A case study on detection and sizing of defects in steam generator tubes using eddy current testing

Eddy current testing (ECT) method is widely used to detect various types of defects occurring in nuclear steam generator tubes. Therefore, the reliability of its detection and sizing accuracy for defects should be validated. For this purpose, two tubes with defect signals were pulled from an operating steam generator and destructively examined. The defect type was a circumferential crack for one tube and an intergranular attack (IGA) for the other tube. The plus point coil probe showed a better capability to detect and size both a circumferential crack and a volumetric IGA than pancake and bobbin coil probe. The destructive results are correlated with the ECT results obtained during the in-service inspection.     

Caution when applying eddy current inversion to stress corrosion cracking

This study evaluates the applicability of computer-aided eddy current inversion techniques to the profile evaluation of stress corrosion cracking in Inconel welds. Welded plate specimens, which model head penetration welds of pressurized water reactors, are fabricated; notches and stress corrosion cracks are artificially introduced into the weld metal of the specimens. Eddy current inspections are performed using a uniform eddy current probe driven at frequencies of 10 and 40 kHz. Since weld noise is observed uniformly along the weld line, a simple signal processing is applied to eliminate it. First, the artificial notches are reconstructed and good agreements between reconstructed and true profiles are provided, which demonstrates that the computer-aided eddy current inversion technique can deal with defects in welds. Then, numerical simulations are performed to evaluate the profiles of the stress corrosion cracks. In the numerical simulations, the stress corrosion cracks are modeled as a conductive region with a fixed width of 0.3 mm. The cross-sectional profiles of the cracks are reconstructed from measured eddy current signals directly above and along a crack. Although eddy current signals calculated from the reconstructed profiles agree well with measured ones, the true profiles revealed by destructive testing are found to be very different from the reconstructed ones. Whereas the most plausible reason for the difference is the unexpectedly volumetric profile of the stress corrosion cracks, this study has revealed that computer-aided eddy current inversion techniques that have been used to consider cracks in thin structures would not at this point be directly applicable to those in thick structures. It is also important to know in advance those crack features that can adversely impact accurate crack sizing including whether a detected crack is volumetric or not, namely there are many parallel cracks in a cluster or not.     

Evaluation of the electromagnetic characteristics of type 316L stainless steel welds from the viewpoint of eddy current inspections

This study evaluated the electromagnetic characteristics of austenitic stainless steel welds from the viewpoint of eddy current testing. Seven welded plate specimens, which were welded using JIS Z3221:2010 YS316L welding metals, were prepared. Two welding metals and several welding conditions were adopted to discuss the generality. The results of eddy current examination of the specimens using three different types of eddy current probes, that is, an absolute pancake probe, a differential plus-point probe, and a uniform eddy current probe, confirmed that the welds cause a large noise especially when the absolute pancake probe was used. The analysis of the signals through finite element simulations revealed that the magnetic property of the welds is not negligible from the viewpoint of eddy current testing. Complementary experiments were conducted using a vibrating sample magnetometer to validate the results.     

Detection of embedded fatigue cracks in Inconel weld overlay and the evaluation of the minimum thickness of the weld overlay using eddy current testing

This study evaluates the applicability of eddy current testing to the detection and sizing of fatigue cracks embedded in Inconel weld overlays. Welded plate specimens, which model head penetration welds and their weld overlays, are fabricated, and fatigue cracks are artificially introduced into the specimens. Eddy current inspections are performed using a uniform eddy current probe driven with 10 kHz, and all of the fatigue cracks are detected with clear signals. Subsequent numerical inversions estimate that the minimum thicknesses of the weld overlays are 1.47, 2.17, and 2.23 mm, whereas true thicknesses revealed through destructive testing are 1.51, 3.25, and 2.10 mm, respectively. Thicknesses are also evaluated using potential drop and ultrasonic testing methods; the results demonstrate that eddy current testing is the most efficient of the three methods.     

