Online monitoring method using Equipotential Switching Direct Current potential drop for piping wall loss by flow accelerated corrosion

The flow accelerated corrosion (FAC) phenomenon persistently impacts plant reliability and personnel safety. We have shown that Equipotential Switching Direct Current Potential Drop (ES-DCPD) can be employed to detect piping wall loss induced by FAC. It has been demonstrated to have sufficient sensitivity to cover both long and short lengths of piping. Based on this, new FAC screening and inspection approaches have been developed. For example, resolution of ES-DCPD can be adjusted according to its monitoring purpose. The developed method shows good integrity during long test periods. It also shows good reproducibility. The Seoul National University FAC Accelerated Simulation Loop (SFASL) has been constructed for ES-DCPD demonstration purposes. During one demonstration, the piping wall was thinned by 23.7% through FAC for a 13,000 min test period. In addition to the ES-DCPD method, ultrasonic technique (UT) has been applied to SFASL for verification while water chemistry was continually monitored and controlled using electrochemical sensors. Developed electrochemical sensors showed accurate and stable water conditions in the SFASL during the test period. The ES-DCPD results were also theoretically predicted by the Sanchez-Caldera's model. The UT, however, failed to detect thinning because of its localized characteristics. Online UT that covers only local areas cannot assure the detection of wall loss.     

On-line monitoring system development for single-phase flow accelerated corrosion

Aged nuclear piping has been reported to undergo corrosion-induced accelerated failures, often without giving signatures to current inspection campaigns. Therefore, we need diverse sensors which can cover a wide area in an on-line application. We suggest an integrated approach to monitor the flow accelerated corrosion (FAC) susceptible piping. Since FAC is a combined phenomenon, we need to monitor as many parameters as possible and that cover wide area, since we do not know where the FAC occurs. For this purpose, we introduce the wearing rate model which focuses on the electrochemical parameters. Using this model, we can predict the wearing rate and then compare testing results. Through analysis we identified feasibility and then developed electrochemical sensors for high temperature application; we also introduced a mechanical monitoring system which is still under development. To support the validation of the monitored results, we adopted high temperature ultrasonic transducer (UT), which shows good resolution in the testing environment. As such, all the monitored results can be compared in terms of thickness. Our validation tests demonstrated the feasibility of sensors. To support direct thickness measurement for a wide-area, the direct current potential drop (DCPD) method will be researched to integrate into the developed framework.     

Stress analysis of non-uniform thickness piping system with general piping analysis software

Most general piping analysis software can only perform ASME design stage type code compliance analysis with uniform pipe wall thickness. However, non-uniform wall thickness, commonly on elbows or bends, can be found in many industrial applications. A typical example is thinned non-uniform thickness at bends or elbows caused by flow accelerated corrosion (FAC). In this paper, an analysis procedure is introduced to enable a general piping software to conduct ASME III class 1 piping analysis with non-uniform wall thickness. The demonstration is performed on CANDU (Canadian Deuterium Uranium) feeder pipes, which have been subjected to FAC caused wall thinning. The results are compared with both conventional uniform thickness piping analysis and non-uniform thickness solid finite element analysis. The comparison shows the validity of the proposed “average-minimum-average” approach by employing the general piping analysis software. The approach remains conservative compared to the benchmark solid finite element analysis results. Meanwhile it provides lower acceptable thickness than the conventional piping analysis.     

Current in-service inspections of austenitic stainless steel and dissimilar metal welds in light water nuclear power plants

A tendency towards growing requirements for the inspection of austenitic piping can be observed in several countries. In Germany the revised KTA rule demands the UT inspection of austenitic and dissimilar metal welds in piping with diameters of 200 mm or more.On the basis of experience gained from austenitic piping with integranular stress corrosion cracking (IGSCC), longitudinal waves and mode conversion techniques are used. Depending on the geometry, material and grain orientation, spurious signals can be observed which require additional evaluation or analysis measurements.A promising new technique is based on horizontally polarized shear (SH) waves generated by electromagnetic acoustic transducers (EMATs). Investigations in the laboratory and field inspections showed that SH waves are well suited for the detection of longitudinal flaws, especially where the weld can be examined from one side only.For the complete solution of a given inspection problem SH waves can be combined with well-known standard techniques in order to provide redundant information for the characterization and sizing of indications.The investigation of possibilities of SH waves showed that the problem of cast austenitic steel inspection might not be solved using this technique. However, measurements using low frequency UT transducers showed promising results.     

