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ALLOY 800 STEAM GENERATOR TUBE STRESS CORROSION CRACKS - DETECTION

A Erhard 1)

F. Otremba 1)

F. Mohr 2)

R. Kilian3)

1) BAM Bundesanstalt für Materialforschung und –prüfung (BAM), Berlin;

2) AREVA NDE Solutions/ IntelligeNDT - Erlangen

3) AREVA NP GmbH Erlangen

ABSTRACT

Intergranular stress corrosion cracking (IGSCC) in Alloy 800 steam generator tubes was until recently an

unknown damage mechanism for this material. But in the meanwhile such cracks were detected within the

tube sheet between upper and lower mechanical tube expansion and in the outer tube bundle periphery.

The detection and the sizing of such defects were not reliable with the common NDT methods. Steam

generator (SG) tube integrity constitutes the main barrier against release of activity to the secondary cir-

cuit. When through wall cracks occur, primary water can leak into the secondary circuit due to the influ-

ence of the pressure difference between the primary and secondary circuit. Such cracks may have safety

relevance if the crack growth is not negligible. Therefore optimized inspection methods are necessary for

tube integrity assessment.

The overall requirements in this particular case are to guarantee the tube integrity in the time be-

tween the periodically in-service inspections until the next inspection by placing special emphases of NDT

methods. In the present contribution, NDT methods for the inspection of defects like IGSCC are present-

ed. Some statements about the root cause for this special degradation mechanism will also be described.

INTRODUCTION

Steam generator (SG) tubes within nuclear Pressurized Water Reactor (PWR) coolant systems are part of

the primary circuit. SG tube walls maintain separation between the primary and the secondary circuits. To

avoid leakage from the primary into the secondary circuit, SG tube integrity is verified using in-service

inspection methods plus surveillance techniques testing for radioactivity in the secondary circuit. Typical

defects include corrosion due to wastage near the top of the tube sheet (TTS) and near the tube support

plates (TSP) plus fretting wear/1-3/ near the TSPs. Stress Corrosion Cracking (SCC) was a problem for

Alloy 600 steam generator tubes /4-7/. To understand the damage mechanism in SG tubes, it is necessary

to have information about Primary Water SCC (PWSCC) commonly observed at the roll transition zone,

at U-bends and tube denting locations, and Outer-Diameter SCC (ODSCC) commonly occurring near

TSP. In /6/ it is mentioned that SCC on both the primary and the secondary sides of SG tubes has become

the principal degradation mode and that this kind of cracking has been observed in Alloy 600 tubes. Due

to recent indications of damage within Alloy 800, starting from the OD of the tubes (secondary circuit),

this degradation mechanism must now to be considered, too. These damage mechanisms are, so far as

known in the moment, related to the design of the steam generator tube supports (Fig 1). There are geo-

metrical parts in the tube sheet area, but limited to the outer tube bundle periphery, as well as in the sup-

port grid area in which deposits can be buildup. In these deposits an enrichment of corrosive species, e.g.

sulphur species, can occur. A decrease of the pH-value is the consequence and subsequently an increasing

the potential for SCC. Finally an increase of stress corrosion cracking growth was detected between two

inspection intervals.

In parallel to the investigation related to the root cause of different SG tube damage mechanisms,

the development of non-destructive testing methods was the challenge. Eddy current testing is currently

the method for SG tube inspection.

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Figure 1: Locations at which ODSCC of Alloy 800 tubing was observed

STEAM GENERATOR TUBE INSPECTION

SG tube inspection of Nuclear Power Plants (NPP) has been required in standards and codes since PWR

plant construction. For this purpose Eddy Current Techniques (ET) are common around the world. Differ-

ent research laboratories have conducted investigations to optimize these techniques, especially for the

detection of cracks, since volumetric defects like fretting or wastage are in general not a challenge for ET,

due to the high sensitivity of Eddy Current (EC) for such defects. Nevertheless, depending on the tech-

nique being employed, the measured ET probe response gives information about the defect location (inner

or outer diameter of the tube), and the relative defect orientation (axial, circumferential). Such signals are

easily sorted without the influence of TTS or TSP structures that also have a significant impact on the EC

signal. If defects are located near geometrical changes or changes of the tube material properties, then

some efforts are necessary to separating and recognizing damage signals from geometrical signals as ex-

plained in /8/. The signal received from IGSCC placed on the OD was superimposed on signals received

from the TSP (TSP is made of carbon steel /8/). In this case, the signal generated by the SCC is not clearly

recognizable any more. The optimization of EC probes for the inspection of these areas was one of the

challenges in the past. In /9/ an optimization procedure is described where the disturbed eddy current field

due to a crack is calculated as well as the impedance changes. In derivation of these theoretical results,

optimizations of the probe coils are presented. The shape and geometry of these coils are more or less

those which are well known conventionally. Further investigations for the optimization of EC probes are

described in /10/. In this presentation, sample calculations were performed with a three-dimensional (3D)

finite-element model to describe the response of an EC probe to defects in SG tubing. Such calculations

could be very helpful in understanding and interpreting EC probe responses to complex tube defect geom-

etries associated with the in-service inspection (ISI) of SG tubes. In general, when designing an EC probe

for a new application, the primary criterion is the detection capability or the sensitivity to the target de-

fects, in our special case intergranular stress corrosion cracking (IGSCC), in the presence of the design

material and geometry. From the mechanism producing IGSCC it is clear that the interaction of ECs with

IGSCC is totally different from the interaction with e.g. wastage, fretting or fatigue cracks.

As mentioned before, the optimization of EC probes contains a high detection capability but also a

good potential for defect sizing. For this task the application of so-called array eddy current probes /11-16/

has some advantages. In /16/ it is mentioned that the biggest evolution in eddy current sensor technology

is the eddy current array technique. Eddy current array technology increases detection capabilities due to

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the kind of coil arrangement (Figure. 2). The principle of those probes is described especially with the

emphasis on crack extent measurement as follows:

Figure 2: Principle of an EC array probe

As shown in the example of Figure 2, the complete eddy current probe consists of a number of sin-

gle coils arranged in three parallel rows (see also picture at the top right). At the bottom of the right hand

side of figure, the flexibility of the probe and cable arrangement at a SG U-bend tube is pointed. The num-

ber of coils depends on the inner diameter of the tube; however with increase in the diameter the number

of coils must also be increased to achieve the same resolution along the surface. At present EC probes with

8 to 18 single coils are available on the market. A further increase in effective coil density is achieved by

switching coils between transmitter (T) and Receiver (R) mode respectively. For detection and estimation

of the length extent of axially oriented cracks in the SG tubes the following alternative measurements may

be conducted:

Signal transmission is initially made using the transmitter coil T1 and signal reception using the re-

ceivers R1 and R2. The measurement steps continue with the transmitter coil changed from T1 to T2 and the

receiver coils to R2 and R3, and subsequently following the algorithm Tn+1, Rn+1 and Rn+2 respectively. By

means of this procedure, an examination of axial defects in the whole tube circumference is achieved.

For the detection and sizing of circumferentially oriented defects the transmit coils are T1 and T2

and the receivers are R4 and R5. The circumferentially oriented display is developed following the Trans-

mitter-Receiver algorithm analogous to the axial sequence as shown above.

A criterion for defect detection is the sensitivity of the EC probe which depends among others on

the aperture size (extension of the coils), the electrical conditions of the specimen and the defect geometry.

The influence of the defect length extension is plotted in Figure 3, as expected theoretically and as re-

ceived during measurements. The theoretical curve describes the measured behavior very well; the sensi-

tivity is increasing with increasing the length extension until a maximum. The maximum is achieved when

the defect length is equal the aperture size. In this particular case the aperture size was approximately 14

mm. The theoretical curve fits the measured well only the sensitivity maximum is shifted to shorter defect

extensions. The theory is more conservative than the measurement. Further the signal decreases lightly for

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Figure 3: Sensitivity distrubution of an EC array probe

Figure 4: Comparison of the results received with EC techniques and fractography /18/

defect length larger than the aperture size and change over to a saturation. The signal maximum can be

explained as the influences of the edges of the defect when the defect length achieves the aperture size.

The black line is presenting the signal of the middle of the defect. This signal is equivalent with the maxi-

mum measured signal when the defect is smaller than the aperture and decrease when the defect length is

larger the aperture. However, the signal saturation is achieved by a little bit longer defect extensions due to

the weaker geometry influence in the middle of the defect. An example of measured results employing the

EC array probe described above is printed in /17/ and shown in Figure 4. In the middle of the figure the

axial orientation of the defect is clearly visible, whereas the measured quasi 3-dimensional image gives the

information of a crack-like defect with two deeper extensions. The evaluation of the depth of these two

measured peaks gives crack depths of 73% and 42% of the wall thickness respectively. For validation of

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the EC inspection results and for investigation of the degradation mechanism, this tube was pulled from

the steam generator. From the fractography on the left hand side, a maximum crack depth of 1.05 mm was

obtained. This is equivalent to a wall thickness reduction of 80% if a wall thickness of 1.30 mm for the SG

tube is assumed. Comparing these results with the eddy current estimate shows good agreement. A cross

section of another IGSCC crack is printed on the right hand side of the figure. The estimation of the small-

er crack depth gives a wall thickness reduction of 0.38 mm. This is equivalent to approximately 30% of

the wall thickness. This bigger difference of the crack between the EC measured and the fractography is

may be due to the interaction between the signals from the two cracks. Further, the investigation has

shown that IGSCC cracks are start in general on corroded surface areas, as also seen in the figure on the

right hand side (see also Figure 6).

Additional results received from a second removed SG tube are shown in Fig. 5. Again, in the mid-

dle the measured 3D eddy current result using the array probe is shown. The wall degradation was esti-

mated at 56%. From the fractography a reduction of the wall thickness due to IGSCC of 0.86 mm was

measured at that position, which is equivalent to a wall thickness degradation of 66%. Depending on the

circumferential position the evaluation of the opened cracking position delivers two intergranular cracks;

one has the mentioned wall thickness reduction of 66% and the second has a reduction of 51% The cir-

cumferential position of this defected area is also visible on the 3D plot at the right hand side. In the EC

signal plot of Fig 5, only one defect is clear recognizable. The two separate cracks in close proximity to

each other cannot be clearly distinguished. The reason in this particular case is the large aperture EC coils

and therefore relatively limited lateral resolution. The weak separation of these two close cracks is obvi-

ously. However, the resolution is always under discussion in the application of nondestructive testing

methods and of course also the application of EC techniques. If the distance between defects is similar to

the aperture size then a separation is impossible. The plotted result is an integral of all signals beneath the

aperture.

Figure 5: Comparison of the results received with EC techniques and fractography /18/

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ROOT CAUSE

The in-service inspection of SG tubes was carried out in accordance with the requirements in the code.

The detection of IGSCC on the outer (OD) tube wall was astonishing. The results of the fractography ex-

plained in the previous section together with the results obtained during the in-service inspection employ-

ing EC array techniques gave not a clear information about the root cause of ODIGSCC mechanism. In

Figure 6, an overview of a damaged area of a pulled tube is shown left hand side above. The corroded area

on the surface is recognizable visually. On the ground of this corroded area, using a higher magnification,

a crack was detected (picture at the right hand side and in the middle below). The same damage situation

was revealed on a second pulled tube. Following years of operation, some difficulties with the condenser

may occur, allowing some leakage coupled with sulphur or other chemical substances that can come into

the secondary medium and hide out between the SG tube and TS. The logical consequence is the concen-

tration of corrosive species e.g. sulphurs in this area increasing the corrosion potential. But in general for

IGSCC generation tensile strength is also a sufficient necessity. To get an answer residual stress measure-

ments were carried out on different SG tubes.

Figure 6: Surface conditions on the cracked area

The residual stress measurements were carried out using tubes of two Alloy 800 heats from the

same manufacturing period and manufacturing plant as the removed damaged tubes. A residual stress pro-

file from the OD to the ID was created using electrochemical metal removal and X-ray residual stress

measurements (Figure 7). The measurements were carried out at three positions at the circumference of

the Alloy 800 tubes. The results showed that compressive stresses in the axial as well as in the circumfer-

ential direction were present. Based on experience with the tube manufacturing process, these compressive

stresses can be attributed to the fabrication of the SG tubes. With increasing measurement depth the com-

pressive stresses decrease. At a depth of approximately 50 µm, a transition from compressive to tensile

stresses could be demonstrated at all measuring positions. This is one explanation for the special degrada-

tion mechanism recognized by the fractography investigations described in the previous section. From this

point it is clear understandable that a corroded surface must be present always to reduce compressive

stresses on the surface. Further results of residual stress distribution have been obtained on SG tubes with

surface treatment using glass bead irradiation. This type of surface treatment was employed for the so

called “Konvoi” NPP produced by the former KWU, whereas only three were build (commercial operation

1988 und 1989). The result of the residual measurements, using the same procedure as mentioned above,

is printed in Figure 8. The comparison with the “older” (marked with A) tubes is obviously. The tubes

treated by glass bead irradiation (marked with B) have approximately a 6 times deeper compressional

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stress zone than the older tubes. Nevertheless, totally avoidance of the generation of ODIGSCC is impos-

sible when chemical substances e.g. due to condenser problems, are into the second circuit.

Figure 7: Residual stresses of SG ab tube (as fabricated)

Figure 8: Residualstresses of a SG tubes

Periodically inspection using non-destructive testing methods is one of the consequences to avoid

leakage of radiation between the primary and secondary circuit. A second method SG tube integrity as-

sessment is verified using surveillance techniques testing for radioactivity in the secondary circuit e.g.

tritium (3H) measurements and hide out return calculations /19/ after shutdown.

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CONCLUSION

The detection of ODIGSCC at SG tubes near the TS is one of the challenges for EC inspection techniques.

In the past, the EC inspection of this area was difficult due to the big carbon steel mass of the TS. With the

application of eddy current arrays the detection of cracks in this area has become more reliable. The re-

sults present in this paper show the advantage of that technique in relation to defect detection, sizing and

characterization by image presentation. Studies of the root cause of the damage mechanism for the genera-

tion of OD IGSCC at the secondary circuit shows, that depending on the treatment of the SG tubes during

fabrication, tensile strength regions some µm beneath the surface are detected and can cause this behavior.

The results are helpful for understanding the mechanism, and an extension to other PWRs with similar SG

tube characteristics seems to be possible.

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