|NDT.net - August 2002, Vol. 7 No.08|
3nd International Conference on NDE in Relation to Structural Integrity for Nuclear and Pressurized Components, Nov 14-16, 2001, Seville Spain
The lifetime extension of the existing installations for electricity production requires the development of innovative techniques for the inspection of critical components. Such techniques must detect changes in the materials before the appearance of macrostructural defects and therefore allow planning actions for failure prevention.
The project GRETE is supported by the European Commission within the Fifth Framework Programme for ŤNuclear Fissionť. The aim of this project is to assess the capability and the reliability of novel non-destructive techniques by means of a round robin exercise concerning neutron irradiation damage in reactor pressure vessels, and thermal fatigue of piping.
Different series of aged samples, metallurgically and mechanically characterised, will be tested by the participants using various non-destructive techniques (i.e. magnetics, thermoelectricity, ultrasounds and dynamic indent).
The purpose of this paper is to present the GRETE project through its objectives, its technical work programme and its fifteen participants. The status and achievements obtained so far are also summarised in this paper.
non destructive testing, material aging, irradiation, fatigue
The material aging of major critical components of nuclear installations due to in-service conditions may lead to a degradation of their mechanical characteristics. The early detection of material changes and their monitoring using innovative non-destructive testing techniques would allow planning actions in order to prevent the apparition of macroscopic damage (e.g. cracks).
One major difficulty in using these particular techniques is to correlate the changes in the measured NDT signals with the microstructural changes occurred in the material due to aging. This problem may be solved through careful microstructural examinations of the material damage.
European Commission supports GRETE project in the framework of the "Nuclear Fission" programme (contract n° FIKS-CT-2000-00086). The purpose of this paper is to present the project through its work-programme and partnership.
The objective of the project GRETE is to illustrate the potential use of NDT techniques for the monitoring of material degradation through two examples:
Neutron irradiation of reactor pressure vessel steel
In the framework of a surveillance programme, the effect of irradiation damage on the mechanical characteristics of the reactor pressure vessel is anticipated through the expertise of irradiated specimens set close to the core of the reactor. These specimens are then evaluated by mechanical testing, therefore breaking them.
The monitoring of irradiation damage by non-destructive techniques would limit the destruction of the specimens, making possible their re-insertion in the reactor for further regular non-destructive measurements.
Thermal fatigue of piping
Cyclic thermal and/or mechanical loads may generate fatigue in the material. Two cases of leakage caused by high cycle thermal fatigue in the nuclear industry were recently reported: in 1998 on the reactor heat removal system of a French 1,450 MWe PWR (CHOOZ-B) and in 1999 on the chemical and volume control system of a Japanese 1,160 MWe PWR (TSURUGA-2).
The monitoring of fatigue damage by non-destructive techniques would prevent any unexpected leakage by the early detection of any significant microstructural change.
In the framework of GRETE, the ability of various NDT techniques to monitor the effect of aging (irradiation and fatigue) will be assessed through experimental measurements to be made on aged specimens representative of materials used in the nuclear industry. The non-destructive techniques that will be examined are different from standard inspection methods. The aim of standard techniques is to detect macroscopic defects like cracks, including for certain applications sizing and imaging. The proposed methods in GRETE are sensitive to any microstructural change in the material leading to a degradation of the mechanical properties of the component long before macroscopic cracks are initiated and eventually grow. However, these indirect methods require a careful interpretation of the signal measured in terms of microstructural evolutions due to aging in the material.
For each damage mechanism, the partners will prepare series of specimens. These specimens will be then tested using non-destructive techniques based on ultrasounds, magnetic properties, thermoelectricity and dynamic indentation. The NDT signals will be correlated with the microstructural changes observed in the material because of aging and with the mechanical properties of the material. At last, possible technology transfer towards the industry will be evaluated for practical use of these techniques by the utilities.