Performance evaluation of eddy current flowmeter in Monju

Measurement of the temperature and flow rate at each fuel subassembly outlet is an effective way for a liquid metal fast breeder reactor to detect a loss of coolant accident or reactivity-initiated accident in the early stage and to understand the reactor’s thermal hydrodynamic performance. Japan Atomic Energy Agency has developed the eddy current flowmeter in practical use and installed 34 of them in the upper core structure of fast breeder reactor, Monju. This report presents data obtained by using the flowmeters in Monju. We observed high linearity between each of the flowmeter’s signal intensity and the primary sodium’s flow rate under 10–100% flow rate condition. High linearity was also observed in a region of low velocity (approx. 0.25 m/s). The fluctuation of flow rate observed by the flowmeters was below 0.2 m/s which is 5% of the time-averaged velocity under a rated condition. These experimental results show that the eddy current flowmeter is an effective tool to detect the changes in relative flow rate.     

Comparison of Inconel 625 and Inconel 600 in resistance to cavitation erosion and jet impingement erosion

Liquid droplet erosion (LDE), which often occurs in bellows made of nickel-based alloys, threatens the security operation of the nuclear power plant. As the candidate materials of the bellows, Inconel 600 and Inconel 625 were both tested for resistance to cavitation erosion (CE) and jet impingement erosion (JIE) through vibratory cavitation equipment and a jet apparatus for erosion-corrosion. Cumulative mass loss vs. exposure time was used to evaluate the erosion rate of the two alloys. The surface and cross-sectional morphologies before and after the erosion tests were observed by scanning electron microscopy (SEM), the inclusions were analyzed by an energy dispersive spectroscopy (EDS), and the surface roughness was also measured by surface roughness tester to illustrate the evolution of erosion process. The results show that the cumulative mass loss of CE of Inconel 625 is about 1/6 that of Inconel 600 and the CE incubation period of the Inconel 625 is 4 times as long as that of the Inconel 600. The micro-morphology evolution of CE process illustrates that the twinning and hardness of the Inconel 625 plays a significant role in CE. In addition, the cumulative mass loss of JIE of Inconel 625 is about 2/3 that of Inconel 600 at impacting angle of 90°, and almost equal to that of the Inconel 600 at impacting angle of 30°. Overall, the resistance to CE and JIE of Inconel 625 is much superior to that of Inconel 600.     

Non-destructive testing of welds with the 3MA-analyzer

For the microstructure and stress determination in the surface near region ( < 1 mm) micromagnetic non-destructive quantities can be used. This contribution gives an actual survey about the possibilities of determining non-destructively hardness and residual stress values in the weld region.     

A quartz-crystal-microbalance technique to investigate ion-induced erosion of fusion relevant surfaces

We describe a highly sensitive quartz-crystal-microbalance technique capable of determining erosion as well as implantation and retention rates for fusion relevant surfaces under ion bombardment. Total sputtering yields obtained with this technique for Ar ion impact on polycrystalline gold and tungsten surfaces are presented. The results compare well with existing experimental data as well as theoretical predictions and thus demonstrate the feasibility of the developed technique. Our setup is capable of detecting mass-changes as small as 10⁻⁵ μg/s, which corresponds to a removal of only 10⁻⁴ W monolayers/s.     

Development,computer simulation and performance testing in sodium of an eddy current flowmeter

Sodium is used as a coolant in Liquid Metal Fast Breeder Reactor (LMFBR). Sodium flow measurement is of prime importance both from the operational and safety aspects of a fast reactor. Various types of flowmeters namely permanent magnet, saddle type and eddy current flowmeters are used in FBRs. From the safety point of view flow through the core should be assured under all operating conditions. This requires a flow sensor which can withstand the high temperature sodium environment and can meet the dimensional constraints and be amenable to maintenance. Eddy current flowmeter (ECFM) is one such device which meets these requirements. It is meant for measuring flow in PFBR primary pump and also at the outlets of the fuel sub-assemblies to detect flow blockage. A simulation model of ECFM was made and output of ECFM was predicted for various flowrates and temperatures. The simulation model was validated by testing in a sodium loop. This paper deals with the design, simulation and tests conducted in sodium for the eddy current flowmeter for use in the Prototype Fast Breeder Reactor (PFBR).     

