QUALIFICATION OF THE ULTRASONIC INSPECTIONS OF VACUUM VESSEL WELDS OF THE ITER REACTOR

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1 More Info at Open Access Database QUALIFICATION OF THE ULTRASONIC INSPECTIONS OF VACUUM VESSEL WELDS OF THE ITER REACTOR ABSTRACT F. Fernández, A. García, M.C. Pérez, R. Martinez-Oña Tecnatom, Spain, G. Pirola, Ansaldo Nucleare, Italy A. Dans, A. Bayón, F4E, Spain The ultrasonic inspection of ITER Vacuum Vessel welds is a challenge, due to the complex geometry of this component and the anisotropy of the austenitic weld material. This inspection could require the combination of several ultrasonic techniques. To detect and characterise defects in these materials, Tecnatom is developing a main ultrasonic inspection technique based on phased array probes. Additionally, other inspection techniques are used, which works with emitter receiver conventional probes. The purpose of these last techniques is to complement the examinations performed with phased array probes in the detection and characterisation of specific defects. The seven sectors of the ITER Vacuum Vessel, which are being manufactured by an Italian Consortium led by Ansaldo Nucleare and including Mangiarotti and Walter Tosto as manufacturing partners, will have available a set of several ultrasonic techniques to guarantee the fulfilment of the RCC-MR Code edition 2007 requirements. The qualification of these inspection techniques is being supervised by the European Domestic Agency Fusion for Energy that is the responsible of managing the production of the Vacuum Vessel. INTRODUCTION The vacuum vessel is an austenitic steel component that houses the fusion reaction. The plasma particles revolve inside a toroidal inner chamber with a D-shaped cross-section. Both the design and manufacturing of the vacuum vessel and the corresponding examinations are in accordance with the requirements of the 2007 edition of the RCC-MR Code. This Code requires that a radiographic examination be performed on all category 1 and 2 welds of the vacuum vessel. Given the geometric characteristics and the construction sequence of the vacuum vessel, certain of these welds are accessible only from one surface. These characteristics mean that it is not possible to perform the volumetric examination of these welds (represented in Figure 1) using radiographic techniques. FORM A 10.0 FORM D 12.0 FORM Z (40.0) (40.0) R (40.0) 1.0 R2.0 R5.0 FORM A1 FILLING BY SMAW (111) PROCESS FORM A2 FILLING BY GTAW MANUAL (141) PROCESS FORM D1 FILLING BY GTAW AUTOMATIC (141) PROCESS FORM D2 FILLING BY SMAW MANUAL (111) PROCESS FORM D3 FILLING BY GTAW MANUAL (141) PROCESS FORM Z2 FILLING BY GTAW AUTOMATIC (141) PROCESS FORM Z3 FILLING BY GTAW MANUAL (141) PROCESS Figure 1. Welds to be inspected ultrasonically The alternative is using ultrasonic techniques, when it is demonstrated that radiographic techniques cannot be developed. This alternative of carrying out the volumetric inspection using ultrasonic techniques entails the following difficulties, among others: 383

2 The welds are manufactured using austenitic stainless steel and several different welding processes (automatic and manual). The habitual thickness of the welds is 60mm. Despite the highly demanding quality requirements applicable to the materials and welding procedures, the inspection of austenitic welds is always a challenge because of the anisotropy of the materials. In certain welds, the accessible surface (i.e., the surface to which the probe is coupled) on either side of the weld is limited. This situation is due to the complexity of the geometry of the vacuum vessel. In order to guarantee the ultrasonic inspection of all the welds, the probes and examination techniques have been designed such that the coupling surface required on either side of the weld is as small as possible. In this respect, certain changes have been made to the design of the vacuum vessel in order to guarantee this minimum accessible surface on all the welds. The probe coupling surface is curved in shape. Given the toroidal shape of the vacuum vessel, there are welds with concave and convex coupling surfaces both in the direction of the weld and in the direction perpendicular to the weld. Although in most cases these curves are negligible for the ultrasonic inspection, given the large dimensions of the vacuum vessel, this is not always the case. The weld centreline may be straight, curved or circular. In order to ensure inspection of all the welds with maximum guarantees, all the examinations performed in situ (i.e., on the actual component) will be carried out using a semi-automatic device that allows the position of the probe to be coded. For each examination performed on each of the welds, a data file will be generated containing the complete ultrasonic signal of each of the probes, along with the positions (coordinates) at which the probe acquired the ultrasonic data. The RCC-MR Code requires each of the defects detected to be characterised, establishing whether these defects are planar or volumetric. All planar defects (cracks and lack of fusion) are unacceptable, regardless of their size and position. In view of the difficulties described above, the European Fusion for Energy Agency, which is responsible for managing the manufacturing of seven sectors of the vacuum vessel, requires the qualification of the ultrasonic volumetric inspections of welds accessible from a single surface. Tecnatom is responsible for such qualification. DEFECTOLOGY Postulated defects in the weld are classified as planar or volumetric. The essential variables of defects currently considered are: size (length and through wall extension), position (base metal, weld metal or weld interface), location along the thickness (scanning surface breaking, embedded, or opposite to scanning surface breaking), orientation (perpendicular or parallel to the weld), skew, and nature (planar or volumetric). A relation of the defects that have been postulated is given in Table 1. To develop and assess the capability of the ultrasonic techniques they must be also considered the imperfections in the root area and the disturbing factors (see Table 1). Table 1. Postulated defect. Weld imperfections and disturbing factors Weld defects Slag inclusion (for SMAW) Volumetric Tungsten inclusion (for GTAW) Volumetric Porosity and gas pore Volumetric Crack Hot crack Lack of fusion Undercut Weld imperfections in the root area Lack of root penetration -- Root concavity -- Excessive penetration -- Shrinkage groove -- Disturbing factors Weld node -- Weld repair -- Misalignment

