Cracks In Tubes Research Articles

A new repair criterion for steam generator tubes with axial cracks based on probabilistic integrity assessment

Steam generator is one of the major components in a nuclear power plant, and it consists of thousands of thin-walled tubes. The operating record of the steam generators has indicated that a number of axial cracks due to stress corrosion have been frequently detected in the tubes. Since the tubes are closely related to the safety and also the efficiency of a nuclear power plant, an establishment of the appropriate repair criterion for the defected tubes and its applications are necessary. The objective of this paper is to develop an accurate repair criterion for the tubes with axial cracks. To do this, a thorough review is performed on the key parameters affecting the tube integrity, and then the probabilistic integrity assessment is carried out by considering the various uncertainties. In addition, the sizes of critical crack are determined by comparing the burst pressure of the cracked tube with the required performance criterion. Based on this result, the new repair criterion for the axially cracked tubes is defined from the reasonably conservative value such that the required performance criterion in terms of the burst pressure is able to be met during the next operating period.

Steam generator tube rupture simulation using extended finite element method

A steam generator (SG) is an important component of any pressurized water reactor. Steam generator tubes represent a primary pressure boundary whose integrity is vital to the safe operation of the reactor. SG tubes may rupture due to propagation of a crack created by mechanisms such as stress corrosion cracking, fatigue, etc. It is thus important to estimate the rupture pressures of cracked tubes for structural integrity evaluation of SGs. The objective of the present paper is to demonstrate the use of extended finite element method capability of commercially available ABAQUS software, to model SG tubes with preexisting flaws and to estimate their rupture pressures. For the purpose, elastic–plastic finite element models were developed for different SG tubes made from Alloy 600 material. The simulation results were compared with experimental results available from the steam generator tube integrity program (SGTIP) sponsored by the United States Nuclear Regulatory Commission (NRC) and conducted at Argonne National Laboratory (ANL). A reasonable correlation was found between extended finite element model results and experimental results.

Some considerations on instability of combined loaded thin-walled tubes with a crack

Instability of a thin-walled stainless steel tube with a crack-shaped defect under combined loading is studied in this paper. Furthermore, the effects of the tube length, crack orientation, and crack length on the buckling behavior of tubes are investigated. The behavior of tubes subjected to combined is analyzed by using the finite element method (by Abaqus software). For cracked tubes with a fixed thickness, the buckling load decreases as the tube length and the ratio of the tube length to its diameter increase. Moreover, the buckling load of cracked tubes under combined loading also decreases with increasing crack length.

Numerical Prediction of Maximum Load-Carrying Capacity of Cracked Alloy 690TT Steam Generator Tubes

This paper presents a finite element (FE) simulation technique to predict maximum load-carrying capacity of cracked steam generator tubes and its application to Alloy 690TT tubes. The simulation method is based on a simplified version of the stress modified fracture strain model. The damage model is determined from tensile test and one cracked tube test data. Predicted maximum pressures are compared with 23 test data of axial through-wall and surface cracked Alloy 690TT steam generator tubes. Comparison with experimental data shows good agreement.

Effect of layerwise structural inhomogeneity on stress- corrosion cracking of steel tubes

Based on X-ray texture and structure analysis data of the material of main gas pipelines it was shown that the layerwise inhomogeneity of tubes is formed during their manufacturing. The degree of this inhomogeneity affects on the tendency of tubes to stress- corrosion cracking under exploitation. Samples of tubes were cut out from gas pipelines located under various operating conditions. Herewith the study was conducted both for sections with detected stress-corrosion defects and without them. Distributions along tube wall thickness for lattice parameters and half-width of X-ray lines were constructed. Crystallographic texture analysis of external and internal tube layers was also carried out. Obtained data testifies about considerable layerwise inhomogeneity of all samples. Despite the different nature of the texture inhomogeneity of gas pipeline tubes, the more inhomogeneous distribution of texture or structure features causes the increasing of resistance to stress- corrosion. The observed effect can be explained by saturation with interstitial impurities of the surface layer of the hot-rolled sheet and obtained therefrom tube. This results in rising of lattice parameters in the external layer of tube as compared to those in underlying metal. Thus, internal layers have a compressive effect on external layers in the rolling plane that prevents cracks opening at the tube surface. Moreover, the high mutual misorientation of grains within external and internal layers of tube results in the necessity to change the moving crack plane, so that the crack growth can be inhibited when reaching the layer with a modified texture.

