Application of eddy current inversion technique to the sizing of defects in Inconel welds with rough surfaces

This paper evaluates the applicability of eddy current inversion techniques to the sizing of defects in Inconel welds with rough surfaces. For this purpose, a plate Inconel weld specimen, which models the welding of a stub tube in a boiling water nuclear reactor is fabricated, and artificial notches machined into the specimen. Eddy current inspections using six different eddy current probes are conducted and efficiencies were evaluated for the six probes for weld inspection. It is revealed that if suitable probes are applied, an Inconel weld does not cause large noise levels during eddy current inspections even though the surface of the weld is rough. Finally, reconstruction of the notches is performed using eddy current signals measured using the uniform eddy current probe that showed the best results among the six probes in this study. A simplified configuration is proposed in order to consider the complicated configuration of the welded specimen in numerical simulations. While reconstructed profiles of the notches are slightly larger than the true profiles, quite good agreements are obtained in spite of the simple approximation of the configuration, which reveals that eddy current testing would be an efficient non-destructive testing method for the sizing of defects in Inconel welds.     

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.     

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).     

Investigation of the behaviour of cracks in a dissimilar weld

Using the finite-element method the behaviour of an axial and a circumferential crack in a dissimilar weld of the ferritic steel X20 CrMoV 12 1 and the austenitic X10 NiCrAITi 32 20 has been investigated. Stresses due to internal pressure and stresses resulting from temperature cycling due to different thermal expansion coefficients have been calculated. The circumferential crack located within the austenite in the region of maximum longitudinal stress is loaded at its deepest point by a stress intensity factor of 473 The axial crack is loaded up to a stress intensity of 325 N mm^−3/2 not at its deepest point, but within the X20 CrMoV 12 1. The resulting crack growth for both cracks has been estimated and compared with a component test with artificial defects in a dissimilar weld undergoing similar loadings.     

The general corrosion behavior of ENiCrFe-7 weld overlay cladding material in deoxygenated high-temperature and high-pressure water

The general corrosion behavior of Alloy ENiCrFe-7 in deoxygenated high-temperature and high-pressure water was investigated. The results showed that the precipitates of Alloy ENiCrFe-7 included niobium carbide and Al-Ti-O compounds, and the weight gain increased fast firstly before 2250 h, then the weight gain slowed down. There were obvious large particles spread on denser oxide film after 3000 h exposure. Ni was present at a single chemical metallic Ni state, Feⁿ⁺ content of the outer layer was close to 60%, which was much higher than that of the matrix. The oxide film consisted of an inner layer and an outer layer, the inner layer was mainly composed of Cr₂O₃ and the outer layer was mainly composed of Fe₃O₄ and FeCr₂O₄. Finally, it is found that the preferential corrosion location of pitting was niobium carbide precipitates by in same site observation, while Al-Ti-O compounds was not dissolved in deoxygenated high-temperature and high-pressure water for 1500 h exposure. The size and number of the pitting was not significantly changed with increasing exposure time.     

The growth of fatigue cracks in the creep range from the point of view of ductile fracture

This paper deals with experimental and numerical investigations of crack growth in 1% CrMoV steels in the creep temperature range. For the load and displacement controlled fatigue tests with predominantly plastic deformation amplitudes, centre cracked panels and CT-specimens of different thicknesses were used.For describing the crack growth per load cycle the crack tip displacement δ_t and the J-integral were applied, whereby the question arises whether the various fracture concepts are applicable unrestrictedly or where they meet their limits of validity.Detailed theoretical-numerical calculations using the finite element method were expected to yield information about the path independence of the J-integral. Special importance was therefore attached to investigate the J-integral, both cyclical and with reference to the instantaneous state of deformation, by comparing the different J-integral values (ΔJ, J) with the experimental method of Dowling/Begley.     

