Inducement of IGA/SCC In I600
Steam Generator Tubing During
Unit Outages
Dave Durance – Bruce Power, Ken Sedman - Bruce Power
Inducement of IGA/SCC In I600 Steam Generator Tubing During
Unit Outages
Overview
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2
The degradation of Unit 4 SG tubing by IGA/SCC has limited both the operating
period and EOL predictions for Unit 4 since restart in late 2003.
The circumferential OD IGA-SCC in the RTZ has been most significant in SG4 with
substantial increases in both initiation and growth rates from 2005 through the spring
of 2007.
A detailed review of operating and shutdown practices indicated that the probable
cause is attack of the SG tubing OD by partially reduced sulfur species such as
tetrathionates and thiosulfates during unit outages
Development of these aggressive species is thought to occur during periods when the
boilers were fully drained for maintenance activities, promoting oxidation of residual
sulfur and sulfides in the TTS (Top of Tubesheet) deposits.
Contradicts the previous assumption that attack occurred during high temperature
operation.
The modification of outage practices to limit secondary side oxygen ingress in the
spring of 2007 has arrested the degradation and has had significant affects on the
allowable operating interval and EOL predictions for the entire unit.
Inducement of IGA/SCC In I600 Steam Generator Tubing During
Unit Outages
Unit 4 Bruce SG Design

• Recirculating design with
external preheaters.
• 4200 SG tubes, inverted U-Bend
arrangement with 7 support
plates (trifoil design).
• 8 Steam Generators/Unit
feeding 2 common steam drums.
• SG Tubing: I600 HTMA,
12.95mm OD, 1.1 mm wall
thickness, hard rolled in tube
sheet
• SG tubing was sensitized during
a vessel stress relieving heat
treatment.
3
Inducement of IGA/SCC In I600 Steam Generator
Tubing During Unit Outages
Bruce 4 SG Operating History
•
•
•
•
Start-Up in 1979.
Significant TTS sludge deposits in the unit.
Significant acid excursion in 1986.
Water lancing/Chemical Clean carried out on TTS in
1993 (95 SG2), hard “collars” remain in HL region.
• Operated until 1997 when the unit was laid up.
Bruce 4 SG Layup
4
• SGs filled with aerated water – later drained (No
Nitrogen Blanketing employed).
• Condition assessment in 2000/2001, inspections
revealed significant pitting and IGA at the TTS due to
the lack of proper layup chemistry, likely due to attack
by reduced sulphur species following prolonged
exposure to oxidizing conditions.
• SGs returned to controlled wet layup conditions 2001.
• Restart of Unit in late 2003 following additional
condition assessment.
Interval Start
Duration
(Months)
Sept 2003
8
June 2004
8.5
April 2005
12.0
June 2006
9
April 2007
5
Sept 2007
6
April 2008
12
Unit 4 Operating History
Since Restart
Inducement of IGA/SCC In I600 Steam Generator Tubing During
Unit Outages
2005 Findings (Following 9 months operation):
•NDE (100% TTS scans in 8 SGs) detected 2 circumferential indications, “crack-like
indications” (None in SG4). 38 SG tubes removed from the Unit for metallurgical evaluation of
the general state of the tubing. 12 removed from SG4, 2 tubes display limited IGA (minor
cracking).
2006 Findings (Following 12 months operation):
•NDE (100% TTS scans in 8 SGs) detected 16 tubes in SG4 with circumferential crack like
indications, 10 tubes removed from SG4 for metallurgical examination confirming NDE
findings. Subsequent operating interval reduced (to 9 months) based on IGA/SCC condition
assessment in SG4.
2007 (Spring) Findings (Following 9 months operation):
•NDE (100% TTS scans in 8 SGs) detected 44 tubes with circumferential indications in SG4, 4
tubes removed from SG4 for metallurgical confirming NDE findings. Subsequent operating
interval reduced (to 6 months) based on IGA/SCC condition assessment in SG4.
5
Inducement of IGA of SG tubing during Outages
Tube R25C5 removed from Unit 4 SG4 in 2006
Tube R33C61 SG4 2007 (PDA at 84%)
6
Tube R31C57 SG4 2007 (PDA 75%)
Inducement of IGA/SCC In I600 Steam Generator Tubing During
Unit Outages
Path Forward Post Spring 2007
Findings:
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7
Significant increase in defect growth
and initiation rate from 2006, does not
correlate to the previous operating
interval duration.
