considered in terms of the system efficiency factor. By combining these three ingredients, the surface breaking crack signals are predicted at different interrogating positions. The accuracy of the proposed models is verified by the initial experiments.

Estimates of Plastic J-Integral and Crack Opening Displacement Considering Circumferential Transition Crack From Surface to Through-Wall Crack in Cylinder

The present paper provides the enhanced estimation of plastic J-integral and crack opening displacement (COD) for circumferential transition crack from surface to through-wall crack in cylinder based on detailed finite element analysis. The effects of circumferential transition crack on plastic J-integral and crack opening displacement have been systematically investigated for practical ranges of cylinder geometries and materials of interest. Then, the plastic influence functions (h1, h2) employed in the GE/EPRI method have been proposed to quantify those effects on plastic J-integral and COD. Furthermore, the J-integral and COD estimations based on the reference stress method using optimized reference load have also been introduced for circumferential non-idealized TWC in cylinder. Then, in order to gain the confidence in the proposed methods, the results from those proposed estimates were compared with elastic-plastic FE results by using actual stress-strain data and Ramberg-Osgood fit constants for TP 316 stainless steel. The present results can be expected to apply on various structure integrity assessments and to accurate plastic J-integral and COD estimation for circumferential non-idealized TWC in cylinder.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

Evaluation of Plastic Collapse Pressure for Steam Generator Tube with Non-Aligned Two Axial Through-Wall Cracks

The of wall thickness criterion which has been used as a plugging rule is applicable only to a single cracked steam generator tubes. In the previous studies performed by authors, several failure prediction models were introduced to estimate the plastic collapse pressures of steam generator tubes containing collinear or parallel two adjacent axial through-wall cracks. The objective of this study is to examine the failure prediction models and propose optimum ones for non-aligned two axial through-wall cracks in steam generator tubes. In order to determine the optimum ones, a series of plastic collapse tests and finite element analyses were carried out for steam generator tubes with two machined non-aligned axial through-wall cracks. Thereby, either the plastic zone contact model or COD based model was selected as the optimum one according to axial distance between two clacks. Finally, the optimum failure prediction model was used to demonstrate the conservatism of flaw characterization rules for various multiple flaws according to ASME code.