|NDT.net - January 2001, Vol. 6 No. 1|
2nd International Conference on NDE in Relation to Structural Integrity for Nuclear and Pressurized Components, New Orleans May 2000.
EFFECT OF CLADDING ON INSPECTION RESULTSM. Biטth, L. Fabbri, J.L. Monjaret
European Commission, JRC-IAM
P.O. Box 2
1755 ZG Petten, The Netherlands
Corresponding Author Contact:
Email: firstname.lastname@example.org, Web: http://www.jrc.nl
Carbon steel components are cladded such as the reactor pressure vessel and other primary circuit components of nuclear power plants to protect them against the primary water corrosion.
Inspection procedures based on NDE are playing an important role in structural integrity assessment and this involves the knowledge of the defects present in the structure in the areas particularly subject to stresses induced by the pressurized thermal shock (PTS): sub clad defects of the planar type generally perpendicular to the surface (under clad cracks, weld cracks, lacks of fusion…), defects in the cladding and cladding defects.
Inspection systems are made of a mix of NDE techniques, setting or calibrating procedures, decision steps, scanning systems, recording and illustration tools and software’s. Moreover, inspection procedures often involve information interpretation of indications relying on the skills of the operator. Inspection procedures are thus not measurement techniques and their performances in defect detection; location, classification and sizing cannot be represented by simple figures and uncertainty margins.
To be able to use NDE inspection data in a structural integrity assessment reasoning or model, it is essential to know whether all the defects above a certain size were detected and reported, and the accuracy on the reported defect sizes in depth and in length and on the ligament, and finally the type of defect.
The present analysis aims at the answer to these questions in the specific case of the cladding inspection. It gives engineering trends, average values, which can be employed by the structural integrity engineer.
The main types of defects susceptible to appear in the cladding area are summarized on tables 1 (manufacturing defects) and 2 (service-induced defects).
|Defect type||Location of defect|
|Lack of bond/fusion||Clad-base metal interface|
|Inclusions||Between strip in strip cladding|
|Volumetric flaws||Clad-base metal interface|
|Cracks in cladding||Usually in the first layer of cladding|
|Crack in ferritic||Heat affected zone in ferritic steel. Parallel to the cladding interface|
|Table 1: Manufacturing defects|
|Type of flaw||Location of flaw|
|Loss of cladding thickness||Cladding|
|Surface-breaking transgranular Cracking||Clad to base metal interface and cladding|
|Surface-breaking intergranular Stress corrosion cracks||Clad to base metal interface and cladding|
|Table 2: Service induced defects|
Despite of the fact that defects are of all kinds, the effectiveness of NDT refers mainly to planar type defects, generally perpendicular to the surface, with a tilt angle from -30 to +30 degrees. Such defects are correctly detected and sized when using specifically designed techniques or usual techniques set at high level of sensitivity. This condition of sensitivity leads to many indications and also to false calls.
Such defects are lacks of fusion and cracks (mechanical fatigue, thermal fatigue, corrosion, reheat, solidification).
Volumetric defects such as inclusions and porosities are also considered but more often as examples as the detection and sizing performance of NDE procedures is often better with such defects compared to planar ones.
An inspection procedure based on NDE techniques is a process involving many (up to
22 in the PISC II exercise) individual techniques, decision steps, calibration
rules, indication treatment, reporting guidelines…
The inspection procedures are also categorized in two groups corresponding to the families of inspection procedures.
For usual reactor pressure vessels with wall thickness ranging from 100 to 250 mm it has to be admitted that no industrially applied NDE technique, even the advanced ones, can claim for better precision than + / - 1mm in the defect depth and length sizing.
Reliability of inspection procedures based on NDE techniques is made of three constitutive elements which are important to be defined correctly to understand the meaning of NDE evaluations and to use NDE reliability data correctly in view of structural integrity assessment.
The relation between these elements is summarized by the following formula:
The fact that the inspection is conducted on the right component or part of the component where defects can develop, is not considered here.
As human factor effects are not really predictable, discussion or use of NDE reliability data is misleading. What counts first is the knowledge of the effectiveness E of the inspection that could be defined as being
A quality assurance programme, knowing that the operator effect could reduce the effectiveness to as little as zero in case of adverse situations, distraction or de-motivation must control the human factor.
This paper considers thus mainly inspection effectiveness and not inspection reliability. The inspection effectiveness is, in turn, considered stressing the value and limits of the intrinsic capability of the techniques. The limiting function g( AP ) of the parameters of the application such as access, surface roughness depends of the given situation.
The most relevant reference to this category of components is the PISC programme.
