Other Major Component Inspection IV Ultrasonic Testing of Copper Welds J. Pitkänen, T. Salonen, Posiva Oy, Finland A. Lipponen, M. Sarkimo, VTT Technical Research Centre of Finland, Finland R. Paussu, Fortum Nuclear Services, Finland
ABSTRACT Copper will be used for nuclear fuel disposal canisters. The copper canister consists of copper lid and copper tube. The canister will be sealed either with electron beam welding or friction stir welding. To ensure the quality of the sealing weld it will be inspected with several NDT-methods. One solution for quality control is to carry out ultrasonic inspection for the sealing weld. In this study the techniques used for ultrasonic testing of those copper welds will be presented. All the welds will be inspected with phased array technology. The copper material is challenging material to be tested. The copper properties affect strongly on the inspection quality and the way to evaluate the inspection has been developed. Items affecting to the inspections will be discussed. One of the largest effects on the inspections is grain size of the copper. The grain size effect will be evaluated for the detectability of defects. The other methods are based on different physical phenomenon like radiographic testing and they are locating partly different type of defects. The detectability of different types of defect will be increased by improving inspection techniques. One way is to use matrix phased array for inspection. This was already realized for study of plate EB-welds. This technique will be added for inspection of real size welds. The main reason is to improve detectability of curved weld defects. For improving techniques and verifying the results the ultrasonic modelling will be used. About 50 real size welds has been inspected with ultrasonic testing using at the beginning conventional techniques and in further phases more advanced techniques. After detection the indications must be analyzed and evaluated. The sizing of indications will be based on information gained study of several method - like TOFD, Sampling phased array, full matrix capture technique or matrix phased array application. Several reference specimen has been manufactured to study detectability and for set up purposes. The FBH of 3 mm will be used for reference defect. In front of weld the detectability is much better compared the detectability behind the weld, because of high attenuation caused by large grains in EB-welds. The effect is about 10 to 16 dB when the weld width is about 10 mm. Some preliminary acceptance criteria for defect sizes will be given. The NDT reliability will be studied using POD (probability of detection) and verified in metallographic studies. The aim is qualify inspections according ENIQ recommendations. INSPECTION OF COPPER WELD IN ENCAPSULATION PLANT The sealing weld is planned in the encapsulation plant as well as the final inspection of the canister sealing weld. For the encapsulation plant a welding and inspection room for quality checking of the canister sealing weld has been designed as shown in Figure 1. The visual, eddy current, ultrasonic and radiography inspections will be executed in the inspection room. All methods have been designed to be in the same room. The basis for the inspection is derived from the following: • • • • •
target of inspection requirements for inspection inspection methods reliability of the methods used experiences from inspections
Figure 1 - The figure shows on the left a weld inspection station from above: A linear accelerator (1), a calibration and assistance ring manipulator (2), a calibration and assistance ring (3), a swivelling bracket (4) for UT, VT and ET testing equipment, a protective wall for the former during RT testing (5), a hatch (6) closing the floor opening. On the right of the figure cameras equipped with swivelling optics for visual inspection of a surface are shown. Visible in the centre are the tank and sensor of an ultrasonic transducer. [1]
DEFECT TYPES IN COPPER WELDS AND ACCEPTANCE OF THE WELD In this study applied welding methods are EBW - Electron Beam Welding and FSW - Friction Stir Welding. The defect types which can occur in these types of welding and also basic principle of welding are shortly described in [2, 3]. The detectable defects set requirements for inspection techniques because the specific and postulated defects are used to plan non-destructive inspections according to ENIQ recommendations. Defect types are mentioned in Figure 2.
Figure 2 - Acceptance and rejection process of canister weld according to evaluation of NDT indications The indications detected in inspections are first evaluated in the screening phase according to acceptance criteria keeping in mind the master requirement of the intact wall thickness. This acceptance and rejection process is shown in Figure 3. The evaluation of the indications shall be carried out by qualified personnel (detection and sizing qualification). The evaluation is described more thoroughly in [2].
