Seong-Sik Hwang

Degradation of Alloy 600 Steam Generator Tubes in Operating Pressurized Water Reactor Nuclear Power Plants

Abstract The Korea Atomic Energy Research Institute (KAERI) and some foreign laboratories have carried out six examinations of pulled tubes from Korean nuclear power plants, since the first commercial operation of Kori Unit 1 in 1978. A total of 15 tubes were extracted that exhibited different types of failures such as pitting, outer diameter stress corrosion cracking (ODSCC), primary water stress corrosion cracking (PWSCC), intergranular attack (IGA), etc. The pitting of the Plant A tube was related to high copper dissolved from the condenser material, and the presence of Cl− and high dissolved oxygen. Transgranular SCC observed at Plant B seemed to be related to lead compounds. ODSCC and IGA in Plant A were deduced to be connected to the formation of an aggressive environment in the crevices. PWSCC in Plant A and Plant C originated from the inherent characteristics of the materials, which were not properly thermally treated. After the failure analysis, the performance of nondestructive testing was evalu...

Effect of Local Strain Distribution on Stress Corrosion Cracking of Cold-Rolled Alloy 690 With Inhomogeneous Microstructure

This article undertakes an investigation of the stress corrosion crack growth behavior of a cold-rolled Alloy 690 (UNS N06690) with microstructural inhomogeneity in the primary water of a pressurized water reactor, and ascertains the relationship between this behavior and the local distribution of the residual strain. Stress corrosion cracking (SCC) was found to propagate in a transgranular mode through a banded region composed of intragranular carbides and small-sized grains, and in an inter-granular mode through a normal matrix region of normal-sized grains, with an abnormally high growth rate when aligning the cold-rolling and crack growth directions with the intragranular carbide bands. The Vickers hardness and the degree of misorientation increased as plastic deformation as a result of cold-rolling proceeded. Moreover, these parameters were consistently higher in the banded region than in the matrix region. From an analysis of the residual strain, it was suggested that higher residual strain near int...

Caustic stress corrosion cracking of alloys 600 and 690 with NaOH concentrations

In order to evaluate the stress corrosion cracking resistance for commercial alloys (C600MA, C600TT, C690TT) and Korean-made alloys (K600MA, K690TT), C-ring tests were performed in a caustic environment of 4, 10, 20, 30, and 50% NaOH solution at 315°C, for 480 h with an applied potential of 125 mV vs. OCP. Different stress corrosion cracking phenomena were observed according to the NaOH concentration. The rate of caustic IGSCC attack did not appear to increase monotonically with caustic concentrations, but peaked at a concentration between 4 and 50% caustic, or approximately 30% NaOH. Intergranular stress corrosion cracking was found for C600MA in 10, 20, and 30% NaOH solutions, while no cracking was observed in the 4 and 50% NaOH solutions. In 30% NaOH solution, transgrnular stress corrosion cracking was detected in C690TT, which may be related with the large amount of plastic strain (150% yield) and the applied potential (125 mV vs. OCP). The overall data clearly indicate that C600MA has the worst SCC resistance while K690TT offers the best resistance. There is also fairly good correlation between the caustic SCC susceptibility and some metallurgical parameters, particularly the grain size and the yield strength at room temperature. Specifically, materials having larger grain size and lower yield strength exhibited higher caustic SCC resistance.

Effect of microstructure on stress corrosion cracking of alloy 600 and alloy 690 in 40% NaOH

Stress corrosion cracking (SCC) behaviors of Alloy 600, Alloy 690 and the Ni-10Cr-10Fe alloy have been studied using a C-ring in 40% NaOH solution at 315°C. The current density of Alloy 690 in polarization curves was higher at 200 mV above corrosion potential than that of Alloy 600. SCC resistance increased with Cr content for the chromium carbide free alloys, probably due to facilitation of SCC crack tip blunting with an increase in Cr content. Both thermally treated Alloy 600 and sensitized Alloy 600 have a comparable amount of intergranular carbide. But the former is more resistant to SCC than the latter, which might be attributed to the presence of the slight Cr depletion around the grain boundary in the former one. Sensitized Alloy 600 showed higher SCC resistance than the solution annealed one due to intergranular carbide in sensitized Alloy 600. This implies that the beneficial effect of intergranular carbide overrides the harmful effects of Cr depletion for sensitized Alloy 600. SCC resistance of Alloy 600 increased with grain size.

Ex situ and in situ characterization of stress corrosion cracking of nickel-base alloys at high temperature

Recent ex situ and in situ characterization of the major factors influencing primary water stress corrosion cracking (PWSCC) and outside diameter stress corrosion cracking (ODSCC) of nickel-base alloys of Alloy 600 and Alloy 690 as the structural materials in pressurized water reactors (PWRs) was introduced to understand cracking mechanisms. In the primary water environment of PWRs, the effect of stress on PWSCC was analyzed using the in situ direct-current potential drop method combined with electrochemical noise measurement. The compositional and microstructural changes around a crack tip including the oxide film formed during crack propagation were evaluated by using ex situ microscopic methods with energy-dispersive spectroscopy (EDS). The equivalent local strain was also evaluated by using the electron backscatter diffraction method as ex situ technique. With the aid of ex situ and in situ characterization, it was proposed that the intergranular precipitation of Cr oxides ahead of the crack tip by O penetration along the grain boundary that make the “oxidized grain boundary” can significantly increase the susceptibility to PWSCC propagation of the alloys. In the secondary water environment of PWRs, the properties of oxide film formed on the surface of the alloys were characterized as a function of the Pb content as impurity by using in situ methods of potentiodynamic polarization measurement and electrochemical impedance spectroscopy and ex situ techniques of EDS and X-ray photoelectron spectroscopy. From these ex situ and in situ characterizations, the degradation model of the passivity of the surface oxide film by Pb incorporation was proposed to understand the ODSCC mechanism.

