inducement of iga/scc in i600 steam generator tubing during unit outages dave durance – bruce...

Inducement of IGA/SCC In I600 Steam Generator Tubing During

Unit Outages Dave Durance – Bruce Power, Ken Sedman -

Bruce Power

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Inducement of IGA/SCC In I600 Steam Generator Tubing During Unit Outages

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.

Overview

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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.

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Unit 4 Operating History Since Restart

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

• 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

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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.

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Inducement of IGA of SG tubing during Outages

Tube R33C61 SG4 2007 (PDA at 84%)Tube R33C61 SG4 2007 (PDA at 84%) Tube R31C57 SG4 2007 (PDA 75%)Tube R31C57 SG4 2007 (PDA 75%)

Tube R25C5 removed from Unit 4 SG4 in 2006

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Bruce A SG4 HL Interpolated Sludge Heights and 2006, 2007 Circumferential IGA/SCC Indications

Path Forward Post Spring 2007 Findings:

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.

Orientation of Pulled Tubes to Significant Future Defects

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Tube Sample Population Plugs in Sludge 2005 2006 2007 significant defects

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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)

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What is the driving the degradation? Growth, initiation rates increasing dramatically, then fall off in fall 2007 Do not correlate with operating interval Isolated to SG4

2005 Outage SG1 SG2 SG3 SG4 SG5 SG6 SG7 SG8 Partially Drained Period (Days) 14 26 26 6 21 10 16 18

Completely Drained/Dry Period (Days) 12 0 0 20 0 11 5 3.5 Wet Layup Period (Days) 7 7 7 7 7 7 7 7 # RTZ Circ Cracks 2006 0 2 0 16 0 0 0 0

2006 Outage SG1 SG2 SG3 SG4 SG5 SG6 SG7 SG8

Partially Drained Period (Days) 38 38 38 20.5 11 11 11 11 Completely Drained/Dry Period (Days) 0 0 0 17.5 0 0 0 0

Wet Layup Period (Days) 13.0 13.0 13.0 13.0 18.0 18.0 18.0 18.0 # RTZ Circ Cracks Spring 2007 0 0 0 44 0 0 0 0

Spring 2007 Outage SG1 SG2 SG3 SG4 SG5 SG6 SG7 SG8

Partially Drained Period (Days) 15 15 15 13 15 15 15 15 Completely Drained/Dry Period (Days) 0 0 0 2 0 0 0 0

Wet Layup Period (Days) 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 # RTZ Circ Cracks Fall 2007 0 0 --- 0 --- --- --- 0

Fall 2007 Outage SG1 SG2 SG3 SG4 SG5 SG6 SG7 SG8

Partially Drained Period (Days) 0 0 0 0 2 2 2 2 Completely Drained/Dry Period (Days) 0 0 0 0 0 0 0 0

Wet Layup Period (Days) 0 0 0 0 0 0 0 0 # RTZ Circ Cracks Spring 2008 0 0 0 0 0 0 0 0

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Proposed Mechanism 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

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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.

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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 • 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

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OPEX Support For RSC Attack a Low Temperature• 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. Defect location2. Flaw morphology3. IGA/SCC rates of initiation and growth

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Defect Location Within SPR 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 full-power attack.

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Flaw Morphology 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.

Micrograph of tube R28C5 Unit 4 SG4SCC crack morphology from PWR SG Tubing Failure

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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)

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Summary of Detected RTZ Indications (Unit 4)

SG1 SG2 SG3 SG4 SG5 SG6 SG7 SG8 SG Total2004 5 0 1 0 0 2 4 0 122005 1 4 0 2 0 2 3 1 132006 5 16 0 40 0 0 21 1 83

Spring 2007 6 19 0 96 0 0 9 1 131Fall 2007 5 28 0 24 0 0 0 21 78

Spring 2008 2 0 0 0 0 0 10 0 12Total (2004-2008) 24 67 1 162 0 4 47 24 329

Total 142 257 16 616 15 35 310 324 1715

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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.

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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.

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.

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Conclusions

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.

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Possible Future Corrosion Studies

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.

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Acknowledgements

Acknowledgements: David Durance (BP NSAS) John Roberts (Cantech) Jeff Gorman (DEI) Bob Tapping (AECL) Peter King (B&W) Ruth Allen (Kinectrics)

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Questions?


Orientation of Pulled Tubes to Significant Future Defects

0

10

20

30

40

50

60

70

80

90

100

05101520253035404550556065707580859095100105110115

Column

Ro

w

Tube Sample Population Plugs in Sludge 2005 2006 2007 significant defects

inducement of iga/scc in i600 steam generator tubing during unit outages dave durance – bruce...

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