Analytical prediction of friction factors and Nusselt numbers of turbulent forced convection in rod bundles with smooth and rough surfaces

A simple analytical method was developed for the prediction of the friction factor, f, of fully developed turbulent flow and the Nusselt number, Nu, of fully developed turbulent forced convection in rod bundles arranged in square or hexagonal arrays. The friction factor equation for smooth rod bundles was presented in a form similar to the friction factor equation for turbulent flow in a circular pipe. An explicit equation for the Nusselt number of turbulent forced convection in rod bundles with smooth surface was developed. In addition, we extended the analysis to rod bundles with rough surface and provided a method for the prediction of the friction factor and the Nusselt number. The method was based on the law of the wall for velocity and the law of the wall for the temperature, which were integrated over the entire flow area to yield algebraic equations for the prediction of f and Nu. The present method is applicable to infinite rod bundles in square and hexagonal arrays with low pitch to rod diameter ratio, P/D<1.2.     

低浓去污预氧化过程中因科镍690合金我因科镍600合金的电化学 …

介绍了低浓去污预氧化过程中两种蒸汽发生器材料因科镍６９０合金及因科镍６００合金（下面简称６９０合金及６００合金）在不同邓氧化剂中脱膜效果（以铬的释放曲线表征）实验、腐蚀电位迁移测试和极化曲线测试。结果表明,６００合金在碱性高锰酸钾（ＡＰ）中的脱膜效果好于酸性高锰酸钾（ＮＰ）中,而６９０合金则在（ＮＰ）中的脱膜效果好于ＡＰ中。并且在ＮＰ中,随着硝酸浓度的增加,６００合金的腐蚀电位向正方向移动,６９０     

Simplified analysis of shrinkage in pipe to pipe butt welds

Generally some shrinkage is typical of butt welding of pipes. Shrinkage due to butt welding could be more pronounced and significant in thin wall stainless steel pipes because the thermal expansion coefficient is roughly one and half times that of carbon steel. An axisymmetric finite element evaluation of hoop shrinkage associated with circumferential butt welds in thin wall stainless steel pipes was performed. Actual shrinkage data for a larger (24 in. diameter, 0.375 in. wall thickness) pipe and a smaller (4 in. diameter, 0.237 in. wall thickness) pipe were utilized. The results indicate that very localized residual stresses in excess of yield strength produced during cooldown of metal in the weld and heat affected zones cause redistribution of the stresses. A simplified elastic–plastic analysis approach was developed with adjustments for section modulus and Poisson’s ratio, and the strains due to radial shrinkage were calculated for inside and outside surfaces of the pipe at the weld center line. From the strain point of view, the strain values in the circumferential direction were about 1.4% for the larger size pipe and 3.4% for the smaller size pipe. The strain values in the axial direction were 2.5% for the larger pipe and 5.9% for the smaller pipe. It is concluded that these levels of strains are not detrimental in nature. However, for the smaller pipe they are on the high side and it is recommended not to use the pipe for elevated temperature service. Residual stresses were also calculated for inside and outside surfaces of the pipe at weld center line using a simplified elastic–plastic approach and a bilinear stress–strain curve and compared with published data indicating a general agreement.     

Model of the flaw size distribution in welds

The principles of probabilistic fracture mechanics are briefly described. It is concluded that the lack of input data prohibits the derivation of absolute reliability numbers. The concept is, however, applicable to sensitivity studies. As one example, a crack generation model is explained, where the crack size distribution is derived from very simple geometrical considerations, but taking into account the real fabrication process.     

