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.     

Screening method for piping wall loss by flow accelerated corrosion

Flow accelerated corrosion (FAC) phenomenon has persisted its impact on plant reliability and personnel safety. Unless we change the operation condition drastically, most parameters affecting FAC will not be effectively controlled. In order to help expand piping inspection coverage, we have developed a screening approach to monitor the wall thinning by direct current potential drop (DCPD) technique. To improve the applicability to the complex piping network such as the secondary cooling water system in PWR's, we devised the equipotential control method that can eliminate undesired leakage currents outside a measurement section. In this paper, we present Wide Range Monitoring (WiRM) and Narrow Range Monitoring (NaRM) with Equipotential Switching Direct Current Potential Drop (ES-DCPD) method to rapidly monitor the thinning of piping. Based on the WiRM results, susceptible locations can be identified for further inspection by ultrasound technique (UT). On-line monitoring of a thinned location can be made by NaRM. Finite element analysis results and a closed-form resistance model are developed for the comparison with measured wall thinning by the developed DCPD technique. Verification experiments were conducted using UT as the reference. The result shows that model predictions and the experimental results agree well to confirm that both WiRM and NaRM based on ES-DCPD can be applicable to FAC management efforts.     

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.     

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.     

Evaluation of flow accelerated corrosion by coupled analysis of corrosion and flow dynamics. Relationship of oxide film thickness, hematite/magnetite ratio, ECP and wall thinning rate

Systematic approaches to evaluate flow accelerated corrosion (FAC) are desired before discussing application of countermeasures for FAC. First, future FAC occurrence should be evaluated to identify locations where a higher possibility of FAC occurrence exists, and then, wall thinning rate at the identified FAC occurrence zone is evaluated to obtain the preparation time for applying countermeasures.Wall thinning rates were calculated with two coupled models:

1.

static electrochemical analysis and

2.

dynamic oxide layer growth analysis.

The anodic current density and the electrochemical corrosion potential (ECP) were calculated with the static electrochemistry model based on an Evans diagram. The ferrous ion release rate, determined by the anodic current density, was applied as input for the dynamic double oxide layer model. Some of the dissolved ferrous ion was removed to the bulk water and others precipitated on the surface as magnetite particles.The thickness of oxide layer was calculated with the dynamic oxide layer growth model and then its value was used as input in the electrochemistry model. It was confirmed that the calculated results (corrosion rate and ECP) based on the coupled models were in good agreement with the measured ones.Higher ECP was essential for preventing FAC rate. Moderated conditions due to lower mass transfer coefficients resulted in thicker oxide layer thickness and then higher ECP, while moderated corrosion conditions due to higher oxidant concentrations resulted in larger hematite/magnetite rate and then higher ECP.     

Aspects of minimizing steel corrosion in liquid lead-alloys by addition of oxygen

Minimizing steel corrosion in liquid lead-alloys by addition of oxygen requires devices for efficient oxygen transfer and reliable oxygen sensors. The accuracy of electrochemical oxygen sensors is analyzed using theoretical considerations and results from experiments in stagnant lead–bismuth eutectic (LBE). Additionally, the feasibility of gas/liquid oxygen-transfer and the long-term performance of electrochemical sensors in flowing liquid metal are addressed on the basis of the operating experience of the CORRIDA loop, a facility for testing steels in flowing LBE.     

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.     

P11钢在湿蒸汽中流动加速腐蚀性能的模拟与实验研究

在核电厂中，二回路碳钢管道的流动加速腐蚀（FAC）对安全生产造成很大影响。根据工程经验和实验室数据，采用Cr含量超过0.1%的碳钢管道可大幅减轻或避免FAC。Cr含量大于1%的SA335-P11钢以其较好的性价比而多被推荐用于核电站蒸汽管道。为了确认P11钢表面氧化膜在高流速湿蒸汽作用下的稳定性，本文采用小型管状试样进行了FAC实验，得到了试样的减重规律和内表面的腐蚀形貌。实验结果表明，试样入口部分形成了明显的FAC形貌，表面氧化膜破坏严重，而在试样的中部，FAC不明显。CFD模拟结果表明，试样入口处局部流速较平均流速高约24%，表面剪切力大，与实验结果一致。     

孔板管道下游流动加速腐蚀速率数值模拟研究

采用计算流体力学方法中的k-ε模型模拟了孔板管道下游管壁与流体间的传质系数分布,并利用Sanchez-Caldera流动加速速率预测模型计算了孔板管道下游的流动加速腐蚀速率分布。结果表明,孔径比的减小会导致流动加速腐蚀敏感部位向孔板下游移动,入口流速的增大对孔板下游流动加速腐蚀敏感部位的位置无明显影响,pH值的增大能有效减小流动加速腐蚀速率。     