Evaluation of the proximity effect on flow-accelerated corrosion

Flow-accelerated corrosion (FAC) is a degradation mechanism that affects carbon steel piping in power plants. The failures and degradation due to FAC have necessitated numerous replacements in many power plants. Several computer codes around the world were developed as part of a systematic program or process to control FAC in power plant utilities. The typical plant model requires the input of the flow parameters, piping configuration and the plant water chemistry. The results on FAC rate are considered the key to proper selection of components for inspection. The lack of information on the effect of the upstream components located in the proximity limited the accuracy of the FAC prediction tools and hence will affect the accuracy in identifying potential inspection locations. In the present study 211 inspection data for 90° carbon steel elbows from several nuclear power plants were used to determine the effect of the proximity between two components on the FAC wear rate. The effect of the velocity as well as the distance between the elbows and the upstream components is discussed in the present analysis. Based on the analyzed trends obtained from the inspection data, significant increase in the wear rate of approximately 70% on average is identified to be due to the proximity.     

Fitness for service assessment of degraded CANDU feeder piping—Canadian regulatory expectations

Allowance for the continued operation of feeder piping at some Canadian CANDU stations, which is experiencing active degradation mechanisms, has been based primarily on augmented inspection practices and conservative fitness for service assessments. The major degradation mechanisms identified to date are: pipe wall thinning due to Flow Accelerated Corrosion (FAC) and service induced cracking due to Intergranular Cracking due to Stress Corrosion Cracking (SCC) and potentially Low Temperature Creep Cracking (LTCC) mechanisms. Given that currently available industry codes and standards do not provide sufficient guidelines/criteria for assessing the degradation of feeder pipes, the Canadian Nuclear Safety Commission (CNSC) has asked the utilities to establish feeder pipe specific procedures to provide reasonable assurance that the risk associated with the feeder degradation is maintained at an acceptably low level. In response to this requirement, the Canadian CANDU industry has developed and continued to update feeder fitness for service guidelines to provide evaluation procedures and industry standard acceptance criteria for assessing the structural integrity of the feeder pipes. The scope and frequency of inspections are determined based on the results of the fitness for service assessments taking into account the relative susceptibility of feeder pipes to each specific degradation mechanism. While industry practices for the management of degraded feeder pipes have, in general, been complied with the regulatory expectations, outstanding issues still remain. Major regulatory concerns include uncertainties associated with limitations in both the inspection techniques and the mechanistic understanding of the degradation processes, which can impede inspection planning and fitness for service assessments.This paper presents the regulator's view of the current situation with respect to degradation of feeder piping, its implications for nuclear safety and the regulatory expectations on industry's management of the critical ageing phenomena.     

The evaluation of erosion/corrosion problems of carbon steel piping in Taiwan PWR nuclear power plant

Taiwan Pressurized Water Reactor (PWR), Maanshan nuclear power plant Units 1 and 2 implemented measurements of the wall thinning of the carbon steel piping under the request of regulation authority to prevent the events due to erosion/corrosion since 1989. At first, the licensee established the comprehensive inspection program by itself. Over 2000 components were inspected per unit and 300–500 pipe components were examined by ultrasonic testing per scheduled outage. The simple predictive methodology determined the operability of each individual piping component in the next fuel cycle. Based on the inspection results, the susceptible pipe components were established. Implementation of effective correction management and an improved inspection program can improve the safety, as well as the efficiency, of long-term reactor operations.     

