Sung-Sik Kang

A study on wear coefficients and mechanisms of steam generator tube materials

Reciprocating sliding wear tests were performed to evaluate wear properties of Inconel 600MA and 690TT steam generator (SG) tube materials against 405 and 409 ferritic stainless steels. With increasing normal loads and sliding amplitudes, the wear rate of tube materials increased but a wear transition occurred only in Inconel 690TT. Subsurface deformation strengthening seemed to be an important factor that determines the wear resistance of tube materials. After the wear test, the worn surfaces were observed to investigate the wear mechanism of tube materials using SEM. The results indicated that there are different mechanisms of wear particle removal between the tube materials. The differences are related to the degree of work hardening due to the differences in chromium content in the tube materials. Based on the present results, wear coefficient values for the life estimation of SG tubes were calculated according to the work-rate model at each test condition. The wear rate is lower for Inconel 690TT compared to that for Inconel 600MA. Finally, parameters that should be considered for evaluation of wear coefficients were discussed.

Estimation of fracture toughness transition curves of RPV steels from Charpy impact test data

Abstract The major concern for reactor pressure vessels (RPVs) in terms of integrity is the reduction in fracture toughness of materials due to radiation embrittlement. In order to ensure the structural integrity of RPVs, a very conservative approach has been employed since the first commercial operation of a nuclear power plant (NPP). RT NDT has been used as a principal parameter to indicate the degree of irradiated degradation in RPV material, which is determined using Charpy impact and drop weight tests based on the ASME code requirements. Charpy test is very practical and easy, but it does not provide the fracture toughness itself. Therefore, the Master Curve method, as a direct method to determine the fracture toughness of RPV, was investigated by a number of researchers during the last decade. An alternative approach is proposed in this paper to estimate the reference transition temperature, T 0 , in the Master Curve method using Charpy impact test data, which are abundant for old NPPs. Two well-known correlations between Charpy absorbed energy and K Ic were used to estimate the fracture toughness transition curves.

Structural integrity assessment of pressure tubes for Wolsong Unit 1 based on operational experiences

Abstract Pressure tube integrity has been considered as a key issue since the first operation of the CANDU reactor. Wolsong Unit 1 has been in service since 1983 and subjected to inspections three times covering 44 tubes. The in-service inspections revealed that a major portion of inspected tubes was in contact with calandria tubes. This is likely to increase the probability of blister formation which is a potential threat to pressure tube integrity. Fortunately, the inspection results indicated that no tube has been affected by blister formation so far. Nevertheless, to reduce the undue risk of blister formation the utility has decided to conduct spacer relocation every year until the entire core is covered. On the other hand, LBB analysis of pressure tubes using AGS performance measured at Wolsong Unit 2 indicated that the operational safety margin was marginal when using 15-year operational data. This raises the concern of pressure tube integrity at Wolsong Unit 1 which has a more than 15-year operation. This paper presents the overall integrity evaluation of Wolsong Unit 1 pressure tubes considering AGS performance test results and operating experience data.

The Effect of Welding Residual Stress for Making Artificial Stress Corrosion Crack in the STS 304 Pipe

The stress corrosion crack is one of the fracture phenomena for the major structure components in nuclear power plant. During the operation of a power plant, stress corrosion cracks are initiated and grown especially in dissimilar weldment of primary loop components. In particular, stress corrosion crack usually occurs when the following three factors exist at the same time: susceptible material, corrosive environment, and tensile stress (residual stress included). Thus, residual stress becomes a critical factor for stress corrosion crack when it is difficult to improve the material corrosivity of the components and their environment under operating conditions. In this study, stress corrosion cracks were artificially produced on STS 304 pipe itself by control of welding residual stress. We used the instrumented indentation technique and 3D FEM analysis (using ANSYS 12) to evaluate the residual stress values in the GTAW area. We used the custom-made device for fabricating the stress corrosion crack in the inner STS 304 pipe wall. As the result of both FEM analysis and experiment, the stress corrosion crack was quickly generated and could be reproduced, and it could be controlled by welding residual stress.

Effect of Post-Weld Heat Treatment Temperature on the Mechanical Properties and Microstructure of Simulated Weld Heat-Affected Zone of SA-516 Grade 70 Carbon Steel

원자력발전소 기기의 제작 및 설치 시 강재에 대한 용접은 필수적으로 수행된다. 그러나 용접 공정 중에는 용접부 주변에 형성되는 급격한 온도구배와 용접 구조 물의 구속 조건에 의해 잔류응력이 발생하게 되며, 용접 열에 의해 상변태가 발생하여 용접 열영향부(heat-affected zone, HAZ)의 충격인성이 저하되는 문제가 있다. 이러한 문제를 방지하고 용접 구조물의 건전성을 확보 하기 위해 원전 기기 제작 기술기준인 ASME Section III NX-4620에서는 용접후열처리(post-weld heat treatment, PWHT)를 요구하고 있으며, 실제로 많은 연구를 통해 PWHT가 용접 잔류응력을 완화시키고 HAZ의 인성을 향상시키는 효과가 있다고 보고된 바 있다. 용접 HAZ의 기계적 특성은 PWHT 온도에 따라 크 게 달라질 수 있기 때문에 적절한 PWHT 조건을 적용 하는 것이 매우 중요하다. 따라서 ASME Section III Table NX-4622.1-1에서는 화학조성별로 모재를 SA-516 Grade 70 탄소강 재현 용접 열 영향부의 기계적 특성과 미세조직에 미치는 용접후열처리 온도의 영향

FEM Model-Based Investigation of Ultrasonic TOFD for Notch Inspection

A two-dimensional numerical model based on the finite element method was built to simulate the wave propagation phenomena that occur during the ultrasonic time of flight diffraction (TOFD) process. First, longitudinal-wave TOFD was simulated, and the numerical results agreed well with the theoretical results. Shear-wave TOFD was also investigated because shear waves have higher intensity and resolution. The shear wave propagation was studied using three models with different boundary conditions, and the tip-diffracted shear-tolongitudinal wave was extracted from the A-scan signal difference between the cracked and non-cracked specimens. This signal showed very good agreement between the geometrical and numerical arrival times. The results of this study not only provide better understanding of the diffraction phenomena in TOFD, but also prove the potential of shear-wave TOFD for practical application.