Figure 4 - Immersion tank having 3 axis - circumferential (rotation), axial and radial and ultrasonic inspection system Multix 2000, on the left. The probe moves on the outer surface of the tube in inspection of EB-weld (in the middle). Probe is positioned so that 1st element is just on the upper corner. Element cover 128 mm from the top, on the right, thus whole weld will covered even with 20 inspection angle Weld inspection techniques The evaluation of ultrasonic inspection is carried out using the latest Civa software version. A- , Band C-scan images are used for the visualization of the indications. In C-scan the whole weld as a top view with possible indications, which has to be analyzed. Using zoom, the size of one separate defect can be determined; by positioning the cursor the A-scan will show the amplitude of the indication related to its position in the weld volume. This amplitude of indication will be compared to the amplitude of the reference defect. If the indication exceeds the registration level, it will be analyzed and the size of the indication will be measured from the corresponding positions in the B- and Cscans. For inspection of EB welds different ultrasonic inspection techniques will be used. All techniques used are shown in Figure 5. The basic inspection technique uses 0 °L, completing inspection techniques using angles of incidence of ±20 °L. For surface and partly for root inspection a technique with an angle of -20° is used. This technique of +20° is also used for sizing purposes. Using 3 different angles satisfies basic standard recommendation in weld inspection using minimum 2 angles having minimum difference equal or more than 15°.
Figure 5 - Ultrasonic inspection techniques of EB-weld
Figure 6 - Ultrasonic inspection of FS-weld For inspection evaluation of an FS weld the following techniques will be applied: the basic inspection technique will be performed using 0 °L surface, the root volume will be inspected using angle scanning. Root and surface inspection for curved defects will be carried out with an angle of 20° as shown in Figure 6. All techniques are visualized in fixed circumferential position showing one electronic scan of that area. The mechanical scan is performed in a similar manner as in the EB weld inspection Different focal laws will be applied to perform the inspections in both weld type inspections. To avoid saturated signals from the key reference reflectors sufficient low gain setting values must be applied. Factors affecting on
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There are a several factors which affect on the detectability in ultrasonic testing. First of all in ultrasonic testing the POD must be tested against to acceptance criteria. If in POD-curve 90% with 95 % confidence limits the detectable defect size is smaller than in acceptance criteria then it is possible to concentrate in improving inspection procedure, sizing techniques, minimizing human factor effect on the reliability. Basic POD -curve and human factors studies has been presented [4, 5]. If POD curves will not be met, inspection technology must be improved to assure good detectability that we can achieve previous mentioned limit set by POD-curve. In ultrasonic inspection there are two main factors which have to be considered are base material from which side the inspection will be carried out (copper tube or copper lid) and which kind of characteristics weld has EB-weld or FS-weld. Both of these factors will in reality affect on the detectability of the flaws if grain size varies extensively. In the reference specimen as a reference reflector will be 3 mm FBH. By measuring the noise level can give the minimum detectable diameter depending on the for instance in EB-welding the ultrasonic attenuation in copper tube before EB-weld, Figure 7. A practical value seems to be 1 mm in front of the weld and 2 mm behind the weld in the optimum case when attenuation is low in tube material. When the attenuation in tube is high, thus detectable size of FBH increases to about 2 mm in front of weld as seen in Figure 7. This means that in inspection attenuation must be taken into account in evaluation of indications and in evaluation of minimum detectable defect size. In evaluation C-scan noise level has been applied, because it seems that noise level based on the A-scan seems to give locally too small noise level, which cannot be used in evaluation of detectability. Attenuation in copper has been studied also in [6, 7].
Figure 7 - Attenuation effect on detectability in copper EB-weld inspection depending if base material has low (on the left) or high (on the right) attenuation Defect sizing For UT the defect sizing is shown in Figure 8. All three directions will be measured. The axial and circumferential size will be received from the C-scan data and the radial size of the defect will be estimated from the B-scan. The amplitude value and dynamic behaviour of the defect will be one part in sizing using the A-scan. The angle inspection (±20°) must be considered in sizing simultaneously with the result of a 0° angle to give the final result of ultrasonic testing. In FSW the defect has been sized using angular scanning.