Development of preventative plugging
strategy, deal with the AAR (SPR)
within the HL of SG4, 400 tubes to
removed from service in the fall of
2007.
Refine deterministic models employed
for condition monitoring practices,
move to probabilistic treatment.
Reduction of operating interval
Further review of OPEX and possible
arresting techniques.
Bruce A SG4 HL Interpolated Sludge Heights and 2006, 2007
Circumferential IGA/SCC Indications
Orientation of Pulled Tubes to Significant Future Defects
100
90
80
70
50
40
30
20
10
0
115 110 105 100 95
90
85
80
75
70
65
60
55
50
45
40
35
30
25
20
15
10
Column
Tube
Sample Population
Plugs in Sludge
2005
2006
2007
significant defects
5
0
Row
60
Inducement of IGA/SCC In I600 Steam Generator Tubing During
Unit Outages
2007 (Fall) Findings (Following 5 months operation):
NDE
(TTS scans in 4 SGs) detected 0 tubes with circumferential crack like indications.
Significant preventative plugging (SG4) performed to reduce AAR in HL RTZ region of boiler.
Performed despite inspection findings due to the short operating interval and the lack of
understanding as to the mechanism.
Subsequent operating interval maintained (to 6 months) based on IGA/SCC condition
assessment for unit.
Contradicts trend established from 2005 to the spring of 2007.
Correlation between SG4 exposure time during outage to oxidizing conditions and the number
of IGA/SCC circ defects in subsequent outages. Suggests growth/initiation does not take place
during operation.
SG1
SG2
SG3
SG4
SG5
SG6
SG7
SG8
SG Total
2004
4
0
1
0
0
2
2
0
9
2005
1
0
0
0
0
0
0
1
2
2006
0
2
0
16
0
0
0
0
18
Spring 2007
0
0
0
44
0
0
0
0
44
Fall 2007
0
0
0
0
0
0
0
0
0
Spring 2008
0
0
0
0
0
0
0
0
0
Total
5
2
1
60
1
2
2
1
73
Summary of Detected Circumferential Indications (Unit 4)
9
Inducement of IGA/SCC In I600 Steam Generator Tubing During
Unit Outages
What is the driving the degradation?
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Growth, initiation rates increasing dramatically, then fall off in fall 2007
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Do not correlate with operating interval
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Isolated to SG4
10
2005 Outage
Partially Drained Period (Days)
Completely Drained/Dry Period (Days)
Wet Layup Period (Days)
# RTZ Circ Cracks 2006
SG1
14
12
7
0
SG2
26
0
7
2
SG3
26
0
7
0
SG4
6
20
7
16
SG5
21
0
7
0
SG6
10
11
7
0
SG7
16
5
7
0
SG8
18
3.5
7
0
2006 Outage
Partially Drained Period (Days)
Completely Drained/Dry Period (Days)
Wet Layup Period (Days)
# RTZ Circ Cracks Spring 2007
SG1
38
0
13.0
0
SG2
38
0
13.0
0
SG3
38
0
13.0
0
SG4
20.5
17.5
13.0
44
SG5
11
0
18.0
0
SG6
11
0
18.0
0
SG7
11
0
18.0
0
SG8
11
0
18.0
0
Spring 2007 Outage
Partially Drained Period (Days)
Completely Drained/Dry Period (Days)
Wet Layup Period (Days)
# RTZ Circ Cracks Fall 2007
SG1
15
0
8.0
0
SG2
15
0
8.0
0
SG3
15
0
8.0
---
SG4
13
2
8.0
0
SG5
15
0
8.0
---
SG6
15
0
8.0
---
SG7
15
0
8.0
---
SG8
15
0
8.0
0
Fall 2007 Outage
Partially Drained Period (Days)
Completely Drained/Dry Period (Days)
SG1
0
0
SG2
0
0
SG3
0
0
SG4
0
0
SG5
2
0
SG6
2
0
SG7
2
0
SG8
2
0
Wet Layup Period (Days)
# RTZ Circ Cracks Spring 2008
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
Inducement of IGA/SCC In I600 Steam Generator Tubing During
Unit Outages
Proposed Mechanism
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A proposed mechanism for this IGA-SCC (IGA) was attack by reduced sulfur
species occurring during unit shutdowns and subsequent startup evolutions not
during high temperature operation.
This mode of attack has been documented for Sensitized MA I600, in the lab
and in operation (US OPEX), attack can be extremely rapid and aggressive.
