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, FinlandR. 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.

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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- , B-

and 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 C-

scans. 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 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 EB-

weld. 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 EB-

weld 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 3D-

defects, 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.

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p. (to be published)

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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

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