A consortium of fifteen partners has been formed to achieve the work programme of GRETE. These partners are utilities or their representatives, research centres and universities. A list is given in Table 1.
|Electricité de France||France|
|Netherlands Energy Research Foundation||The Netherlands|
|VTT Manufacturing Technology||Finland|
|Fraunhofer Institut für Zerstörungsfreie Prüfverfahren||Germany|
|Joint Research Centre of the European Commission||The Netherlands|
|AEA Technology National NDT Centre||United Kingdom|
|Centro de Investigaciones Energéticas Medioambientales y Tecnológicas||Spain|
|Austrian Research Centre||Austria|
|Atomic Energy Research Institute||Hungary|
|University of Hannover||Germany|
|Paul Scherrer Institute||Switzerland|
|National Research Institute||Czech Republic|
|Siempelkamp Prüf- und Gutachter- Gesellschaft mbH||Germany|
|Siemens Nuclear Power||Germany|
|Table 1: Partners of GRETE|
|Automated ball indenter||NRI-Rez|
|Magnetic Barkhausen Noise||
|Micromagnetic measurements||Fraunhofer Institute|
|Non Linear Harmonic Analysis of Eddy Current signals||University of Hannover|
|Table 2: NDT techniques for the characterisation of neutron irradiation damage.|
|Magnetic Barkhausen Noise||Fraunhofer Institute AEKI|
|Micromagnetic measurements||Fraunhofer Institute|
|Non Linear Harmonic Analysis of Eddy Current signals||University of Hannover|
|Fluxgate, Giant Magnetic Resistor, Superconducting Quantum Interference Device||
|Ultrasonic Scattering or Backscattering||
|Table 3: NDT techniques for the characterisation of fatigue damage|
The non-destructive techniques that will be used by each partner in the framework of this project are reported in Table 2 (neutron irradiation damage) and Table 3 (fatigue damage).
A brief description of each technique is proposed here below.
Magnetic Barkhausen Noise
The magnetic Barkhausen effect is observed as transient pulses induced across a search coil placed near or around the ferromagnetic material undergoing a change in magnetisation. These pulses can either be observed individually by counting and amplitude sorting or as a RMS signal as a function of the applied magnetic field. The BE signal arises from irreversible magnetic domain wall movements as domain walls become successively pinned and jump over obstacles in the material. These obstacles are typically dislocation defects, second phases or grain boundaries and consequently the technique is particularly sensitive to the microstructure and mechanical properties of the component. The technique is also sensitive to the internal stress state because of the partial domain alignment along the maximum principal stress axis. Thus, tensile and compressive stresses usually increases and decrease the BE signal respectively.
Non Linear Harmonic Analysis
This technique utilises the whole magnetic hysteresis loop and the way in which it is influenced by the microstructural changes due to degradation. An oscillating sinusoidal magnetic field is applied to the material, and this is modified by the material that acts as a transfer function, so that a detector coil picks up a distorted signal, which is analysed for amplitude and phase of different harmonics of the original signal frequency. To calibrate, the variation of these parameters is fitted using a "multidimensional regression analysis" to provide the best correlation with material property. Some degree of selectivity to the different mechanical properties is achieved.
The 3ma Analyser System
The 3MA analyser system (Micromagnetic, Multiparameter, Microstructure and Stress Analysis) has been developed by the Fraunhofer Institute for Non-Destructive Testing (IzFP) at Saarbrücken in Germany. As its name implies, the instrument measures a combination of different magnetic parameters, enabling some degree of separation between variations in the stress and microstructure states. The 3MA analyser employs the techniques of magnetic Barkhausen, coercivity (derived from Barkhausen profiles) and magnetic field frequency harmonics. The instrument is designed for use in a wide range of applications including: detection of different heat treatments, residual stresses, hardness gradients and parameters loosely related to strength and toughness.
To achieve some quantitative measurement the 3MA analyser must be calibrated against samples containing the variations of interest. Indeed a great deal of work has been done by the researchers in investigating a large range of materials and heat treatment conditions. Recently, new approaches have been developed which concentrate on using linear multiple regression or neural network algorithms to calibrate the system for limited well defined set of specimen or component conditions. These calibrations rely on detailed mathematical variations formalism that notably does not involve any empirical or fundamental understanding of the physical principles of the magnetic techniques.