Fatigue cracks in Eurofer 97 steel: Part II. Comparison of small and long fatigue crack growth

The fatigue crack growth rate in the Eurofer 97 steel at room temperature was measured by two different methodologies. Small crack growth data were obtained using cylindrical specimens with a shallow notch and no artificial crack starters. The growth of semicircular cracks of length between 10-2000 μm was followed in symmetrical cycling with constant strain amplitude (R_ε = −1). Long crack data were measured using standard CT specimen and ASTM methodology, i.e. R = 0.1. The growth of cracks having the length in the range of 10-30 mm was measured. It is shown that the crack growth rates of both types of cracks are in a very good agreement if J-integral representation is used and usual assumptions of the crack closure effects are taken into account.     

In-service inspection of PWR steam generators by eddy currents; French achievements in defect detection and evaluation

In a nuclear plant the steam generator tubes must be efficiently inspected. The highest possible detection sensitivity is necessary to get a clear decision for plugging the rejected tubes. For this reason multi-frequency eddy current examinations have been developed at the Commissariat à l'Energie Atomique and industrialized by Intercontrôle. Defects are characterised case by case as the needs arose on the site: sludge height determination; measurement of tube deformation under pressure of oxides; determination of tube degradation due to the pressure of foreign bodies; detection of cracks under coatings; detections of cracks at the end of flanging. Problems unsolved by standard probes were dealt with by rotating-head machines. This report sums up part of the research undertaken in the field of eddy current testing.     

Numerical evaluation of the growth rate material parameters in fatigue propagation of surface flaws

Fatigue propagation of a surface flaw in a plate was studied using the Paris model. A large variation was obtained for the coefficient of the Paris model along the crack front when the classical technique was used. A new technique, which gives a fairly constant value for this coefficient, was evaluated and discussed. Both techniques gave virtually the same and fairly constant value for the exponent of the Paris model along the crack front.     

The evaluation on the effect of shell thickness reduction for ITER vacuum vessel sector 1 and 6

The structural analyses of vacuum vessel have been performed to investigate the effect of shell thickness reduction on structural integrity. The finite element models of vacuum vessel considering original design and thickness reduction have been developed. The expected maximum thickness reduction possibly caused by forming and bending processes during fabrication was applied to the curved region of the analysis models. The linear elastic and nonlinear limit analyses have been performed. The structural integrity of main vessel including lower port stub extension has been verified in accordance with the requirements of RCC-MR. It is concluded that the inner and outer shells of main vessel still have enough strength margins under pressure and VDE (Vertical Displacement Event) load conditions in spite of thickness reduction. These results have been reviewed and approved by ITER Organization and ANB (Agreed Notified Body).     

Combined analysis and evaluation of piping systems using the computer

The organization and theoretical aspects of a computer program designed primarily for the analysis and evaluation of power piping with particular emphasis on nuclear class I and II piping are described. The program includes a one-dimensional (radial coordinate in a cylindrical system) thermal analysis which computes the required entries in eqs (10), (11) and (13) of section NB 3600. Six types of loadings may be considered simultaneously: deadweight, sustained design mechanical loads, thermal expansion, thermal anchor movement, earthquake-response spectra or time history, and other anchor movement. For dynamic analysis the generalized Rayleigh-Ritz method using unit loads to generate the trial vectors is utilized. This method provides a consistent reduction of both the stiffness matrix and the mass matrix. The Ritz method is not without its problems, however, in the selection of the so-called ‘master nodes’. While getting enough master nodes is generally not a problem (easier than lumping masses) it is easy to over-specify and therefore introduce ill conditioning. To overcome this shortcoming a method is described for checking the trial vectors for their degree of parallelism in kinetic energy space, and for automatically removing those degrees of freedom which may lead to difficulties. Sample solutions with the program are presented.     