A probabilistic model of wall thinning in CANDU feeders due to flow-accelerated corrosion

Wall thinning due to flow-accelerated corrosion (FAC) is a pervasive form of degradation in feeder pipes of the primary heat transport system of CANDU reactors. Prediction of the end-of-life of a feeder from wall thickness measurement data is confounded by the sampling and temporal uncertainties associated with the FAC degradation phenomenon. This paper presents a probabilistic model of wall thinning due to FAC, and calibrates it with a set of feeder wall thickness measurements obtained from a CANDU plant. The proposed model derives the feeder lifetime distribution, which is useful in developing optimum strategies for life-cycle management of the feeder system.     

流速加速腐蚀引起的碳钢管壁减薄

论述了核电厂高能碳钢系统中流速加速腐蚀（ＦＡＣ）引起的碳钢管壁减薄,介绍了问题的出现,ＦＡＣ的腐蚀机理及其影响因素,ＣＡＮＤＵ ６核电站一回路出口供水管管壁减薄的原因,以及秦山三期ＣＡＮＤＵ机组解决出口供水管管壁减薄的改进措施,以保证核电厂安全可靠经济地运行。     

Predicting the wall thinning engendered by erosion-corrosion using CFD methodology

Erosion-corrosion (EC) is a serious degradation mechanism of piping, especially for nuclear power plants since it may result in the piping damage, plant shutdown, or personnel injury. The majority of this paper investigates the dependence of wall thinning on the hydrodynamic characteristics using the computational fluid dynamics (CFD) methodology. Four piping systems in a pressurized water reactor (PWR) power plant are selected in this investigation. Based on the plots showing the measured wall thinning with the calculated hydrodynamic parameters, the relationship between them is clearly revealed. Utilizing the characteristics of near-wall turbulence kinetic energy, an envelopment model is proposed herein to conservatively predict the amount of wall thinning distributed on the pipe wall. This estimation model can simply predict the possible distributions of severe EC wear sites and subsequently assist the plant staff to schedule the pipe wall monitoring program in the measured range of pipe wall for the fittings.     

Distribution of Elements in Phases Produced by Reaction between CaO-Stabilized Zirconia and Simulated High-Level Radioactive Waste

A study for identifying the relationship between turbulent parameters and local wall thinning inside a carbon steel piping was performed. Experiments and numerical analyses for several types of downscaled piping components were conducted, and the obtained results were compared. Based on the results indicating that flow behaviors inside piping components can be sufficiently simulated by numerical analysis, numerical analyses for the models magnified to the actual sizes of plants were carried out. To determine the relationship between turbulent parameters and local wear rates, numerical analyses were performed for 17 piping components of 7 types installed in a feedwater system. Turbulent parameters resulting from numerical analyses were compared with the local wear rates based on the measured thickness data. From thecomparison of the results, the vertical flow velocity component (V_r) flowing again to the wall after separation due to geometrical configurations or direct collision against the wall at an angle of some degrees was found to be analogous to the configuration of local wall thinning. From the least-squares fitting result, it was derived that the average relationship between V_r and the local wear rate is proportional to 0.55-fold V_r adding 0.1 to the standard deviation of 0.65.     

Crack detection and sizing technique by ultrasonic and electromagnetic methods

Improvements in defect detection and sizing capabilities for non-destructive inspection techniques have been required in order to ensure the reliable operation and life extension of nuclear power plants. For the volumetric inspection, the phased array UT technique has superior capabilities for beam steering and focusing to objective regions, and real-time B-scan imaging without mechanical scanning. In contrast to the conventional UT method, high-speed inspection is realized by the unique feature of the phased array technique. A 256-channel array system has developed for the inspection of weldment of BWR internal components such as core shrouds. The TOFD crack sizing technique also can be applied using this system. For the surface inspection, potential drop techniques and eddy current techniques have been improved, which combined the theoretical analysis. These techniques have the crack sizing capability for surface breaking cracks to which UT method is difficult to apply. This paper provides the recent progress of these phased array and electromagnetic inspection techniques.     