Physics of the Superconducting Quantum Interference Device
The Superconducting Quantum Interference Device (SQUID) is the most sensitive magnetic field sensing element known. One version of the SQUID, the thin-film DC SQUID, has two Josephson weak links interrupting a superconducting loop. The maximum supercurrent that can be passed through such a loop before a voltage develops (the critical current) is periodic in the magnetic flux passing through the loop, with period F0=2×10-15 T×m2. Typically SQUID electronics DC-bias the device close to the superconducting critical current, apply an AC modulation bias field to the loop, and feed back on a DC bias field to keep the voltage output at the modulation frequency constant. The DC feedback field is then directly proportional to the magnetic flux through the loop.
The SQUID sensor requires a cryogenic environment, since it must be superconducting to operate.
Physics of the Giant Magnetoresistance
The Giant Magnetoresistance (GMR) effect is a very large change in electrical resistance that is observed in a ferromagnet/paramagnet multilayer structure when the relative orientations of the magnetic moments in alternate ferromagnetic layers change as a function of applied field. Changes in resistance with magnetic field of up to 70% (typically up to 20%) were observed. The basis of the GMR is the dependence of the electrical resistivity of electrons in a magnetic metal on the direction of the electron spin, either parallel or antiparallel to the magnetic moment of the films (layers). Electrons that have a parallel spin undergo less scattering and therefore have a lower resistance. When the moments of the magnetic layers (e.g. NiFe) are antiparallel at low field, there are no electrons that have a low scattering rate in both magnetic layers, causing an increased resistance. At applied magnetic fields where the moments of the magnetic layers are aligned, electrons with their spins parallel to these moments pass freely through the solid, decreasing the electrical resistance. The resistance of the structure is therefore proportional to the cosine of the angle between the magnetic moments in adjacent magnetic layers.
GMR magnetic field sensors have recently been evaluated for use in many applications (e.g. geophysical exploration) and found to have a noise floor of 0.1 to 1.0 nT in an unshielded, unfiltered system.
The system is based on the Seebeck effect, which lead to a thermoelectric power in metals. Currently two devices have been developed, the first by EdF together with the Technical University INSA de Lyon, and the second by the JRC.
Laboratory measurements have established the variation of the voltage generated when a temperature gradient is applied to a metal, varies with hardness, toughness and with Cu content of reactor pressure vessel steels. The generated voltage drop DV is measured to give the coefficient DV/DT = DTEP.
As an example EdF has built a portable TEP system, which can be used on large components after some surface preparation. It has been demonstrated by the measurement of damage on a cast duplex steel elbow. The JRC device has shown its capability to detect materials damage induced by irradiation.
Ultrasonic Scattering or Back-Scattering
Ultrasonic scattering and Back-scattering are techniques based on the measurement of ultrasonic waves scattering back to the transducer from the microstructure of the material. The measurement can be performed based on the reflection received from the back-wall of the sample or on the back-scattered signals coming from the interior of the sample. These basic techniques can be complemented by measurement of sound velocity and center frequency of the back-wall echo. The measurements can be performed also in industrial environment with reasonable accuracy. For measurement of sound velocity special techniques based on the use of electromagnetic acoustic transducers (EMAT) are available. Under laboratory conditions the velocity of leaky surface acoustic wave can be measured precisely by using the V(z)-technique.
|Partner||Steel grade||Irradiation conditions||
Fluence (1019 n/cm2)|
[E > 1 MeV]
|UNI-||A533 cl. 1||VVER-2||254||0.0||0.94||5.7||10.2||-||8|
|TECNATOM||A533 cl. 1||PWR||290||-||0.7||2.1||5.2||-||6|
|Table 4: Irradiated specimens for NDT testing|
a: annealed 160 hours at 475°C after irradiation (2.3 1019 n/cm2)
b: irradiated (1.8 1019 n/cm2) + annealing 160 hours at 475°C + irradiation (2.0 1019 n/cm2)
c: irradiated (2.3 1019 n/cm2) + annealing 160 hours at 475°C + irradiation (3.8 1019 n/cm2) + annealing 160 hours at 475°C
A total number of 96 specimens will be used for NDT testing. EDF, TECNATOM, the University of Hannover and the National Research Institute provide such irradiated specimens (see Table 4). They are made of materials representative of reactor pressure vessel (A533 cl.1, A508 cl.3 and 12Kh2NMFA steels). The samples from EDF, TECNATOM and the National Research Institute belong to the surveillance programme of commercial reactors (PWR and VVER) whereas samples from the University of Hannover were irradiated in an experimental reactor (VVER).