Modelling the early development of secondary side stress corrosion cracks in steam generator tubes using incomplete random tessellation

A thorough understanding of the secondary side stress corrosion cracking of Inconel 600 in steam generator (SG) tubes seems to be still somewhat in the future. Especially the early phase of the development of cracks, also called the initiation phase, is beyond the present state-of-the-art explanations. An effort was, therefore, made to propose modelling and visualisation of the kinetics of secondary side stress corrosion crack initiation and growth on the grain-size scale:

An incomplete random tessellation is used to approximate the random planar grain structure.

The crack initiation is modelled by random processes, taking into account the most important factors such as proximity of the aggressive medium and the orientation of the grain boundaries relative to the stress field.

The stochastic process describing crack growth accounts for crack branching, coalescence and interference between neighbouring cracks.

Several numerical examples are provided to demonstrate the versatility of the proposed method. Reasonable qualitative agreement with metallographic results is shown.     

Measurement of transient thickness between the body and graze layers of ancient porcelains using microprobe EDXRF technique

The oxide contents of TiO2, MnO, SrO and Fe2O3 in the body and graze layers of the Jiao-Tan-Xia (JTX) and Lao-Hu-Dong (LHD) porcelains in Southern Song Dynasty (1127-1279 A.D.) have been determined using an International Eagle-Ⅱ μ-probe EDXRF spectrometer. The results show that the contents in the body are much different from those in the graze one. Therefore, the transient thickness (TT) between the body and graze layers can be measured through determination of a distance of the drift change in the chemical contents. The TT average for the JTX porcelains is 161μm, while that for the LHD porcelains is 258μm, which are consistent with a range of 0.15-0.3mm inthe Ru-Yao porcelains. The different TT is related to the variances in firing temperature and raw material for manufacturing the respective porcelains.     

Considerations on fatigue stress range calculations in nuclear power plants using on-line monitoring systems and the ASME Code

Nuclear power plants are generally designed and inspected according to the ASME Code. This code indicates stress intensity (S_INT) as the parameter to be used in the stress analysis of components. One of the particularities of S_INT is that it always takes positive values, independently of the nature of the stress (tensile or compressive). This circumstance is relevant in the Fatigue Monitoring Systems used in nuclear power plants, due to the manner in which the different variable stresses are combined in order to obtain the final total stress range. This paper describes some situations derived from the application of the ASME Code, shows different ways of dealing with them and illustrates their influence on the evaluation of the fatigue usage factor through a case study.     

Analysis of the deformation and damage development of a dissimilar weld operating in the creep regime

The analysis of the deformation and damage behaviour of stress rupture tests with specimens out of the dissimilar metal weld seam 12% Cr-steel welded with a nickel base electrode for alloy 800 exhibits two competing processes:

• - Crack initiation occurs along the melting line due to high thermal stresses;

• - Creep deformation and damage concentrates in a heat affected zone of the ferritic 12% Cr steel due to long term stresses. The velocity of stress relaxation determines the resulting damage mechanism. At high temperatures with predominant creep deformation the cracks initiated in the melting line arrest and the creep deformation is concentrated in the heat affected zone (HAZ). At lower temperature the fracture area along the melting line increases. Long term tests at 535°C lead to lower stress rupture values compared to the scatterband of X 20 CrMoV 12 1 due to the reduced cross section after crack initiation in the melting line.

The analysis of stress rupture tests leads to the conclusion that grinding of melting line cracks is a reasonable measure because of sufficient stress relaxation.     

Estimate of the space station shielding thickness at a USLab site using ALTEA measurements and fragmentation cross sections

In this work we presented a method to estimate a local shielding thickness of the International Space Station (USLab), expressed as equivalent of aluminum. The calculation took advantage of total and partial charge-changing cross sections, both measured experimentally and computed theoretically, to obtain the total survival fractions of ions with charge 5?Z?26 when transversing the space craft hull. The combination of these values with the measured relative abundances outside the space station provided the estimates of the internal relative abundances at several thicknesses. The results have been compared with data measured by ALTEA experiment in the USLab to find the thickness that minimizes the discrepancy between the calculated data and the measurements: the result yielded to an estimate of the shielding thickness at ALTEA site of about 5 cm of aluminum equivalent.     