Numerical Modeling as the Method to Determine the Parameters of Tube Bending with Local Induction Heating

The numerical analysis results of bending with local induction heating of 10CrMo9-10 steel tubes have been presented in the paper. The goal of conducted simulations was to determine the process parameters that allow to obtain tube bends with geometric features which meet requirements of relevant standards. The risks of wrinkling, excessive tube bend ovalization and cracking (according to Cockroft-Latham criterion) have been analysed. The obtained results prove that numerical modeling can be used to minimize the costs of tube bending technology implementation without the need of further process parameters corrections.

Concrete filled elliptical steel tubular members with large diameter-to-thickness ratio subjected to bending

Concrete filled elliptical steel tubes, hereafter called CFESTs, are elliptical steel tube members filled in with concrete. The CFEST belongs to a family of concrete filled steel tubes, the so-called CFT, having good deformability and large seismic strength due to confined effect between the tube and in-filled concrete. The present study aims to investigate experimentally the characteristics of the CFEST members under pure bending. The selected testing parameters are diameter-to-thickness ratio of elliptical steel tube and loading directions, namely, the minor and major axes directions. From the test results, both local buckling and cracking of the steel tube can be observed in compressive and tensile regions, respectively. Obtained pure bending strength of the CFEST is strongly affected by diameter-to-thickness ratio. Pure bending capacity of the CFEST is also compared to that of the circular CFT. Methods to predict pure bending strength of the CFEST based on concrete strength, yielding and fracture points of the steel tube and confinement effect are described. Moreover, pure bending strength of the CFEST members is mainly discussed in comparison to that of ordinary CFT members. Additionally, biaxial stress behavior of the steel tube induced by in-filled concrete is also mentioned.

Implementation of condition-dependent probabilistic risk assessment using surveillance data on passive components

A great deal of surveillance data are collected for a nuclear power plant that reflect the changing condition of the plant as it ages. Although surveillance data are used to determine failure probabilities of active components for the plant’s probabilistic risk assessment (PRA) and to indicate the need for maintenance activities, they are not used in a structured manner to characterize the evolving risk of the plant. The present study explores the feasibility of using a condition-dependent PRA framework that takes a first principles approach to modeling the progression of degradation mechanisms to characterize evolving risk, periodically adapting the model to account for surveillance results. A case study is described involving a potential containment bypass accident sequence due to the progression of flow-accelerated corrosion in secondary system piping and stress corrosion cracking of steam generator tubes. In this sequence, a steam line break accompanied by failure to close of a main steam isolation valve results in depressurization of the steam generator and induces the rupture of one or more faulted steam generator tubes. The case study indicates that a condition-dependent PRA framework might be capable of providing early identification of degradation mechanisms important to plant risk.

Failure analysis of stress corrosion cracking in heat exchanger tubes during start-up operation

The cracking failure of a new heat exchanger during first start-up operation has been analyzed. Through the investigation of the operating history of the equipment, analysis of the chemical compositions of tube material and corrosion products, metallographic test of specimens with cracks, the cracking mode can be described as the Stress Corrosion Cracking (SCC) of austenitic stainless steel. This kind of cracking was induced by the chloride in high temperature steam and tensile stress. The residual tensile stress due to seal expansion has been proved by numerical calculation. The pre-heating steam which was polluted by the catalyst with chloride is the main reason for the tube cracking in this case.