Uncertainty and risk associated with the Charpy impact energy of multi-run welds

A model for determining the uncertainty and risk associated with the Charpy impact energy of multi-run welds is developed. Using the model, it is demonstrated that a ductile-to-brittle transition region determined from fitting Charpy impact energy at several test temperatures is associated with large uncertainty. The uncertainty in the location of the transition region is strongly dependent on the number of test temperatures, the way the multi-run weld is sampled by the Charpy V-notch, the choice of the test temperatures and the variation of the impact energy at the test temperatures. With increasing the number of test temperatures, the uncertainty associated with the location of the transition region can be reduced significantly.A conservative model of the variation of the Charpy impact energy at a specified test temperature is also developed based on determining an upper bound of the variance of the Charpy impact energy. On this basis, a risk assessment model is developed for detecting impact toughness degradation with a high degree of confidence.A general equation is derived regarding the probability of failure from multiple impacts following a homogeneous Poisson process in a finite time interval. On its basis a method for setting risk-based reliability requirements is also proposed regarding the maximum acceptable number density of impacts in the time interval. The reliability requirements guarantee that if the number density of the impacts is within a specified envelope, the risk of impact failure in the time interval will remain below a maximum acceptable level.     

Matching of ultrasonic flaw sizing methods to the defect type and location

Sizing techniques are discussed; they are specialized techniques for fitness for purpose inspection on defects detected by other ultrasonic methods. Since amplitude evaluation is somewhat reserved for quality assurance applications and can only give a rough classification. Nevertheless it is necessary to point out that this is the most important approach in today's NDT-practice. The sizing accuracy which can be expected for amplitude evaluation is in general not better than 50% whereas with other specialized procedures 10%–15% sizing accuracy can be expected.     

超灵敏小型回旋加速器质谱计研制中计算机技术的应用

介绍SMCAMS物理设计和磁场测量阶段中一系列的计算机技术的应用,包括电磁场的数据计算,粒子动力学计算,注入引出系统计算和磁场测量与垫补中的应用.     

Measurements of the acoustic field on austenitic welds: a way to higher reliability in ultrasonic tests

In nuclear power plants many of the welds in austenitic tubes have to be inspected by means of ultrasonic techniques. If component-identical test pieces are available, they are used to qualify the ultrasonic test technology. Acoustic field measurements on such test blocks give information whether the beam of the ultrasonic transducer reaches all critical parts of the weld region and which transducer type is best suited. Acoustic fields have been measured at a bimetallic, a V-shaped and a narrow gap weld in test pieces of wall thickness 33, 25 and 17 mm, respectively. Compression wave transducers 45, 60 and 70° and 45° shear wave transducers have been included in the investigation. The results are presented: (1) as acoustic C-scans for one definite probe position, (2) as series of C-scans for the probe moving on a track perpendicular to the weld, (3) as scan along the weld and (4) as effective beam profile. The influence of the scanning electrodynamic probe is also discussed.     

Comparison of critical circumferential through-wall crack lengths in welds between pieces of straight pipes to welds between straight pipes and bends, with and without internal pressure at force- and displacement-controlled bending load

In many research projects methods to calculate critical circumferential through-wall cracks have been developed and verified. During the last years, the differentiation between force- and displacement-controlled loading has been shown to be of significant importance. So it was looked at with more interest in new analytical methods to calculate the critical crack length. Most of the approaches applied in the safety analysis of piping systems assume defect at welds connecting pieces of straight pipes. But in nearly all cases in modern power plants the true position of the welds in the piping system is not correctly represented, as in those systems only few welds connect parts of straight pipes. Most of the connections are situated between pipes and bends, bends with elongated ends, nozzles or T-parts. This paper presents a non-linear finite element (FEM) study covering an essential part of the relevant piping parameters of nuclear power plants primary and secondary system. It compares defects in circumferential welds between straight pipes to those joining pipes to elbows. In the case of displacement controlled loading, e.g. as due to restrained thermal expansion, which is one of the most severe load cases for most of the welds, we find, that the calculated J-integral values, and so the critical crack length are of comparable size. At force-controlled loading the codes require stronger limitations to the allowable forces and moments. In the regime of allowable loads, we find that the critical crack sizes in welds near bends are not significantly longer than the ones connecting straight pipes. In the cases where we have to consider in the safety analysis of piping systems, it is a realistic approach to use the methods accepted for welds between pipes to calculate the critical crack length in welds near bends.     

冷坩埚技术在核废物处理中的应用

阐述了冷坩埚的技术原理,概述了国外冷坩埚技术在核废物处理中的应用和发展,指明了在核废物处理研究中应用冷坩埚技术的重要性,展望其应用前景。