Figure 8 - Defect sizing in EB-weld and in FS-weld (upper) and case of sizing a long defect and a curved defect in root of EB-weld Because of 3D defects in copper welds using either EB-welding or FS-welding, matrix phased array offers possibility to improve defect detectability by adjusting ultrasound field in optimal way to defect orientation, Figure 9. Other important defect detection improving factor in matrix phased array is that better focus can be achieved when focussing is two active in direction. In linear phased array we have one passive direction, which cannot be focused and that passive direction acts like in conventional ultrasonic probes. Matrix phased array technology has been applied until now for thick plate inspection (60 mm). For thick plate EB-weld was manufactured a reference specimen which contains defects having 2.5 mm and 5 mm depths in front of weld and behind the weld.
Figure 9 - Ultrasonic matrix phased array bases on 2D-matrix containing elements in 2 active directions (in the upper left). This makes possible to steer ultrasonic beam in 3D as shown in image
The 2.5 mm defect depth having length 5 mm is detected with 20 dB signal to noise ratio, which means that about 1 mm notch can be detected behind the weld, Figure 10. The sizing and detectability of defects will be evaluated until to the end of year 2011 by comparing detected indications to metallographic results. The process of this study has been reported in [8].
Figure 10 - Indication from reference notches behind and in front of weld using matrix phased array with 50 angle and focused on the root of the weld Further studies will be carried out using advanced sizing methods. These studies will be carried out in 2011 and 2012 and following methods will be applied like TOFD, SAFT, Sampling phased array and full matrix capture (FMC) and evaluated as well sizing using raw data.
QUALIFICATION REQUIREMENT FOR WELD INSPECTION The qualification according to ENIQ recommendations is divided into the following subareas: -
input information inspection procedure technical justification based on the study of the inspection procedure open and/or blind trials.
The qualification body should accompany the qualification already from the beginning. The qualification body should comment on and check that the qualification test specimens for open and blind trials are valid for the qualification purposes. In the technical justification one important part is to study parameters, which are assumed to have an effect on the inspection performance. These parameters can be divided into two groups: essential and influential parameters, Figure 11 [9]. It is vitally important to define inspection objectives for qualification and the requirements for NDT techniques to be used. Such requirements are, for example, type and minimum size of defects to be detected and also the accuracy that is needed in the defect sizing. During the definition of inspection target values viewpoints of both structural integrity and inspection shall be taken into account.
Figure 11 - Parameters affecting inspection according to ENIQ
EXPERIENCES OF COPPER WELD MEASUREMENT About 40 real size welds and several hundred plate welds have been inspected. Ultrasonic inspection studies of copper EB-weld were started already 1990's. Posiva has started to test also some FS welds with ultrasonic and eddy current testing methods. The inspection techniques have been developed for ultrasonic inspection from conventional probes to phased array probes. This is due to the fact the phased array probes give more possibilities to change and adjust the beam sound field to different requirements. This phased array technology is also itself going through a change from linear probes to matrix probes. Posiva will test in 2010 matrix probes for weld testing. The main interest is to improve the detectability of curved defects or defects in bad orientation to the linear phased array sound field. Statistically it seems that ultrasonic inspection finds defects similar to radiography, but in closer study of the indications it was found that the defect indications differ from positions. This is explainable due to the inspection limitations of both methods. Posiva is verifying now detectability also by metallography of the indications. In studies it has been shown that defects have been found and the welds can be categorized according to quality, but some techniques need to be improved. Also, some welding parameters, such as the penetration of depth, can be shown by ultrasonic testing. Larger grain size in the tube makes it more difficult in some cases to distinguish the depth of penetration of the welding. It also seems that a combination of inspection results improves the quality of the inspection. This is under study and will be reported in 2012.