Cracking of AISI 309S Stainless Steel Lance Tube Used for Coal Injection in Blast Furnace

The cracking of coal injection lance tubes in a blast furnace has been analyzed in this paper. Most modern blast furnaces have facilities for injecting pulverized coal as an auxiliary fuel. The coal is injected in front of the tuyeres through lance tubes using nitrogen as a carrier gas, and the injected coal is burnt in contact with the hot blast (blast temperature ≈ 1225 °C) enriched with oxygen and humidity. The injection lances were failing prematurely within 2 months of installation. The material of the lance tube was AISI 309S grade austenitic stainless steel. Visual observation showed multiple branched cracks on the tube wall. Fractography of the crack surface under scanning electron microscope showed cleavages indicating brittle fracture. Microstructural examination revealed branched transgranular cracks initiating from the outer surface which was in contact with the hot blast. Energy dispersive spectroscopy on the cracks near the outer surface showed presence of corrosive elements like chlorine and sulfur in the corrosion product, which may have been introduced by the raw materials such as coal and coke used in the blast furnace. Measurement of stress indicated significant amount of internal stress within the tube. Sulfidation was also observed at the inner surface of lance tube. Analyses of the failure, particularly, multi-branched transgranular cracks of austenitic stainless steel suggested stress corrosion cracking induced by exposure to a chlorine-containing environment; hot blast conditions (temperature, humidity, and oxygen enrichment) aggravated the cracking. A suitable material (Inconel 600) was recommended for the application which increased the service life from 2 to 4 months. Microstructural characterization exhibited oxidation associated with voids at the grain boundary suggesting creep of the Inconel 600 lance tube after 4 months.

Numerical Analysis to Predict Load-Bearing Capacity of Flawed Steam Generator Tubes in Nuclear Plants

This paper presents a prediction of load-bearing capacity of flawed steam generator tube using numerical analysis. Alloy 690TT steam generator tube with diameter of 19.05mm and thickness of 1.07mm widely used in nuclear power plants is considered in this study. Sound tubes and through-wall cracked tubes are used in tensile tests. Finite element analysis applying a damage mechanics is performed to predict ductile fracture and load-bearing capacity of the cracked tube. A strain-based fracture model is determined and applied to calculate ductile damage during the simulations. The sound tube and through-wall cracked tube tensile tests are analyzed to determine the fracture model as well as proper element size for Alloy 690TT. Crack growths are simulated by implementing stress reduction technique for failed finite element. Predicted maximum load-bearing capacity are compared with published test data.

DETECTION OF ODSCC IN SG TUBES DEPENDING ON THE SIZE OF THE CRACK AND ON THE PRESENCE OF SLUDGE DEPOSITS

It was discovered in a Korean PWR that an extensive number of very short and shallow cracks in the SG tubes were undetectable by eddy current in-service-inspection because of the masking effect of sludge deposits. Axial stress corrosion cracks at the outside diameter of the steam generator tubes near the line contacts with the tube support plates are the major concern among the six identical Korean nuclear power plants having CE-type steam generators with Alloy 600 high temperature mill annealed tubes, HU3&4 and HB3∼6. The tubes in HB3&4 have a less susceptible microstructure so that the onset of ODSCC was substantially delayed compared to HU3&4 whose tubes are most susceptible to ODSCC among the six units. The numbers of cracks detected by the eddy current inspection jumped drastically after the steam generators of HB4 were chemically cleaned. The purpose of the chemical cleaning was to mitigate stress corrosion cracking by removing the heavy sludge deposit, since a corrosive environment is formed in the occluded region under the sludge deposit. SGCC also enhances the detection capability of the eddy current inspection at the same time. Measurement of the size of each crack using the motorized rotating pancake coil probe indicated that the cracks in HB4 were shorter and substantially shallower than the cracks in HU3&4. It is believed that the cracks were shorter and shallower because the microstructure of the tubes in HB4 is less susceptible to ODSCC. It was readily understood from the size distribution of the cracks and the quantitative information available on the probability of detection that most cracks in HB4 had been undetected until the steam generators were chemically cleaned.

INFLUENCE OF SIGNAL-TO-NOISE RATIO ON EDDY CURRENT SIGNALS OF CRACKS IN STEAM GENERATOR TUBES

This work presents the influence of noise originating from the tube itself on the detectability and sizing accuracy for laboratory-induced outer diameter axial cracks in nuclear steam generator tubes. The variations of signal amplitude and phase angle of the same cracks were analyzed when increasing the signal-to-noise ratio of the tube itself from 9 to 18. It was experimentally verified that the detectability for small cracks was enhanced by increasing the signal-to-noise ratio. The phase angle also rotated to a value representing the actual position and depth of a crack when increasing the signal-to-noise ratio.