Theory: Sulphate reduced to sulphide/sulphur during hot operation. Oxygen
(air) oxidises Sulphur/Sulphide to Thiosulphate/polythionate and this exposure
causes VERY rapid IGA in sensitized alloys.
For this to be a viable cause of the SG tubing defects, specific conditions must
be met as follows:
1.
An inventory of sulfur compounds within the HL TTS sludge-pile region
2.
Exposure to oxidizing conditions
3.
Susceptible SG tubing microstructure.
4.
Residual stress and possibly additional operating stresses
Inducement of IGA/SCC In I600 Steam Generator Tubing During
Unit Outages
Inventory of Sulfur Compounds:
•
The presence of sulfide compounds in the sludge material of the Bruce A boilers has
been confirmed through analysis of the sludge deposits on removed tubes
Sulfides and possibly elemental sulfur accumulate in the sludge and crevices at the
TTS as a result of the normal concentration processes that act in this area, particularly
in the HL region of the SG.
The sources of the sulfur are considered to be the low levels of sulfates that entered
the SGs with the feedwater, and also a sulfate ingress event that occurred in 1986..
•
•
Exposure to oxidizing conditions:
•
During shutdown periods when the SGs were not filled with treated water the TTS
area was exposed to oxidizing conditions for extended periods.
The oxidizing conditions that occurred during the fully drained periods caused
oxidation of sulfides or elemental sulfur at the TTS, putting the sulfur into aggressive
•
oxidation states, such as those of tetrathionates and thiosulfates.
12
Inducement of IGA/SCC In I600 Steam Generator Tubing During
Unit Outages
Susceptible SG tubing microstructure:
•
•
•
The high temperature mill annealed Inconel 600 tubing (600HTMA) in the Bruce 4
steam generators was sensitized during the vessel post weld stress relief.
Formation of chromium carbide precipitates at the grain boundaries reduces the
chromium concentrations at the GB increasing the susceptibility to attack by reduced
sulfur species.
Confirmed by lab tests of removed tubes from Unit 4
Residual/Operating Stress
•
•
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Residual stress is present in the RTZ due to the hard rolling to expand the tube in
the tubesheet
DEI suggested that the IGA attack may not occur until, or shortly following
restart, and that the additional operating stresses may be paying a role
Inducement of IGA/SCC In I600 Steam Generator Tubing During
Unit Outages
OPEX Support For RSC Attack a Low Temperature
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The previous observations above provide a basis for low temperature attack by
reduced sulfur species during unit shutdowns (and possibly during restart).
To provide a further technical basis for this failure mode Bruce A observations
were supplemented with US industry OPEX with regards to:
1.
2.
3.
14
Defect location
Flaw morphology
IGA/SCC rates of initiation and growth
Inducement of IGA/SCC In I600 Steam Generator Tubing During
Unit Outages
Defect Location Within SPR
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Circ. OD IGA/SCC in SG4 is not
consistent with PWRs high attack
Model boiler tests and PWR plant
experience indicate that, at high
power, impurities concentrate in the
top 1 to 2 cm (½ to 1 inch ) of the
sludge, which acts as a dryout zone,
and that IGA/SCC tends to occur in
this region.
Circ cracks in PWR SGs concentrate
near the edges of the sludge pile
where the 1 to 2 cm dryout zone
corresponds to the roll transition
zone.
In the central deep sludge pile
IGA/SCC tends to occur at the top
of the sludge and to be axial. In
contrast in Bruce 4 SG4 the
circ.defects are concentrated in the
center of the sludge pile. close to the
TTS.
The cracking pattern at Bruce 4 is
consistent with startup attack but not
Inducement of IGA/SCC In I600 Steam Generator Tubing During
Unit Outages
Flaw Morphology
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The flaw morphology exhibited by the circumferential OD IGA/SCC in
Boiler 4 is characterized by very broad bands of IGA in the roll transition
aream rather than by narrow IGA with leading fingers of SCC and thus is
more consistent with IGA caused by exposure to reduced sulfur species at
low temperature than it is with IGA/SCC at high temperature.
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SCC crack morphology from PWR SG Tubing
Failure
Micrograph of tube R28C5 Unit 4 SG4
Inducement of IGA/SCC In I600 Steam Generator Tubing During
Unit Outages
Modeling Progression of Degradation
 As shown during the 2006 and Spring 2007 inspections, 16 and 44 tubes with
circumferential OD IGA/SCC at the roll transition zone at the TTS were
detected in the most severely affected SG (SG4). Following normal industry
practice for PWRs, this degradation was modeled as increasing with
increasing service time, measured in effective full power years (EFPY). A null
result in the Fall 2007 and Spring 2008 inspections contradicts the models
put forth.