SUMMARY AND CONCLUSIONS Nuclear fuel will be seal welded in encapsulation plant. Seal welding of nuclear fuel copper canister can be made by EB-welding or FS-welding. Defect types of those welding methods are base for selection of inspection techniques. Ultrasonic inspection techniques have been applied to inspection of those weld types. Weld inspection is based on using 3 different inspection angles (0, ±20). On the other hand the evaluation of signals is specially affected by base material structure, which can have very variable grain size distribution. Base material is copper lid in FS-weld and copper tube in EBweld. Large grain size attenuates ultrasonic signals and at the same time decreases defect detectability. In this study it gave clearly detectability decrease from 1 mm to 2 mm in front of EBweld when the attenuation was about 13 dB in the tube material. This effect will be measured during inspection and can be roughly estimated from the measurements of real components afterwards even locally. Very important fact for the inspection is the form of defects. Some of the defect types are 3Ddefects, which mean that they are curved in that manner, that inspection techniques with for instance one angle inspection cannot fully characterize defect and its size. Linear phased array inspections are limited to focus only in one direction. The improvement can be carried out either using matrix phased array technology which has been started applying this technique for EB-weld reference plate inspection having thickness of 60 mm. The evaluation of defect sizing for those copper weld types will be carried out in the next 2 years. One used technology is certainly matrix phased array and special sizing techniques will be applied in the future studies like SAFT, Sampling phased array and full matrix capture and TOFD. The sizing of indications can be carried out with good reliability even with simple sizing techniques but as mentioned in case of 3D-defects the sizing can be limited. This realiability study will continue also in sizing concerning POS (Probability of sizing) and human factors affecting sizing. Nevertheless ultrasonic testing is not only testing method, the reliability will be improved with other testing methods, like visual testing, eddy current testing and radiographic testing.
REFERENCES 1) Suikki, M., & Wendelin T., 2009, Inspection station design for canister sealing weld, Posiva Working report 2009-71, December, 34 p. 2) Pitkänen J, Inspection of bottom and lid welds for disposal canister. Posiva Report 2010-04, 100 p. (to be published) 3) Pitkänen, J., Salonen, J., Sandlin, S. & Ronneteg, U., 2007, Defect Detectability in EB-welded Copper Disposal Canister with 9 MeV Accelerator, 6th International Conference on NDE in Relation to Structural Integrity for Nuclear and Pressurized Components, Budapest 12th -14th October 2007, 14 p. 4) Bertovic, M., Müller, C., Fahlbruch, B. Pitkänen, J. & Ronneteg, U., 2010, Human Factors Approach to the Reliability of NDT in Nuclear Waste Management in Sweden and Finland, 8th International Conference on NDE in Relation to Structural Integrity for Nuclear and Pressurized Components, Berlin 30th September-1st October 2010, 10 p. 5) Kanzler, D., Milsch, S., Pavlovic, M, Müller, C. & Pitkänen, J., 2010, Concept of total reliability of NDT methods for inspection of the EB weld of the copper canister used for a long-term storage of spent nuclear fuel, 8th International Conference on NDE in Relation to Structural Integrity for Nuclear and Pressurized Components, Berlin 30th September-1st October 2010, 6 p. 6) Pitkänen, J., Arnold, W. & Hirsekorn, S., 2007, The Effect of Grain size to Defect Detectability in Copper Components In Ultrasonic Testing, 6th International Conference on NDE in Relation to Structural Integrity for Nuclear and Pressurized Components, Budapest 12th -14th October 2007, 14 p. 7) Pavlovic, M., Böhm, R., Müller, C., Pitkänen, J, & Ronneteg, U., 2010, Investigation of the influence of the Material Attenuation on the POD of the Flaws, 8th International Conference on NDE in Relation to Structural Integrity for Nuclear and Pressurized Components, Berlin 30th September-1st October 2010, 9 p. 8) Pitkänen, J., Paussu, R., Pohjanne, P., Virkkunen, I,, Kemppainen, M., Lipponen, A., Sarkimo, M., Simola., K. & Reddy, K.-M.., 2010, Metallographic Study of Detected Indications in EBCopper Welds for Verifying the NDT Reliability of Inspections, 8th International Conference on NDE in Relation to Structural Integrity for Nuclear and Pressurized Components, Berlin 30th September-1st October 2010, 12 p. 9) ENIQ, 1998, Technical Justification, Pre-trials, ENIQ Report 10 EUR 18114 EN, November 1998, 132 p.