Ultrasonic signal characteristics by pulse-echo technique and mechanical strength of graphite materials with porous structure

Ultrasonic testing (UT) is an important non-destructive method to detect internal flaws and is widely applied to product control in industrial fields. In an investigation on ultrasonic signal characteristics in porous ceramics, the present authors developed an ultrasonic wave propagation model for the pulse-echo technique by improving an existing one for the transmission technique. A wave-pore reflection process was taken into account in the improvement. In the developed model, both diffusion and scattering losses can be treated as important factors of ultrasonic wave attenuation. The model was demonstrated by experimental data on ultrasonic signal characteristics of nuclear grade graphite. As an application of the model, the authors proposed a new approach combined UT signal with fracture mechanics to evaluate the mechanical strength of porous ceramics from UT signal. The combined approach was tried to apply to the acceptance test and the in-service inspection conditions of graphite components in the High Temperature Engineering Test Reactor (HTTR) as an example. This paper presents the developed propagation model for the pulse-echo technique as well as the combined approach. Moreover, both acceptance test and in-service inspection techniques of graphite components in high temperature gas-cooled reactors (HTGRs) using the combined approach was also proposed in this paper.     

OPDE—The international pipe failure data exchange project

Certain member countries of the Organization for Economic Cooperation and development (OECD) in 2002 established the OECD pipe failure data exchange project (OPDE) to produce an international database on the piping service experience applicable to commercial nuclear power plants. OPDE is operated under the umbrella of the OECD Nuclear Energy Agency (NEA). The Project collects pipe failure data including service-induced wall thinning, part through-wall crack, pinhole leak, leak, and rupture/severance (i.e., events involving large through-wall flow rates up to and beyond the make-up capacity of engineered safeguards systems). The part through-wall events include degradation in excess of design code allowable for pipe wall thinning or crack depth. OPDE also addresses such degradation that could have generic implications regarding the reliability of in-service inspection.Currently the OPDE database includes approximately 3,700 records on pipe failure affecting ASME Code Class 1 through 3 and non-safety-related (non-Code) piping. This paper presents the motivations and objectives behind the establishment of the OPDE project. The paper also summarizes the unique data quality considerations that are associated with the reporting and recording of piping component degradation and failure. An overview of the database content is included to place it in perspective relative to past efforts to systematically collect and evaluate service experience data on piping performance. Finally, a brief summary is given of current database application studies.     

A two-phase methodology to predict FAC wear sites in the piping system of a BWR

Flow accelerated corrosion (FAC) wear is a serious degradation problem especially for nuclear power plants since it may result in the plant damage as well as risk to personnel. In this paper, a methodology which includes the two-phase hydrodynamic CFD models and FAC models, is proposed to predict severe FAC wear sites. Based on hydrodynamic simulation results, the present CFD models can precisely capture the two-phase characteristics within the piping system, which include the centrifugal effect, the gravitation effect and the imbalance of phase and mass separation in a T-junction, etc. Coupled with these flow characteristics, the appropriate FAC indicators can predict the possible locations of severe FAC wear. This methodology was validated against the measured results of wear site distributions for the piping system in a boiling water reactor (BWR) power plant. Good agreement between measurements and predictions at severe wear sites indicate that the present models can capture the characteristics of severe FAC wear and can help assist in the pipe wall-monitoring program for a nuclear power plant.     

Acceptance Size of Erosion Thinning in Carbon Steel Pipes Subjected to Internal Pressure and Tensile Load

Structural integrity evaluation of local wall thinning caused by erosion is important for maintaining the integrity of piping systems in power generating plants. The pipe of interest is a STS 42 carbon steel pipe loaded by an axial load and subjected to an internal pressure. Acceptable thinning length and width were determined from the allowable size of circumfer- encial and axial cracks in pipes, and the wall thickness is determined from the local membrane stress rule. Thus the acceptable extent and depth of wall thinning were proposed. Double-ended fracture can then be prevented if the local wall thinning is kept within this acceptable size.     