Structural Integrity Estimates of Steam Generator Tubes Containing Wear-Type Defects

It is requested that steam generator tubes with defects exceeding 40% of wall thickness in depth should be plugged to sustain all postulated loads with appropriate margin. This critical defect size has been determined based on a concept of plastic instability, however, which is known to be too conservative for some locations and types of defects. The application of this concept may even cause premature retirement of steam generator tubes. In reality, a reliable structural integrity estimation for steam generator tubes containing a defect has received increasing attention. Although several guidelines have been developed and used for assessing defect containing tubes, most of these guidelines are focused on stress corrosion cracking or wall-thinning phenomena. Because some of steam generator tubes fail due to fretting and so on, specific integrity estimation schemes for relevant defects are required. In this paper, more than a hundred three-dimensional finite element analyses of steam generator tubes under internal pressure condition are carried out to simulate the failure behavior of steam generator tubes with specific defect configurations: elliptical wear-type, tapered wedge-type, and flat wear-type defects. After investigating the effect of key parameters such as defect depth, defect length, and wrap or tapered angle on equivalent stress across the ligament thickness, burst pressure estimation equations are proposed in relation to material strengths. Predicted burst pressures agreeded well with the corresponding experimental data, so the proposed equations can be used to assess the structural integrity of steam generator tubes with wear-type defects.

Comparison of Experimental and Numerical Analysis Data for BMI Mock-Up With Dissimilar Metal Welds

During the last decade, several defects due to primary water stress corrosion cracking (PWSCC) have been reported at bottom-mounted instrumentation (BMI) and control rod drive mechanism nozzles. The exact locations were dissimilar metal weld parts which are greatly important because the cracking could lead to leakage of primary coolant. The PWSCC of BMI mock-up’s penetration with dissimilar metal welds was examined by using doped steam test method by Korea Atomic Energy Research Institute. In this work, numerical analyses are performed for the same environment condition with the doped steam test. With respect to the numerical analyses, heat transfer analyses are carried out based on thermal conduction. The welding paths are simulated by using lumped path method for conservative evaluation and model change (remove/rebirth) method. Then residual stress analyses are conducted using the heat transfer analysis results, in which annealing effect of welding process simulation is considered for resetting the plastic deformation. However, the plastic behaviour of steels during phase transformations is not considered with experimental data. In addition, the consequence of weld residual stress that is known as the cause of PWSCC is being investigated.© 2008 ASME

PWSCC Assessment of Dissimilar Weld on Steam Generator Drain Nozzle in PWR

Recently, it is reported that axial and circumferential PWSCCs (primary water stress corrosion crackings) occurred on the dissimilar welds of steam generator drain nozzles in PWR. In this study, the PWSCC is assessed via residual stress analysis, PWSCC initiation review and growth assessment. The residual and operating stress distributions are calculated by finite element method. Based on the stress analysis results, PWSCC initiation locations are predicted by comparing between the stresses and PWSCC threshold stress value. The predicted locations are compared with the examination results by using penetration, eddy current and destructive tests. It is found that the PWSCC initiation prediction has good agreement with the examination results. Also, PWSCC growth is assessed by using the PWSCC growth analysis procedure presented in ASME B&PV Code, Sec.XI. Finally, it is identified that the penetration time via growth assessment has reasonable agreement with the real one.Copyright

Structural Integrity Evaluation of SG Tube with Surface Wear-type Defects

During the last two decades, several guidelines have been developed and used for assessing the integrity of a defective steam generator (SG) tube that is generally caused by stress corrosion cracking or wall-thinning phenomenon. However, as some of SG tubes are also failed due to fretting and so on, alternative failure estimation schemes are required for relevant defects. In this paper, parametric three-dimensional finite element (FE) analyses are carried out under internal pressure condition to simulate the failure behavior of SG tubes with different defect configurations; elliptical wear, tapered and flat wear type defects. Maximum pressures based on material strengths are obtained from more than a hundred FE results to predict the failure of SG tube. After investigating the effect of key parameters such as defect depth, defect length and wrap angle, simplified failure estimation equations are proposed in relation to the equivalent stress at the deepest point in wear region. Comparison of failure pressures predicted by the proposed estimation scheme with corresponding burst test data showed a good agreement.

Evaluation of Plastic Collapse Behavior for Multiple Cracked Structures

Until now, the 40% of wall thickness criterion, which is generally used for the plugging of steam generator tubes, has been applied only to a single cracked geometry. In the previous study by the authors, a total number of 9 local failure prediction models were introduced to estimate the coalescence load of two collinear through-wall cracks and, then, the reaction force model and plastic zone contact model were selected as the optimum ones. The objective of this study is to estimate the coalescence load of two collinear through-wall cracks in steam generator tube by using the optimum local failure prediction models. In order to investigate the applicability of the optimum local failure prediction models, a series of plastic collapse tests and corresponding finite element analyses for two collinear through-wall cracks in steam generator tube were carried out. Thereby, the applicability of the optimum local failure prediction models was verified and, finally, a coalescence evaluation diagram which can be used to determine whether the adjacent cracks detected by NDE coalesce or not has been developed.