3 INSPECTION TECHNIQUES. PHASED ARRAY PROBE Taking into account all the ultrasonic examination requirements indicated above, the basic ultrasonic technique proposed is the pulse-echo technique, using a phased array probe performing the inspection in the two directions (perpendicular and parallel to the weld) and in the four possible orientations (Y+, Y-, X+ and X-). This probe is of the dual type (i.e., with the emitting and receiving stages separate) and is mounted on an angle wedge. The advantage of using a phased array probe is that this type of probes allows scanning movements to be minimised (this being adequate in view of the small space available on either side of the weld), and also allows ultrasonic beams with different angles and with both types of wave (longitudinal and transversal) to be refracted in a single examination. With dual probes, the maximum sensitivity occurs in the zone in which the ultrasonic beams of the emitter and receiver stages cross. The advantage of this design is that it improves the signal-noise ratio of the ultrasonic signal in the crossover zone. The focal laws are calculated for steering of the inspection volume with refracted beams varying within a given range of angles. A first set of ultrasonic beams covers the upper third of the inspection volume with longitudinal waves; a second set covers the intermediate area and lower third with longitudinal waves; and a third covers the lower third with transverse waves. In this way, a dual array probe replaces an entire series of conventional probes, allowing inspection times to be reduced. In order to detect defects parallel to the weld, the probe is displaced along either side of this weld (see Figure 2). To detect defects perpendicular to the weld, the probe is passed along the weld in the two possible orientations (X+ and X-) (see Figure 3). Inspection of the upper part of the inspection Inspection of the middle and lower part of the inspection Inspection of the lower part of the inspection Figure 2. Scanning for detection of defects parallel to the weld Inspection of the upper part of the inspection Inspection of the middle and lower part of the inspection Inspection of the lower part of the inspection Figure 3. Scanning for detection of defects perpendicular to the weld 385

4 OTHER TECHNIQUES. CONVENTIONAL PROBES As a complement to the phased array probe technique described above, other inspection techniques using conventional probes are being developed. The purpose of these techniques is to complement the examinations performed using phased array probes for the detection and characterisation of certain specific cases. These probes are as follows: Creep wave probes with the pulse-echo technique for the detection of defects open to the scan surface, both perpendicular and parallel to the weld. Although phased array probes detect these defects, the use of creep wave probes notably improves the signal-noise ratio with which they are detected. Consequently, it has been proposed that the inspection of all the vacuum vessel welds include an examination with these probes in the four directions (Y+, Y-, X+ and X-, see Figure 4 and Figure 5). Creep waves. Inspection of the upper part of the inspection Creep wave Figure 4. Scanning for detection of defects open to the inspection surface Creep waves. Inspection of the upper part of the inspection Creep wave Figure 5. Scanning for detection of defects open to the inspection surface Low frequency probes with pulse-echo technique for the detection of defects perpendicular to the weld and open onto the surface opposing that used for the inspection. The detection of these defects may be performed with phased array probes by positioning the probe on the base material and orienting the position of the probe with respect to the weld centreline. However, the detection of these defects by positioning the probe just above the centreline of the weld (i.e., with the ultrasonic signal always travelling through the weld) is possible only with this type of probes (see Figure 6). Inspection of the lower part of the inspection Figure 6. Scanning for detection of defects open to the opposite surface 386

5 Focussed probes with the pulse-echo technique for the characterisation of defects (through the search for diffraction signals). With phased array probes it is possible to observe the diffraction signals of planar defects. The signal-noise ratio of these diffraction signals may be improved in different ways. For example, the signal may be improved by using matrix phased array probes or root angled wedges. However, the simplest way to improve this signal-noise ratio is by using conventional probes focussed at different depths (see Figure 7). Figure 7. Scanning for characterization of defects (focused probes) Shear wave probes with the tandem technique. The objective of this technique is to confirm that defects characterised as being planar (due to possible diffraction signals having been observed) are in fact planar. The criterion postulated for use with this technique is to consider the amplitude of the signal reflected by a planar defect to be greater than the amplitude of the signal reflected by a volumetric defect. The range of thicknesses in which this technique may be applied is approximately between 0mm and 40mm (see Figure 8). T R Figure 8. Scanning for characterization of defects (tandem probes) LLT probes with the auto or round trip tandem technique. The objective of this technique is the same as for the tandem technique. The range of thicknesses in which this technique may be applied is approximately between 30mm and 60mm (see Figure 9). R T Figure 9. Scanning for characterization of defects (LLT probes) 387

6 CONCLUSIONS The volumetric examination of certain vacuum vessel welds may be performed using ultrasonic techniques from one only of its surfaces. In this case, the difficulties due to the limited space available for performance of the examinations, the complexity of the geometry of the vacuum vessel and the stringent requirements of the RCC-MR Code are to be added to the habitual difficulties involved in inspecting austenitic welds. In view of all these difficulties, the European Fusion for Energy Agency requires the qualification of the volumetric ultrasonic inspections of the vacuum vessel welds accessible from a single surface. Tecnatom is responsible for this qualification. It has been proposed that the basic ultrasonic inspection of the vacuum vessel welds be performed using the pulse-echo technique with phased array probes. This basic inspection will be complemented with other techniques and probes to detect and characterise specific defects. REFERENCES 1. RCC-MR, Design and Construction Rules for Mechanical Components of Nuclear Installations, Section 3: Examination Methods and Section 4: Welding. Edition CEN/TR 14748:2004, Non-destructive testing Methodology for qualification of non-destructive tests. 388