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INSPECTION OF AUSTENITIC WELDS IN ITER VACUUM VESSEL WELDS WITH

ULTRASONIC PHASED ARRAY TECHNOLOGY.

A. García, F.J. Fernández, M.C. Pérez, R. Martínez-Oña, Tecnatom, Spain; G. Pirola,

Ansaldo Nucleare, Italy.

ABSTRACT

The ultrasonic inspection of austenitic welds has always been a great challenge due to the dispersion and

attenuation of the ultrasonic beam in these materials. In the case of the ultrasonic inspection of ITER

Vacuum Vessel welds, the complex geometry of this component adds to these difficulties.

Seven sectors of the ITER vacuum vessel will be manufactured by an Italian Consortium led by

Ansaldo Nucleare and including Mangiarotti and Walter Tosto as manufacturing partners; Tecnatom will

be in charge of performing the ultrasonic qualification.

To detect and characterise defects in these materials, Tecnatom is developing an ultrasonic

inspection technique based on the phased array technology using dual probes. This technique will be

qualified under the supervision of the European Domestic Agency “Fusion for Energy”, responsible for

managing the production of the seven sectors of the vacuum vessel mentioned above.

This paper will show the main tasks achieved in the development and implementation of this

inspection technique, which have been as follows:

• Specification of phased array probes adapted to the component to be inspected (defined by

its geometry and materials) and the type of defects postulated (defined by their type, size, shape and

position). As a support for the definition of the array probes, simulation tools have been used to predict

the shape of the ultrasonic beam.

• Design and manufacture of scanners, specific for the ultrasonic examination of the

components subject to inspection.

• Performance of ultrasonic examinations on test pieces with realistic defects for the

optimisation and tuning of equipment and inspection techniques.

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.

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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:

• 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 (as is the case for the flexible

housings welds, which measure 210mm in diameter) (see Figure 2). 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.

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Figure 2. Welds of form A with straight and circumferential trajectories.

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.

INSPECTION TECHNIQUES

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 active surface of the probe is made up of two lines of transducers with

up to 28 elements (see Figure 3). 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.

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Longitudinal waves.

Inspection of the upper part of the

inspection volume.

Longitudinal waves.

Inspection of the middle and lower

part of the inspection volume.

Transversal waves.

Inspection of the lower part of the

inspection volume.

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Figure 3. Prototype dual probe for vacuum vessel weld inspections.

SCAN PLANS WITH PHASED ARRAY PROBES

In order to detect defects parallel to the weld, the probe is displaced along either side of this weld (see

Figure 4). To detect defects perpendicular to the weld, the probe is passed along the weld in the two

possible orientations (X+ and X-) (see Figure 5). With a view to improving the technique, this

examination is also performed by skewing the probe to facilitate the detection of defects located

perpendicular to the weld and close to the root of the weld.

Scan Y+Y+Y+Y+

Scan line number 1 Scan line number 2 Scan line number 3

Figure 4. Scanning for the detection of defects parallel to the weld.

Scan X-Y+Y+Y+

Scan line number 1 Scan line number 2 Scan line number 3

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Figure 5. Scanning for the detection of defects perpendicular to the weld.

The simulation of the beam radiated by an array probe (see

Figure 6) helps to understand the results of ultrasonic data acquisitions. This tool may be used to

improve the quality of the inspection, for example by improving the design of a prototype probe by

contrasting the results of the simulations with the results of the inspections performed. The modelization

is also useful for the assessment of assumptions that it has not been possible to verify with the probes on

the test pieces, such as for example how defects not included in the test pieces would be detected and

characterised or how probes mounted on curved wedges would work during inspection.

Figure 6. Simulation of beam radiated by an array probe.

TEST PIECES

The test pieces that will be used during the qualification are representative of the welds present on the

vacuum vessel. These test pieces incorporate reflectors representative of the planar and volumetric defects

postulated for these welds.

First, some test pieces are foreseen for the characterisation of all weld forms. Test pieces will

subsequently be manufactured to validate the detection and characterisation of planar (embedded and

open) and volumetric defects. An example of these test pieces has been represented in

Figure 7.

Figure 8 represents an example of ultrasonic data acquisition on a test piece. It is also expected that

test pieces will be manufactured with what are known as disturbing factors, these being defined as weld

characteristics potentially having an effect on the quality of the ultrasonic examinations for defect

detection and characterisation. Three disturbing factors have been identified: weld nodes, weld

misalignments and weld repairs.

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Figure 7. Diagram of two of the test pieces.

Figure 8. Example of the ultrasonic data acquired from a test piece.

OTHER TECHNIQUES

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-).

• 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

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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.

• 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.

• Transverse 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.

• 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.

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 2007.

2. CEN/TR 14748:2004, Non-destructive testing – Methodology for qualification of non-destructive

tests.