Correction Techniques of Radiation Induced Errors for Raman Distributed Temperature Sensor and Experiment at the Experimental Fast Reactor: JOYO

Optical fibers have advantages like flexible configuration, intrinsic immunity for electromagnetic fields etc., and they have been used as optical fiber sensors. By some of these techniques, continuous or discrete distribution of physical parameters can be measured. Here, in order to apply Raman distributed temperature sensor (RDTS) to the monitoring of nuclear facilities: some correction techniques for radiation induced errors were investigated. It has been shown that, when uniform loss distribution can be assumed, simple correction technique with two thermocouples can be applied. Moreover, if loop type arrangement is applied, even when the loss distribution is not uniform, radiation induced errors can be canceled.

For the demonstration of the feasibility of this technique, measurements using a commercial RDTS sys-tem were carried out along the primary piping system of the experimental fast reactor: JOYO. During the continuous measurements with the total dose of more than 10⁷R, the radiation induced errors showed a saturating tendency. The correction technique with two thermocouples was applied and its feasibility has been demonstrated. Although the time response of the system should be improved, the RDTS can be expected as a noble temperature monitor in nuclear facilities.     

Comparison of multiple support excitation solution techniques for piping systems

Design and analysis of nuclear power plant piping systems exposed to a variety of dynamic loads often require multiple support excitation analysis by modal or direct time integration methods. Both methods have recently been implemented in the computer program KWUROHR for static and dynamic analysis of piping systems, following the previous implementation of the multiple support excitation response spectrum method (see papers K6/15 and K6/15a of the SMiRT-4 Conference).The results of multiple support excitation response spectrum analyses can be examined by carrying out the equivalent time history analyses which do not distort the time phase relationship between the excitations at different support points.A frequent point of discussion is multiple versus single support excitation. A single support excitation analysis is computationally straightforward and tends to be on the conservative side, as the numerical results show. A multiple support excitation analysis, however, does not incur much more additional computer cost than the expenditure for an initial static solution involving three times the number, L, of excitation levels, i.e. 3L static load cases. The results are more realistic than those from a single support excitation analysis.A number of typical nuclear plant piping systems have been analyzed using single and multiple support excitation algorithms for: (1) the response spectrum method, (2) the modal time history method via the Wilson, Newmark and Goldberg integration operators and (3) the direct time history method via the Wilson integration operator. Characteristic results are presented to compare the computational quality of all three methods.     

Electroless nickel-plating for the PWSCC mitigation of nickel-base alloys in nuclear power plants

The feasibility study has been performed as an effort to apply the electroless nickel-plating method for a proposed countermeasure to mitigate primary water stress corrosion cracking (PWSCC) of nickel-base alloys in nuclear power plants. In order to understand the corrosion behavior of nickel-plating at high temperature water, the electrochemical properties of electroless nickel-plated alloy 600 specimens exposed to simulated pressurized water reactor (PWR) primary water were experimentally characterized in high temperature and high pressure water condition. And, the resistance to the flow accelerated corrosion (FAC) test was investigated to check the durability of plated layers in high-velocity water-flowing environment at high temperature. The plated surfaces were examined by using both scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) after exposures to the condition. From this study, it is found that the corrosion resistance of electroless nickel-plated Alloy 600 is higher than that of electrolytic plating in 290 °C water.     

Development of Ultrasonic Testing Equipment Incorporating Electromagnetic Acoustic Transducer

An ultrasonic testing equipment for use in in-service inspection of nuclear power plant piping has been developed, which comprises an angle-beam electromagnetic acoustic transducer mounted on a vehicle for scanning the piping surface to be inspected. The transducer functions without direct contact with the piping surface through couplant, and the vehicle does not require a guide track installed on the piping surface, being equipped with magnetic wheels that adhere to the piping material, permitting it to travel along the circumferential weld joint of a carbon steel pipe. The equipment thus dispenses with the laborious manual work involved in preparing the piping for inspection, such as removal of protective coating, surface polishing and installation of guide track and thereby considerably reduces the duration of inspection. The functioning principle and structural features of the transducer and vehicle are described, together with the results of trial operation of a prototype unit, which proved a 1 mm deep notch cut on a test piece of 25 mm thick carbon steel plate to be locatable with an accuracy of ±2 mm.     