SG1
SG2
SG3
SG4
SG5
SG6
SG7
SG8
SG Total
2004
4
0
1
0
0
2
2
0
9
2005
1
0
0
0
0
0
0
1
2
2006
0
2
0
16
0
0
0
0
18
Spring 2007
0
0
0
44
0
0
0
0
44
Fall 2007
0
0
0
0
0
0
0
0
0
Spring 2008
0
0
0
0
0
0
0
0
0
Total
5
2
1
60
1
2
2
1
73
Summary of Detected Circumferential Indications (Unit 4)
18
Inducement of IGA/SCC In I600 Steam Generator Tubing During
Unit Outages
2004
2005
2006
Spring 2007
Fall 2007
Spring 2008
Total (2004-2008)
Total
SG1
5
1
5
6
5
2
24
142
SG2
0
4
16
19
28
0
67
257
SG3
1
0
0
0
0
0
1
16
SG4
0
2
40
96
24
0
162
616
SG5
0
0
0
0
0
0
0
15
SG6
2
2
0
0
0
0
4
35
Summary of Detected RTZ Indications (Unit 4)
19
SG7
4
3
21
9
0
10
47
310
SG8
0
1
1
1
21
0
24
324
SG Total
12
13
83
131
78
12
329
1715
Inducement of IGA/SCC In I600 Steam Generator Tubing During
Unit Outages
Defect Growth Rates
Low Temperature RSC Attack
 Laboratory tests and PWR plant experience indicate that sensitized Alloy
600 tubes of the type in the Bruce 4 boilers are susceptible to IGA/SCC
in environments with reduced sulfur species, low pH, and oxidizing
conditions. Growth rates in sensitized Alloy 600 can be very rapid such
that significant growth, in the range of magnitudes observed in SG4, is
possible during a short startup period.
High Temperature Attack During Operation
 The growth rates determined for SG4, assuming that the defects grew at
high temperature and high power, are about 10 times higher than
expected based on PWR experience.
20
Actions
The approaches being used to minimize occurrence of circumferential
OD IGA/SCC and also volumetric attack in the TTS sludge pile region
are based on the conclusion that most of the flaw growth has at low
temperature by reduced sulfur species coupled with oxidizing conditions.
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Wet layup of boilers during outage (with hydrazine (≥ 50 ppm)).
Promotes reducing conditions and acts as a cathodic depolarizer.
Eliminate draining of boilers except for critical operations until
EOL.
Following any drained periods return to controlled wet lay-up ASAP.
Laying up boilers, after drain down and refilling, for max period prior
to start-up.
Conclusions
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The Bruce 4 RTZ Circumferential OD IGA/SCC defects occurred
during startup evolutions following outages and not during operation as
previously assumed.
Specifically, these defects are considered likely the result of exposure to
oxidizing conditions and the presence of aggressive reduced sulfur
species in RTZ area.
This assumption is supported by rate of attack, defect morphology,
defect location and exposure data from the 2005 and 2006 outages.
Supports longer operating periods and has significant impact on Unit 4
EOL, assuming Spring 2009 inspection results remain consistent with the
degradation hypothesis.
22
Possible Future Corrosion Studies
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23
Like to be able to understand how the Oxygen exposure
drove the IGA degradation (does oxygen need to diffuse to
the RTZ or not)
Like to get a good understanding of the Kinectics regarding
rate of attack.
When does the growth occur, during layup exposure or start
up ( can we soak and avoid damage?).
What are the key ingredients, Cu, previous acid exposure,
nature or depth of sludge pile.
Nitrogen or Argon blanketing recommendations.
Acknowledgements
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Acknowledgements:
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24
David Durance (BP NSAS)
John Roberts (Cantech)
Jeff Gorman (DEI)
Bob Tapping (AECL)
Peter King (B&W)
Ruth Allen (Kinectrics)
Inducement of IGA/SCC In I600 Steam Generator Tubing During
Unit Outages
Questions?
25
Orientation of Pulled Tubes to Significant Future Defects
100
90
80
70
50
40
30
20
10
0
115 110 105 100 95
90
85
80
75
70
65
60
55
50
45
40
35
30
25
20
15
10
Column
Tube
Sample Population
Plugs in Sludge
2005
2006
2007
significant defects
5
0
Row
60