Biological studies using mammalian cell lines and the current status of the microbeam irradiation system,SPICE

The development of SPICE (single-particle irradiation system to cell), a microbeam irradiation system, has been completed at the National Institute of Radiological Sciences (NIRS). The beam size has been improved to approximately 5 μm in diameter, and the cell targeting system can irradiate up to 400–500 cells per minute. Two cell dishes have been specially designed: one a Si₃N₄ plate (2.5 mm × 2.5 mm area with 1 μm thickness) supported by a 7.5 mm × 7.5 mm frame of 200 μm thickness, and the other a Mylar film stretched by pressing with a metal ring. Both dish types may be placed on a voice coil stage equipped on the cell targeting system, which includes a fluorescent microscope and a CCD camera for capturing cell images. This microscope system captures images of dyed cell nuclei, computes the location coordinates of individual cells, and synchronizes this with the voice coil motor stage and single-particle irradiation system consisting of a scintillation counter and a beam deflector. Irradiation of selected cells with a programmable number of protons is now automatable. We employed the simultaneous detection method for visualizing the position of mammalian cells and proton traversal through CR-39 to determine whether the targeted cells are actually irradiated. An immuno-assay was also performed against γ-H2AX, to confirm the induction of DNA double-strand breaks in the target cells.     

A comparative study of variable and constant thickness high precision triangular plate bending elements in the analysis of variable thickness plates

For the analysis of plate bending and vibration problems high precision triangular elements are very attractive. The effect of thickness variation can be included in these elements very easily at the expense of the computer time, for analysing variable thickness plates. On the other hand, variable thickness plates can be analysed as stepped plates with constant thickness elements with less computer time. In this paper a comparative study of the use of variable thickness and constant thickness elements for analysing variable thickness plates is attempted by solving three typical problems. The results show that for static problems there is a marginal accuracy when using variable thickness elements as against average thickness elements. However, for vibration problems even though there is not much difference in the fundamental frequency parameter obtained by using both elements, the higher modes obtained by using variable thickness elements are very accurate.     

Atom probe characterization of copper solubility in the Midland weld after neutron irradiation and thermal annealing

An atom probe field ion microscopy characterization has been performed to determine the copper matrix concentration in a submerged arc beltline weld of the Midland Unit 1 pressurized water reactor after four conditions: unirradiated, unirradiated and annealed for 168 h at 454°C, neutron-irradiated in a test reactor to a fluence of 1.1×10²³ n m⁻² (E>1 MeV) at a temperature of 288°C, and neutron-irradiated and annealed for 168 h at 454°C. Atom probe analysis of the unirradiated material revealed a substantial depletion of the copper in the matrix to 0.119±0.007 at.% Cu from the bulk value of between 0.18 and 0.28 at.% Cu. Annealing the unirradiated material produced intragranular copper-enriched precipitates and reduced the matrix copper level by 25% to 0.088±0.012 at.% Cu. Neutron irradiation also produced copper-enriched precipitates and reduced the matrix copper level by almost 50% over the stress relieved material to 0.058±0.008 at.% Cu. Annealing the neutron-irradiated material reduced the matrix copper level further to 0.050±0.010 at.% Cu. These results indicate that the annealing treatment coarsens the copper-enriched precipitates produced during neutron irradiation with a slight decrease in the matrix copper content.     

The effect of inspection and repair on the size distributions of the weld imperfections in nuclear power plant pipes

The weld defects appearing in the pipes of the main steam system in a BWR power plant were studied. The effect of inspection rejection and repair on the imperfection size distribution was analysed. The size distributions before and after the inspection rejection as well as after the repair procedure were of the form a + b/x², where x is the imperfection size and a and b are coefficients. The portion of the rejected defects had the size dependence near to the form of the cumulative Gaussian function. The effect of repair on the initial size distribution had the form of the cumulative Poisson distribution.