DETECTION OF ODSCC IN SG TUBES DEPENDING ON THE SIZE OF THE CRACK AND ON THE PRESENCE OF SLUDGE DEPOSITS

It was discovered in a Korean PWR that an extensive number of very short and shallow cracks in the SG tubes were undetectable by eddy current in-service-inspection because of the masking effect of sludge deposits. Axial stress corrosion cracks at the outside diameter of the steam generator tubes near the line contacts with the tube support plates are the major concern among the six identical Korean nuclear power plants having CE-type steam generators with Alloy 600 high temperature mill annealed tubes, HU3&4 and HB3∼6. The tubes in HB3&4 have a less susceptible microstructure so that the onset of ODSCC was substantially delayed compared to HU3&4 whose tubes are most susceptible to ODSCC among the six units. The numbers of cracks detected by the eddy current inspection jumped drastically after the steam generators of HB4 were chemically cleaned. The purpose of the chemical cleaning was to mitigate stress corrosion cracking by removing the heavy sludge deposit, since a corrosive environment is formed in the occluded region under the sludge deposit. SGCC also enhances the detection capability of the eddy current inspection at the same time. Measurement of the size of each crack using the motorized rotating pancake coil probe indicated that the cracks in HB4 were shorter and substantially shallower than the cracks in HU3&4. It is believed that the cracks were shorter and shallower because the microstructure of the tubes in HB4 is less susceptible to ODSCC. It was readily understood from the size distribution of the cracks and the quantitative information available on the probability of detection that most cracks in HB4 had been undetected until the steam generators were chemically cleaned.

Failure Analysis and Welding Repair for Water Wall Tube Crack of Supercritical Boiler

Through methods, such as microscope analysis, mechanical analysis, chemical analysis, metallography examination etc, the causes resulting in water wall tube cracking of the supercritical boiler have been analyzed. The result indicates this water wall tube cracking is the high temperature creep fracture. The higher tensile stress, microstructure aging and high temperature strength degradation are the primarily inducement of cracking failure. In welding tube, the measures of reducing residual stress, such as lower welding heat input, reducing thickness of welding layer, assigning reasonably heat treatment power, lengthening holding time and so on, were taken. The welding repair succeeded greatly.

원전 증기발생기 전열관의 확관방법에 따른 응력부식균열 저항성 연구

원자력발전소의 증기발생기 전열관은 가동 중에 다양한 형태의 부식 손상이 발생한다. 전열관의 외면에 발생하는 응력부식균열은 2차측 응력부식균열이라 불리는데 주로 전열관의 확관천이지역에서 발생한다. 그 원인은 이 지역의 기하학적 특성과 관련된 슬러지의 침적에 의한 불순물의 농축과 증기 발생기 제작과정에서 확관에 의한 잔류응력이다. 특히 잔류응력은 확관방법에 따라 방향성 및 그 크기가 달라지는데 전열관에 발생하는 균열의 방향 및 발생빈도는 이와 관련이 있다. 현장 경험에 따르면, 폭발확관된 전열관은 수압확관된 전열관에 비해 확관천이 부위에서 원주방향 균열이 잘 발생하는 것으로 나타났다. 따라서 본 연구에서는 예민화된 증기발생기 전열관에 대한 응력부식균열 시험을 통해 확관법에 따른 특정방향 균열의 발생빈도 및 균열 크기를 비교하였다. 또한 균열이 발생된 전열관의 파단면 검사를 통해 균열 양상과 수화학 환경 중의 특정 성분의 영향을 관찰하였다. The steam generator tubes of nuclear power plants have various types of corrosion failures during the plant operation. The stress corrosion cracking which occurs on the outer surface of tube is called the secondary side stress corrosion cracking and mainly occurs in the expansion-transition area of tube. The causes are the concentration of impurities by the sludge pile-up related to the geometry of its region and the residual stress by tube expansion in the process of steam generator manufacturing. Especially the directionality and sizes of residual stresses are differed according to the tube expansion methods and the direction and the frequency of tube cracks depend on their characteristics. In bases on the plant experiences, it is notified that circumferential cracks of tubes expanded with explosive expansion method are dominantly occurred compared to those of tubes done with hydraulic expansion one. Therefore in this study, according to tube expansion methods frequencies and sizes of tube cracks with specific direction are compared by means of accelerated immersion test and also the crack morphology and the specific chemicals from water-chemistry environment are observed through the fracture surface examination.