The mechanical properties of these materials have already been measured in previous studies. They will be made available for the GRETE project; the irradiated samples for NDT testing are Charpy-V notch specimens, and in order to avoid local deformations those broken in the brittle domain have been selected. Non-irradiated specimens of the same material are also available for ND testing.
The NDT measurements on irradiated specimens will be performed in the hot cell facility of the Netherlands Energy Research Foundation. The test campaign is organised as a "blind exercise", the participants will not know the material and the ageing condition of the specimen under testing.
Two grades of austenitic stainless steel will be provided by EDF (AISI 304L) and by SIEMPELKAMP (AISI 321). These steels are widely used in power generating systems for piping and support structures.
Hourglass specimens designed according to ASTM E606 will be manufactured and tested (Low Cycle Fatigue) by the partners according to the experimental matrix in Table 5. In addition, two series of specimens will be cold rolled in order to see the effect of the presence of martensite (about 30%) on the fatigue behaviour of the material. The applied amplitude range of strain, will be ą 0.4 %.
The fatigue testing will be performed at room temperature and at 300°C (in service temperature) at a frequency of one Hertz. Three usage factors have been chosen: 0.6, 0.8 and 1.0.
|Partner||Steel grade||Material conditions||
|Table 5: Experimental matrix for fatigue testing|
From the total amount of 112 specimens manufactured, 96 will be fatigue tested. After that, EDF and the Paul Scherrer Institute will observe the microstructural changes related to fatigue damage (i.e. dislocation network and martensitic phase). The Technical University of INSA of Lyon (France) will support EDF in its microstructural examination and analysis.
The dislocation network will be observed through Transmission Electron Microscopy, Neutron Diffraction and advanced X-Ray diffraction methods. The presence of martensitic phase will be detected and its amount measured using metallography, X-ray diffraction and neutron diffraction techniques. Martensitic constituents and their distribution will be observed through Scanning and Transmission Electron Microscopy, Neutron and X-ray diffraction methods.
Scanning Electron Microscopy will be also used to assess the fracture mode, to identify the location and the distribution of micro-cracks and to identify the microstructural components that have played a role in the micro-crack initiation.
In the meantime, each partner (as specified in Table 3) will perform NDT measurements on 56 fatigue specimens.
The data obtained through non-destructive testing will be compared with the results of microstructural examination and mechanical test. The partners will try to establish a relationship between the microstructural changes, the NDT signals and the mechanical properties of the material.
Quantitative criteria (accuracy, reproducibility, dispersion of data) will be set and used to characterise the performance of each technique for monitoring materials damage.
The assessment of the capability and the reliability of innovative and reliable inspection techniques will achieved through:
To conclude the project recommendations will be drawn to exploit the potential use of the most promising non-destructive techniques. The industrial feasibility of non-destructive measurements on real components has to be evaluated and on site practical problems have to be taken into account.
In the GRETE project the main European actors in the field of the non-destructive assessment of material damage are collected. All participants possess a good knowledge and experience of aging issues in the industry, good skills in structural integrity, material science and non-destructive techniques. Moreover, each partner has a special knowledge and experience in one or more innovative, or at least non-conventional, non-destructive technique.
The main objective of GRETE is to assess the capability and the reliability of innovative inspection techniques by means of a round robin exercise.
Even if the intended field of application of the NDT methods participants in GRETE is nuclear power plants, the potential for using the same technique in different fields of industry is also remarkable. Off-shore and steel manufacturing, aircraft maintenance, integrity of railways, high-temperature piping of conventional power plants, process piping containing toxic gas/liquid and critical components of vehicles used for public transportation are just a few examples of areas where the same techniques could be used to improve safety and economical operation
The project could the precursor of a EU standardisation in the field of structure ageing assessment. On the basis of the obtained results, a EU guideline or validation procedure will be drafted.
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