Several other programmes were conducted but either produced results of lesser
statistical significance or generally confirmed or anticipated the PISC
From PISC, conclusions were drawn on the detection, location and sizing capability:
Fig 1: Detection probability of ASME-type procedures with recording level at 20% DAC as a function of the defect through wall size for the three categories of defects. |
A): smooth, planar for ultrasonic wavelength, sharp crack edges; B): hybrid defects or rough defects like hot tears; C): volumetric defects.
Inspection of the sub clad area with specialized procedures
Few blind tests results relevant with the under clad defects theme and suitable inspection procedures were produced up to now by RRTs conducted by independent institutions: PISC has included several near surface defects in the assemblies. However, either, the inspection procedures did not specifically address the sub clad defect in the context of PTS, or the statistical significance of results is not established: their evaluation was not conducted in the suitable context or only on very few sub clad defects.
The NESC exercise (5) addresses the case of the PTS type defects detection and sizing but the RRT involving specific inspection procedures was conducted on 20 defects only.
Very safe detection is reached for defects of a size in depth of about 10 mm for perpendicular planar defects using specific and advanced inspection techniques.
Information resulting from qualification programmes and trials in France, Sweden, USA, and Germany for European and Eastern plants, can be used as well.
Based on the above source of information, it is anticipated that inspection procedures are or will be qualified and applied along the new qualification requirements in several countries for the safe detection of sub clad defects as small as 5mm in depth and for sizing with uncertainties limited to few mm.
Inspection of the cladding
Again, few neutral exercises addressed the theme of cladding inspection in itself. PISC plates contained some defects in the cladding: porosities, few cracks, disbonding of the cladding.
Due to the frequent rough surface of the cladding, inspection of the cladding itself is generally ineffective. Very specific techniques show good detection performance if the clad surface is machined or ground. Such inspection techniques are the ones that can be used for thin walled cast austenitic components or welds.
In ideal conditions, defects of the order of 50% of the clad thickness are well detected. Sizing capability should not be considered.
The reliability of inspection procedures depends heavily on the influential parameters:
Correct classification of defects
The results of RRTs, when considering the classification of defects, as a performance variable were generally not convincing. Results of good procedures indicate that the correct sequence to follow should be: detection - classification - sizing. For heavy section components, the use of specific techniques, on request, improved the differentiation between planar and volumetric flaws.
|Fig 2: Combined effect on tilt and skew angles on the defect response in amplitude, for smooth planar cracks with sharp crack tips in the vessel wall.|
Obviously, the knowledge of the ligament is essential to perform an integrity analysis. Performant NDE techniques can evaluate such values in the same way that they size a defect: by the detection and location of a crack tip. Ligament measurement capabilities are thus corresponding to the ones of capable sizing techniques in heavy section steel component.
Conclusions were that most of the parameter influences are dominated or reduced by the calibration of the technique if done properly. Variations measured, after calibration, using the amplitude of response as variable are generally less than 6 dB.
Importance of the cladding
The effects of the cladding characteristics on the inspection performance of underclad zones were measured in PISC II (6). The importance of these effects depends on the type of cladding and on the NDE techniques used.
Ultrasonic techniques can find important disturbances created by the complex structureexture represented by the cladding strips or wires and heat affected zones. The back-wall echo, used as a measure of the attenuation, has shown in some cases of wrong selection of transducer frequencies, up to 30 dB drops in locations corresponding to the overlay of passes (channels). The figure 3 illustrates this influence. Beam distortions are also introducing important sizing errors or even false calls. Such local variations have to be considered in qualification plans.
|Fig 3: Combined effect on tilt and skew angles on the defect response in amplitude, for smooth planar cracks with sharp crack tips in the vessel wall.|
The PISC II parametric study concluded on this subject that the existence of a stainless steel cladding may have important effects on the beams of ultrasonic transducers, local variations of amplitude of 30 dB being observed at 2 MHz on a plane infinite reflector with a shear wave contact probe working in emission-reception.
Very tight defects
When defects are very tight with contacts between the faces, UT waves can propagate through the defect. The effect of compressive stresses on planar defect response to ultrasonic testing can lead to loss of signal amplitude, which is in some cases unacceptable for industrial inspection procedures.
Importance of human factors
Action 7 of PISC II analyzed some human factors that influence the inspection results. Conclusions were detailed in specific reports. Human errors were encountered at different levels of the inspection process during calibration, during scanning, by miss-observation of the screen, and during evaluation. These observations on the importance of human factors were noted in several exercises including the one in NESC and suggest that similar events can happen in the reality of industrial inspections.
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