An approximative solution for limit load of piping branch junctions with circumferential crack and finite element validation

This paper is concerned with the prediction of limit load of the piping branch junctions with circumferential crack under internal pressure. Recently, we have developed a new approach for predicting the limit load of two-cylinder intersection structures with diameter ratio larger than 0.5, which has been successfully applied to defect free cases under various loading conditions. In the present work, we consider the extension of the approach to cover cracked piping branch junctions. On the basis of stress analysis in the vicinity of intersection line, a closed form of limit load solution for piping branch junctions with circumferential crack was developed. Then, 36 finite element (FE) models of piping branch junction with various dimensions of structure and crack were analyzed by using nonlinear finite element software. The limit loads from FE analysis and the proposed solution are compared with each other. Overall good agreement between the estimated solutions and the FE results provides confidence in the use of the proposed formulae for defect assessment of piping branch junctions in practice.     

压水堆核电机组二回路给水系统弯头减薄原因分析

2012年3月,某核电厂大修期间对二回路部分管线进行了现场壁厚测量,发现电动主给水泵系统弯头存在壁厚减薄现象。文章对其中的一根弯头在实验室进行了失效分析。利用超声波测厚仪在实验室对换下的弯头进行壁厚测厚并利用等离子光谱发生仪等设备及分析手段对异常减薄部位和减薄一般部位进行了分析研究。结果表明管壁异常减薄是由于流动加速腐蚀（FAC）引起的。最后,根据分析结果,结合国内外的最新研究进展,对管道的管理及变更提出了建议。     

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.     

Analysis and test correlation of flexible and stiff piping systems

An in situ pipe test program was conducted to provide a basis for evaluating piping analysis methodologies and design philosophies. In this program, a 20.3-cm boiler feedwater line with two fundamentally different support systems was tested and analyzed. One system employed hanger supports and was very flexible. The second system employed strut and snubber supports and was relatively stiff. Snapback and forced vibration tests were performed on the piping systems. The test results were used to determine piping damping values and to correlate with analyses. These analyses were used to evaluate current piping analysis methodologies and their analytical models. Also, parametric studies were performed with the analytical models to evaluate the effect of different support systems on the pipe behavior for thermal and seismic loads. In addition, the seismic analysis results were compared to quantify the differences between direct time integration and response spectra analysis methods.     

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.     

Correlation of flow accelerated corrosion rate with iron solubility

Flow accelerated corrosion (FAC) of the carbon steel is one of the most important subjects in the coolant systems of the power plants. FAC is influenced by the composition of the material, the flow condition, temperature, and the water chemistry conditions. It is considered that the solubility of iron (Fe) is the most important factor in the water chemistry parameters affecting FAC. In the present study, the effects of temperature and pH on the Fe solubility were evaluated in consideration of the hydrolysis reactions of the ferrous iron, the dissolution equilibria of Fe₃O₄, FeO, and Fe(OH)₂, and the charge balance. The correlation between the Fe solubility and the FAC behavior was discussed. It has been suggested that the product of the Fe solubility equilibrated with Fe₃O₄ and the mass transfer coefficient can explain the temperature and pH dependence of FAC. These results indicate the presence of the magnetite on the surface of the carbon steel. Diffusion of the Fe from the saturated layer to the bulk solution determines the FAC rate from the water chemistry aspect.     

Decreasing snubber inservice inspection costs through snubber reduction and improved test limits

Snubber inservice inspection (ISI) requirements, along with a history of snubber malfunctions, has made inspection and maintenance of snubbers a significant part of a nuclear power plant's ISI budget. These expenses can be minimized through snubber reduction and the use of improved test limits for snubber functional testing. This paper presents a snubber overview and reviews snubber ISI requirements. Examples are given of the high cost that maintaining a snubber in an operating nuclear plant represents.Snubber reduction refers to reducing a plant's snubber population by eliminating snubbers shown not to be required to restrain piping for design basis dynamic loadings, and by replacing snubbers with other types of restraints, such as rigid struts. Snubber reduction is discussed in terms of what makes removing snubbers practical along with approaches to, and results of recently implemented snubber reduction programs.Improved or increased test limits for snubber functional testing are discussed along with an approach to, and results of an Electric Power Research Institute sponsored program to develop improved limits that would not significantly affect piping response. Improved piping acceptance criteria can be used to justify the use of increased test limits provided by snubber manufacturers. An additional use is to justify the operability of piping on which faulty snubbers were found.