Volumetric Flaws in the

Preheater Section of

Darlington NGS Steam

Generator Tubes

June 16, 2010

Volumetric Flaws in the Preheater Section of

Darlington NGS Steam Generator Tubes

by

S. Sullivan, E. Cartar, T. Harasym and I. Vela – OPG

and

J. Huggins, G. Goszczynski – Kinectrics

Third International CANDU In-Service Inspection and NDT in Canada 2010 Conference – June 14-17, 2010

Outline of Presentation

• Darlington Steam Generators

• Tube Inspection Technologies deployed in OPG

Steam Generators

• NDE Signals from Preheater (PH) Volumetric

Flaws

• Destructive Examination (DE) of removed tubes

• Potential Causes

• Summary

3

Darlington NGS Steam

Generators

No leaks have ever occurred due to

early detection of flaws

4663 Incoloy 800 Tubes

Eleven 410 Stainless Steel Lattice Bar

Support Structures (“Egg Crates”)

Eight 410 Stainless Steel U-Bend Anti-Vibration Fan Bar structures (AVBs)

U-Bend Auxiliary AVB Retrofit Structures

(have significantly reduced tube fretting)

Preheater Section with Eleven

410 Stainless Steel Drill-Hole (Baffle) Plates4

Support Plates in DNGS SGs

5

Lattice Bar Structures for straight-leg

tube sections

Drill-hole Baffle plates used in preheater

section – Baffle plates are windowed

Windows in Darlington PH

Baffle Plates

6

Tube Inspection Technologies

Deployed by OPG

• Basic – Zetec Bobbin Eddy Current Testing (ET) Probe

• Enhanced – Zetec 3x12 X-Probe Eddy Current Array

• High Resolution Characterization – Kinectrics Tiny Rotating

Ultrasonic Tube Inspection Equipment (TRUSTIE)

- Normal Beam

- Axial Shear Wave

- Circumferential Shear Wave

7

Bobbin Eddy Current Probe

• Advantages: Inexpensive, Fast Scanning

• Disadvantages: Limited Sensitivity at Expansions and Support

Plates, Insensitive to Circumferential Cracks, Limited Resolution for Flaw Characterization

8

Bobbin Eddy Current Signals

9

Calibration Signals –

Multi-Channel/frequency

display

Bobbin Eddy Current Signals

Signal from Darlington Unflawed Baffle Plate Location10

Bobbin Eddy Current Signals

Signal from Volumetric Flaw at Preheater Baffle Plate11

Distortion due to

Volumetric Flaw

X-Probe Eddy Current Array

12 Coil Layout for X-Probe Array for Darlington/CANDU 600 SG Tubes

24 Axial T/R pairs use

end row coils.

12 Circumferential T/R

pairs are pairs within the

same row.

Coil Layout

End Bracelet Coilsare Axial T/R Pairs

Center Bracelet Coils are Circumferential T/R Pairs

X-Probe Signals

Signals from Volumetric Flaw at PH Baffle Plate Location13

X-Probe Signals

14 Signal from a 15% tw Volumetric Flaw at PH Baffle Plate

Ultrasonic Testing of SG Tubes

15

Kinectrics Tiny Rotating Ultrasonic Tube Inspection Equipment (TRUSTIE)

Normal Beam

Shear for Axial Flaws Shear for Circ Flaws

TRUSTIE Signals

(Normal-Beam Scans)

16• UT C-Scan displays of OD indications at PH support plate

locations showing various stages of degradation morphology

Baffle

Plate

Baffle

Plate

TRUSTIE Signals

17

• UT Shows flaw signals, and gap between tube and

support plate when external region is flooded

C-Scan B-Scan

Baffle

Plate

UT Characterization of PH

Flaws

• These flaws are NOT pit-like or fret-like

• Maximum UT Depth Measurement of 52% of the tube wall

• Flaw signals have unique triangular shape

• Flaw signals occur at locations that appear to correlate to the gap between the tube and support plate

18

Complementary Information

19

Unit YearX-Probe

Indications

Bobbin

IndicationsUT Depth Range

Largest UT Flaw not

detected by ET

D2 2007 55 10 13-52% tw 6% tw

D1 2008 20 5 10-40% tw 7% tw

D3 2009 48 2 8-34% tw 11% tw

D4 2010 47 2 6-26% tw 15% tw

Total 170 19 6-52% tw 15% tw

Information on the detection capabilities of ET using information from

the more sensitive UT data

Removed Tube R3 C51 from D1

SG4 (2008)

20

• UT Measured flaw shape confirmed by DE. Maximum

Flaw Depth by DE = 43% tw.

UT C-Scan Image

Photograph of Flaw at P05

SP Location

Destructive Evaluation (DE)

• Optical Images showing depth

profiles at 2nd, 3rd, and 4th grinds

• Higher magnification image

showing microstructure

21

Morphology and Mechanism

• Maximum Wall Loss of Flaws in Tube R3 C51 from DNGS 1, SG 4 in

2008:

– at P05 = 43% tw (UT ~ 40% tw)

– at P06 = 8% tw (UT ~ 7% tw)

• No Evidence of Cracking or Intergranular Attack

• General pattern of flaws and elemental analysis at and away

from flaws strongly suggests that a hydrodynamic process

(possibly cavitation) contributed to the formation of the flaws and NOT a corrosion mechanism.

22

Mechanism – Factors for

Assessment

• A CANDU Owners Group (COG) R&D Project (by AECL) has

been initiated to further assess a possible flaw mechanism

(cavitation):

– Detailed modelling of thermal-hydraulic conditions in SG preheater

zone (temperatures, pressures, flows)

– Computational fluid dynamics to study effects on boiling of flow

rate through the clearance, primary fouling, secondary fouling,

clearance width, tube eccentricity, tube skewdness

23

Flaws at Odd Numbered Baffle

Plates (P05, P07, P09)

24

10

Row

10 20 30 40 50 60 70 80 90 100 110 120Column

20

30

40

50

60

70

80

90

100

Flaws at Even Numbered Baffle

Plates (P04, P06, P08, P10)

25 D931 D811 D721 Inspections

Tubesheet Outlet

P02-4-6-8-10 Composite (Crossover Compensated)

10

Row

10 20 30 40 50 60 70 80 90 100 110 120Column

20

30

40

50

60

70

80

90

100

Flaws in High Row Tubes at P03

Baffle Plate Locations

26

10

Row

10 20 30 40 50 60 70 80 90 100 110 120Column

20

30

40

50

60

70

80

90

100

Growth Monitoring

• Growth monitoring is based on Signal-to-Signal

comparisons – that is comparison of current flaw

signals with signals from past inspections

• Signal-to-Signal Analysis has been mostly based on

Bobbin Eddy Current Probe Signals

• Signal-to-Signal Analysis has indicated that these

flaws were present since 2001 and have exhibited

SLOW to NO growth

• More UT scan/re-scan data in the future will give

higher precision and higher confidence in our

understanding of the growth of these flaws27

Summary

• A unique flaw mechanism has been discovered in

Darlington NGS steam generator tubes.

• OPG’s use of complementary NDE technologies and

Destructive Analysis of pulled tubes has enabled the

detection (with X-Probe) and characterization (with

TRUSTIE and DE) of new flaw mechanisms in a safe,

reliable and efficient manner.

28

Path Forward

• