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Secondary side corrosion in steam generator tubes: lessons learned in France from the in-service inspection results
Nuclear Engineeri.ng Nuclear Engineering and Design 168 (1997) 255-259
ELSEVIER
andDesngn
Secondary side corrosion in steam generator tubes: lessons learned in France from the in-service inspection results Robert Comby EDF, Central Laboratories, 21 AlleO PriveO, Carrefour Pleyel, 93206 St Denis, Ckdex 1, France
Abstract Non-destructive testing (NDT) has proved to be very important in the maintenance of steam generator tubing. This is particularly true in the case of secondary side corrosion, because this type of degradation leads to various morphologies which are often complex (intergranular attack) (IGA), intergranular stress corrosion cracking (IGSCC), or a mixture of both. Their detection and characterization by the usual NDT techniques have been achieved through numerous laboratory studies, which were conducted in order to determine the performance and limitations of NDT. Pulled tube examination in a hot laboratory was very valuable, for both NDT and fracture mechanics aspects. The eddy current bobbin coil probe, used for multipurpose inspection of tubes, allows the detection of IGA-SCC at the tube support plate elevation. In France, the use of rotating probes is not required for that type of degradation, since the repair criterion is based on bobbin coil results only. The bobbin coil is also used for detection of IGSCC occurring in free spans, within sludge deposits. The eddy current rotating probe allows, in that case, characterization of main cracks. Concerning the outer diameter initiated circumferential cracks which occur at the top of the tube sheet, only the rotating probe is used. An ultrasonic (UT) inspection was performed several times, in order to obtain information on UT capabilities. The goal of tube inspection is obviously knowledge of the status of steam generators, but also to follow up degradations and to estimate their revolution, and to verify the beneficial effect of some corrective measures, e.g. boric acid injection. © 1997 Elsevier Science S.A.
I. Introduction With the primary to secondary leakage monitoring, tube inspection during refuelling outages is a necessary action to guarantee safety and availability of plants. Metallographic examination of pulled tubes has proved its effectiveness for knowledge of the morphology of defects and their behaviour under normal and accidental conditions. However, only non-destructive testing ( N D T ) can give the overall status of around 800 000 tubes in the operating French steam generators.
Steam generator maintenance activities are often referred to N D T results: • signal parameters are correlated with fracture mechanics results, in order to define a repair criterion (generally tube plugging, sometimes sleeving) based on these parameters; • successive inspections allow the data to be used for degradation growth rate estimation, which will be taken into account for the repair criterion definition and for lifetime prediction; • destructive examination of pulled tubes, in addition to the above mentioned aspects, is conducted in order to determine NDT
0029-5493/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PI1 S0029-5493(96)01 366-0
R. Comby / Nuclear Engineering and Design 168 (1997) 255-259
256
performances, i.e. detection threshold, ability to distinguish various types of defects, sizing accuracy of these defects; • these three points are thus taken into account to establish the maintenance strategy and inspection scope, and for each type of degradation, the quantity of tubes to be inspected during each outage and right probe will be defined (D6chelotte, 1995). This paper will be limited to the description of inspection techniques, for each of the three following defect types: • intergranular attack (IGA) stress corrosion cracking (SCC) at the tube support plate elevation; • intergranular SCC (IGSCC) in free spans, within sludge deposits; • outer diameter (OD) circumferential cracking at the top of the tube sheet.
2. Intergranular attack-stress corrosion cracking at the tube support plate elevation Found for the first time in France in 1986, this type of degradation is easily detected by using the
E1
E2
E3
4CE 0.515V -88"
CE 0.845V -93"
1.0 Volt/cm
1.0 Volt/cm
ll .o
Volt/cm
4CE 0.775V -84"
CF 1.285V -84" )
i
Fig. 1. Example of IGA-SCC signals at two support plate evaluations.
%
11111
•
E
M
i i
80 ~
I" m~
6O
•
•
m-
• •
m i
=o
~ 4o 20
0
20
40 estimated
60 d e p t h ( b o b b i n coil )
80
1O0 %
Fig. 2. IGA-SCC at tube support plate: depth sizing accuracy.
eddy current bobbin coil. This probe is systematically used as the worldwide multipurpose tool for tube inspection. Since safety studies and burst pressure tests on pulled tubes have been made, only indications above a given threshold have to be reported. These indications are clearly identified by the bobbin coil probe, as shown in the example in Fig. 1. The IGA-SCC indication is automatically selected by the rule-based analysis software. For each tube inspected, signals related to intersections with support plates are printed after automatic analysis. The results are then validated by specialists by simply checking the printing. If any doubt remains, a manual analysis can be performed. Signal voltage and phase angle are measured on two channels and used to estimate the depth in the tube thickness. A comparison between estimated and true depth has shown that this technique is able to estimate the true depth with an accuracy of _+ 20% (Fig. 2). The dispersion is due to the complex morphology of defects which are composed of both IGA and multiple IGSCC. Nevertheless, only the signal voltage at the main frequency is used for the plugging criterion. Despite the fact that signal voltage is not really representative of any physical defect feature, this parameter appears to be more robust in defect rejection. This approach has been recently adopted by the US. For that kind of defect, bobbin coil examination is the only method required by the inspection
R. Comby /Nuclear
Engineering and Design
168 ( 1 9 9 7 ) 2 5 5 - 2 5 9
257
i,i ,,: 14~. ,,.,, D e b u t : 5 mm ! F A
XFA
J
11 I I
I r I I I I I
Ii
- - - "~ ~7"7~'7777-'-'-'-7.7-7 . . . . . .
SSS
- S 7?5 S - S Z ' I I I I I
YFA
.
~
~
......................
75" ...................
CB:
~p: ~:
2.B
BC
I I I I
cm/cm Y:
8.50
O/cn
43 595 mU
Debut:
5 me
8.58
U/cm
I I I I I I
Fig. 3. Example of IGSCC signal, within sludge pile above the tube sheet. specifications. In some cases, an eddy current rotating probe is used to give a better idea of the defect distribution around the circumference, or to verify that no circumferential crack occurred. In addition, some correlation was found between local signal voltage and flaw depth (Prieux et al., 1994).
• tubes with macrocracks detected by the rotating probe, regardless of the length; • tubes containing, in the same area, both corrosion (initiated from the secondary side) and axial cracks (initiated from the primary side).
4. Outer diameter circumferential cracking at the top of the tube sheet 3. Intergranular stress corrosion cracking within sludge deposits This type of secondary side corrosion was found for the first time in 1988 with the rotating probe. The degradation consists in a network of numerous short axial cracks. Sometimes, several shallow cracks meet, thus producing a single macrocrack. The affected area along the tube axis can be quite long, i.e. similar to the deposit height. For that reason, preliminary inspection based on a bobbin coil probe in absolute mode is performed. Fig. 3 shows an example with the Lissajous pattern and its signal components. Burst tests were performed on pulled tubes and led to three plugging criteria: • tubes with bobbin coil indication above a given voltage limit;
OD circumferential cracking occurs generally within the last millimetres below the secondary side of the tube sheet.-The defect consists in a network of shallow circumferential cracks, sometimes reaching each other, sometimes separated by axial ligaments. This cracking is often present over 360 ° with various depths around the circumference (Cattant and Mercier, 1993). This morphology is considered as a disadvantage for detection by NFT. Ligaments between individual cracks allow the eddy currents to circulate and thus can reduce the detection capability. Obviously, the bobbin coil is not suitable for circumferential crack detection and the use of rotating probes is necessary. Imaging signal processing can be useful to compare the capabilities of probes and then to develop software suitable for field
R. Comby /Nuclear Engineering and Design 168 (1997) 255 259
258
Fichier E : k I R ~ S T L ~ I I Z F T 2 O B . S T L - Canal: 2
Min : -351.1 mV
,
¥
Max: t77.7 mV
Tours : 1Fo2 - 52.J52
Amplitude : 517.7 mV
~rmla : 164.4 °
1113 mV~ 1 7 1 6 "
>~--~
,3
! Uj ¸
Amplitude : 513.2 mV
Fig. 4. Eddy current rotating probe signal on a circumferential crack (obtained with laboratory software).
R. Comby / Nuclear Engineering and Design 168 (1997) 255-259 360° x 760pro
1,00 0,90 0,80 o,7o
0,60
i : ected crack
>" 0,50 o~ 0,40
non detected crack
0,30 0,20 0,10 o,o0 0
20
40 Cracked area
60
80
1O0
(%)
Fig, 5. OD circumferential crack detection as a function of crack features.
inspections. An example is given in Fig. 4. In France, all tubes made of mill-annealed Inconel 600 are inspected at every refueling outage, in their hot leg. Defect growth rate is difficult to estimate, since correct sizing is not achievable with common probes. This is the reason why all tubes with detectable cracks are plugged. Data provided by destructive examination of 35 cracked tubes have been analysed. 18 tubes were pulled because of circumferential cracks detected by the probe. 17 tubes were pulled for other reasons, with a crack observed in laboratory and not previously reported by NDT. The detection (yes or no) was compared with the crack morphology, represented by two parameters: • the percentage of cracked area in a cross-section of the tube; • the symmetry factor, defined as the ratio between the percentage of cracked area and the maximum depth. The results are presented in Fig. 5. All cracks with a minimum depth greater than 750 jam, or a depth greater than 600 pm along a 40 ° arc length, are
259
detected. A statistical approach shows that cracks reaching 35% of cross-sectional area are detected with a probability of 95% (Comby, 1996). Since 1990, several ultrasonic (UT) inspections have been performed on a small number of tubes, in addition to eddy current examination. The goal was to obtain some experience about UT capabilities. Some cracks, not reported by eddy current rotating probes, were detected by UT. Their small depth, measured on pulled tubes, did not modify the previous conclusions regarding eddy current performances. Nevertheless, in several cases, indications reported by UT did not correspond to actual degradations. These false calls were probably due to deposits or tube profile disturbances. In other cases, some UT indications (due to circumferential cracks, as confirmed by destructive examination) were not previously considered as representative of real cracks by the analysts. Finally, it was decided that UT inspection was not reliable enough to be implemented during field inspections. In order to anticipate the possible implementation of new techniques, Electricit6 de France is currently working on the improvement of eddy current rotating probes. In addition, a new ultrasonic probe was recently qualified.
References F. Cattant and L. Mercier, OD circumferential cracking at top of tube sheet; various features observed on pulled tubes, EPRI IGA/IGSCC Workshop, Minneapolis, MN, 1993. R. Comby, In-service inspection of SG tubes: NDE Performance demonstration based on pulled tubes destructive evaluation, EPRI Steam Generator NDE Workshop, Long Beach, 1996. J. D6chelotte, Acceptability of degradation, tube plugging criteria, SFEN Conf. on Secondary Side Corrosion in Steam Generators, Paris, 1995. B. Prieux, F. Vaillant, F. Cattant and P. Lemaire, Secondary side cracking at Saint-Laurent BI NPP, AFEN Int. Conf. Fontevraue 3, Fontevraud, 1994.
ELSEVIER
andDesngn
Secondary side corrosion in steam generator tubes: lessons learned in France from the in-service inspection results Robert Comby EDF, Central Laboratories, 21 AlleO PriveO, Carrefour Pleyel, 93206 St Denis, Ckdex 1, France
Abstract Non-destructive testing (NDT) has proved to be very important in the maintenance of steam generator tubing. This is particularly true in the case of secondary side corrosion, because this type of degradation leads to various morphologies which are often complex (intergranular attack) (IGA), intergranular stress corrosion cracking (IGSCC), or a mixture of both. Their detection and characterization by the usual NDT techniques have been achieved through numerous laboratory studies, which were conducted in order to determine the performance and limitations of NDT. Pulled tube examination in a hot laboratory was very valuable, for both NDT and fracture mechanics aspects. The eddy current bobbin coil probe, used for multipurpose inspection of tubes, allows the detection of IGA-SCC at the tube support plate elevation. In France, the use of rotating probes is not required for that type of degradation, since the repair criterion is based on bobbin coil results only. The bobbin coil is also used for detection of IGSCC occurring in free spans, within sludge deposits. The eddy current rotating probe allows, in that case, characterization of main cracks. Concerning the outer diameter initiated circumferential cracks which occur at the top of the tube sheet, only the rotating probe is used. An ultrasonic (UT) inspection was performed several times, in order to obtain information on UT capabilities. The goal of tube inspection is obviously knowledge of the status of steam generators, but also to follow up degradations and to estimate their revolution, and to verify the beneficial effect of some corrective measures, e.g. boric acid injection. © 1997 Elsevier Science S.A.
I. Introduction With the primary to secondary leakage monitoring, tube inspection during refuelling outages is a necessary action to guarantee safety and availability of plants. Metallographic examination of pulled tubes has proved its effectiveness for knowledge of the morphology of defects and their behaviour under normal and accidental conditions. However, only non-destructive testing ( N D T ) can give the overall status of around 800 000 tubes in the operating French steam generators.
Steam generator maintenance activities are often referred to N D T results: • signal parameters are correlated with fracture mechanics results, in order to define a repair criterion (generally tube plugging, sometimes sleeving) based on these parameters; • successive inspections allow the data to be used for degradation growth rate estimation, which will be taken into account for the repair criterion definition and for lifetime prediction; • destructive examination of pulled tubes, in addition to the above mentioned aspects, is conducted in order to determine NDT
0029-5493/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PI1 S0029-5493(96)01 366-0
R. Comby / Nuclear Engineering and Design 168 (1997) 255-259
256
performances, i.e. detection threshold, ability to distinguish various types of defects, sizing accuracy of these defects; • these three points are thus taken into account to establish the maintenance strategy and inspection scope, and for each type of degradation, the quantity of tubes to be inspected during each outage and right probe will be defined (D6chelotte, 1995). This paper will be limited to the description of inspection techniques, for each of the three following defect types: • intergranular attack (IGA) stress corrosion cracking (SCC) at the tube support plate elevation; • intergranular SCC (IGSCC) in free spans, within sludge deposits; • outer diameter (OD) circumferential cracking at the top of the tube sheet.
2. Intergranular attack-stress corrosion cracking at the tube support plate elevation Found for the first time in France in 1986, this type of degradation is easily detected by using the
E1
E2
E3
4CE 0.515V -88"
CE 0.845V -93"
1.0 Volt/cm
1.0 Volt/cm
ll .o
Volt/cm
4CE 0.775V -84"
CF 1.285V -84" )
i
Fig. 1. Example of IGA-SCC signals at two support plate evaluations.
%
11111
•
E
M
i i
80 ~
I" m~
6O
•
•
m-
• •
m i
=o
~ 4o 20
0
20
40 estimated
60 d e p t h ( b o b b i n coil )
80
1O0 %
Fig. 2. IGA-SCC at tube support plate: depth sizing accuracy.
eddy current bobbin coil. This probe is systematically used as the worldwide multipurpose tool for tube inspection. Since safety studies and burst pressure tests on pulled tubes have been made, only indications above a given threshold have to be reported. These indications are clearly identified by the bobbin coil probe, as shown in the example in Fig. 1. The IGA-SCC indication is automatically selected by the rule-based analysis software. For each tube inspected, signals related to intersections with support plates are printed after automatic analysis. The results are then validated by specialists by simply checking the printing. If any doubt remains, a manual analysis can be performed. Signal voltage and phase angle are measured on two channels and used to estimate the depth in the tube thickness. A comparison between estimated and true depth has shown that this technique is able to estimate the true depth with an accuracy of _+ 20% (Fig. 2). The dispersion is due to the complex morphology of defects which are composed of both IGA and multiple IGSCC. Nevertheless, only the signal voltage at the main frequency is used for the plugging criterion. Despite the fact that signal voltage is not really representative of any physical defect feature, this parameter appears to be more robust in defect rejection. This approach has been recently adopted by the US. For that kind of defect, bobbin coil examination is the only method required by the inspection
R. Comby /Nuclear
Engineering and Design
168 ( 1 9 9 7 ) 2 5 5 - 2 5 9
257
i,i ,,: 14~. ,,.,, D e b u t : 5 mm ! F A
XFA
J
11 I I
I r I I I I I
Ii
- - - "~ ~7"7~'7777-'-'-'-7.7-7 . . . . . .
SSS
- S 7?5 S - S Z ' I I I I I
YFA
.
~
~
......................
75" ...................
CB:
~p: ~:
2.B
BC
I I I I
cm/cm Y:
8.50
O/cn
43 595 mU
Debut:
5 me
8.58
U/cm
I I I I I I
Fig. 3. Example of IGSCC signal, within sludge pile above the tube sheet. specifications. In some cases, an eddy current rotating probe is used to give a better idea of the defect distribution around the circumference, or to verify that no circumferential crack occurred. In addition, some correlation was found between local signal voltage and flaw depth (Prieux et al., 1994).
• tubes with macrocracks detected by the rotating probe, regardless of the length; • tubes containing, in the same area, both corrosion (initiated from the secondary side) and axial cracks (initiated from the primary side).
4. Outer diameter circumferential cracking at the top of the tube sheet 3. Intergranular stress corrosion cracking within sludge deposits This type of secondary side corrosion was found for the first time in 1988 with the rotating probe. The degradation consists in a network of numerous short axial cracks. Sometimes, several shallow cracks meet, thus producing a single macrocrack. The affected area along the tube axis can be quite long, i.e. similar to the deposit height. For that reason, preliminary inspection based on a bobbin coil probe in absolute mode is performed. Fig. 3 shows an example with the Lissajous pattern and its signal components. Burst tests were performed on pulled tubes and led to three plugging criteria: • tubes with bobbin coil indication above a given voltage limit;
OD circumferential cracking occurs generally within the last millimetres below the secondary side of the tube sheet.-The defect consists in a network of shallow circumferential cracks, sometimes reaching each other, sometimes separated by axial ligaments. This cracking is often present over 360 ° with various depths around the circumference (Cattant and Mercier, 1993). This morphology is considered as a disadvantage for detection by NFT. Ligaments between individual cracks allow the eddy currents to circulate and thus can reduce the detection capability. Obviously, the bobbin coil is not suitable for circumferential crack detection and the use of rotating probes is necessary. Imaging signal processing can be useful to compare the capabilities of probes and then to develop software suitable for field
R. Comby /Nuclear Engineering and Design 168 (1997) 255 259
258
Fichier E : k I R ~ S T L ~ I I Z F T 2 O B . S T L - Canal: 2
Min : -351.1 mV
,
¥
Max: t77.7 mV
Tours : 1Fo2 - 52.J52
Amplitude : 517.7 mV
~rmla : 164.4 °
1113 mV~ 1 7 1 6 "
>~--~
,3
! Uj ¸
Amplitude : 513.2 mV
Fig. 4. Eddy current rotating probe signal on a circumferential crack (obtained with laboratory software).
R. Comby / Nuclear Engineering and Design 168 (1997) 255-259 360° x 760pro
1,00 0,90 0,80 o,7o
0,60
i : ected crack
>" 0,50 o~ 0,40
non detected crack
0,30 0,20 0,10 o,o0 0
20
40 Cracked area
60
80
1O0
(%)
Fig, 5. OD circumferential crack detection as a function of crack features.
inspections. An example is given in Fig. 4. In France, all tubes made of mill-annealed Inconel 600 are inspected at every refueling outage, in their hot leg. Defect growth rate is difficult to estimate, since correct sizing is not achievable with common probes. This is the reason why all tubes with detectable cracks are plugged. Data provided by destructive examination of 35 cracked tubes have been analysed. 18 tubes were pulled because of circumferential cracks detected by the probe. 17 tubes were pulled for other reasons, with a crack observed in laboratory and not previously reported by NDT. The detection (yes or no) was compared with the crack morphology, represented by two parameters: • the percentage of cracked area in a cross-section of the tube; • the symmetry factor, defined as the ratio between the percentage of cracked area and the maximum depth. The results are presented in Fig. 5. All cracks with a minimum depth greater than 750 jam, or a depth greater than 600 pm along a 40 ° arc length, are
259
detected. A statistical approach shows that cracks reaching 35% of cross-sectional area are detected with a probability of 95% (Comby, 1996). Since 1990, several ultrasonic (UT) inspections have been performed on a small number of tubes, in addition to eddy current examination. The goal was to obtain some experience about UT capabilities. Some cracks, not reported by eddy current rotating probes, were detected by UT. Their small depth, measured on pulled tubes, did not modify the previous conclusions regarding eddy current performances. Nevertheless, in several cases, indications reported by UT did not correspond to actual degradations. These false calls were probably due to deposits or tube profile disturbances. In other cases, some UT indications (due to circumferential cracks, as confirmed by destructive examination) were not previously considered as representative of real cracks by the analysts. Finally, it was decided that UT inspection was not reliable enough to be implemented during field inspections. In order to anticipate the possible implementation of new techniques, Electricit6 de France is currently working on the improvement of eddy current rotating probes. In addition, a new ultrasonic probe was recently qualified.
References F. Cattant and L. Mercier, OD circumferential cracking at top of tube sheet; various features observed on pulled tubes, EPRI IGA/IGSCC Workshop, Minneapolis, MN, 1993. R. Comby, In-service inspection of SG tubes: NDE Performance demonstration based on pulled tubes destructive evaluation, EPRI Steam Generator NDE Workshop, Long Beach, 1996. J. D6chelotte, Acceptability of degradation, tube plugging criteria, SFEN Conf. on Secondary Side Corrosion in Steam Generators, Paris, 1995. B. Prieux, F. Vaillant, F. Cattant and P. Lemaire, Secondary side cracking at Saint-Laurent BI NPP, AFEN Int. Conf. Fontevraue 3, Fontevraud, 1994.