Steam generator small bore piping socket weld inspection using the phased array ultrasonic technique

As nuclear power plants age, the likelihood of failures in the small bore piping used in those plants caused by exposure to mechanical vibrations during plant operations increases. While small bore piping failures rarely cause plant shutdown, the management of small piping has been a keen area of interest since their repair or maintenance requires a reactor trip. Steam generator (SG) drain pipe socket welds are small diameter piping connections (nominal pipe schedule 3–4 inches) susceptible to mechanical vibration. SG drain pipe socket weld failures have caused coolant leakage. Therefore, more reliable inspection technologies for small bore piping need to be developed to detect problems at an early stage and prevent pipe failures. This research aims to improve the reliability and accuracy of small bore piping inspections through the design, manufacture and application of a new phased array ultrasonic testing technique and inspection system for SG drain line socket welds.     

Determination of rupture frequency for feeders subject to flow accelerated corrosion

Feeder pipe wall thinning due to flow accelerated corrosion (FAC) has been identified as a degradation mechanism that may affect the operating life of outlet feeder pipes. A large number of feeders are expected to require costly repair or replacement over the remaining life of the station if a conservative FAC rate is used in the deterministic structural integrity assessment. The paper presents a preliminary probabilistic framework for determining the rupture frequency of feeders subject to FAC based on commercial probabilistic fracture mechanics code WinPRAISE 2007. The obtained information can be used as inputs for developing risk-informed feeder life management plans. Darlington Unit 2 is selected to demonstrate the proposed method.     

Effect of length of thinning area on the failure behavior of carbon steel pipe containing a defect of wall thinning

The present study performed pipe failure tests using 102 mm-Sch. 80 carbon steel pipe with various simulated wall thinning defects, to investigate the effect of axial length of wall thinning and internal pressure on the failure behavior of pipe thinned by flow accelerated corrosion (FAC). The tests were conducted under loading conditions of four-point bending with and without internal pressure. The results showed that a failure mode of pipe with a defect depended on the magnitude of internal pressure and axial thinning length as well as stress type and thinning depth and circumferential angle. Both load carrying capability (LCC) and deformation capability (DC) were depended on stress type in the thinning area and dimensions of thinning defect. For applying tensile stress to the thinned area, the dependence of LCC on the axial length of wall thinning was determined by circumferential thinning angle, and the DC was proportionally increased with increase in axial length of wall thinning regardless of the circumferential angle. For applying compressive stress to thinned area, however, the LCC was decreased with increase in axial length of the thinned area. Also, the effect of internal pressure on failure behavior was characterized by failure mode of thinned pipe, and it promoted crack occurrence and mitigated a local buckling of the thinned area.     

Predicting local distributions of erosion–corrosion wear sites for the piping in the nuclear power plant using CFD models

The erosion–corrosion (E/C) wear is an essential degradation mechanism for the piping in the nuclear power plant, which results in the oxide mass loss from the inside of piping, the wall thinning, and even the pipe break. The pipe break induced by the E/C wear may cause costly plant repairs and personal injures. The measurement of pipe wall thickness is a useful tool for the power plant to prevent this incident. In this paper, CFD models are proposed to predict the local distributions of E/C wear sites, which include both the two-phase hydrodynamic model and the E/C models. The impacts of centrifugal and gravitational forces on the liquid droplet behaviors within the piping can be reasonably captured by the two-phase model. Coupled with these calculated flow characteristics, the E/C models can predicted the wear site distributions that show satisfactory agreement with the plant measurements. Therefore, the models proposed herein can assist in the pipe wall monitoring program for the nuclear power plant by way of concentrating the measuring point on the possible sites of severe E/C wear for the piping and reducing the measurement labor works.