Development of probability of detection functions for evaluating integrity of axial outside diameter stress corrosion cracks in steam generator

The reliability of an eddy current testing (ECT) inspection system depends upon the inspection technique and quality of analyst. In evaluating the integrity of a steam generator (SG) tube, degradation detection and sizing accuracy are considered performance measures of the nondestructive evaluation (NDE) system. A probability of detection (POD) model serves as a functional measure of the ability of an NDE system to detect degradation. It is one of the inputs in the operational assessment, and it is used to estimate the degradation during service via ECT of the SG tube. In this study, the POD functions of the inspection technique and analyst were obtained to quantitatively analyze the ECT bobbin probe for axial outside diameter stress corrosion cracks in SG tubes. This should serve to evaluate the integrity of the SG tubes. The depth and amplitude of defects were used as parameters of the POD model. Hit (detection) and miss (no detection) binary data obtained from destructive and nondestructive inspection of cracked tubes were also used.

Analysis of Welding Crack in Thermal Sprayed Water Wall Tubes and the Establishment of Welding Procedure

The cause of welding cracks in thermal sprayed water wall tubes was analyzed, the choice of nondestructive testing method was discussed, and a welding procedure for the water wall tube with thermal sprayed coatings was established successfully.

Design and Development of a Control and Drive System for Cable Pusher Module for PFBR Steam Generator Tube Inspection

Abstract The Steam Generator of the PFBR is a vertical heat exchanger with sodium flowing in the shell side, and water in tube side. The tubes have an expansion bend to accommodate the thermal expansions and periodic in-service inspection is to be carried out to check the reduction in tube wall thickness and crack using Eddy Current Probes. Cable Pusher Module is used to insert and retract the probe in to the tube having 23 meter length. This paper presents the detailed design and development of CPM which includes the control system, software and mechanical aspects. CPM transmits the parameters like odometer reading, slip, velocity and time of inspection through serial interface. Graphical User Interface is developed using VB 6.0 Compiler for data logging. Slip of the motor is plotted for the tube length and this suggests an optimum design for the mechanical pulley arrangement of CPM.

Utility of a scoring balloon for a severely calcified lesion: bench test and finite element analysis

We aimed to investigate the effectiveness of a scoring balloon catheter in expanding a circumferentially calcified lesion compared to a conventional balloon catheter using an in vitro experiment setting and elucidate the underlying mechanisms of this ability using a finite element analysis. True efficacy of the scoring device and the underlying mechanisms for heavily calcified coronary lesions are unclear. We employed a Scoreflex scoring balloon catheter (OrbusNeich, Hong Kong, China). The ability of Scoreflex to dilate a calcified lesion was compared with a conventional balloon catheter using 3 different sized calcium tubes. The thickness of the calcium tubes were 2.0, 2.25, and 2.5 mm. The primary endpoints were the successful induction of cracks in the calcium tubes and the inflation pressures required for inducing cracks. The inflation pressure required for cracking the calcium tubes were consistently lower with Scoreflex (p < 0.05, Student t test). The finite element analysis revealed that the first principal stress applied to the calcified plaque was higher by at least threefold when applying the balloon catheter with scoring elements. A scoring balloon catheter can expand a calcified lesion with lower pressure than that of a conventional balloon. The finite element analysis revealed that the concentration of the stress observed in the outside of the calcified plaque just opposite to the scoring element is the underlying mechanism of the increased ability of Scoreflex to dilate the calcified lesion.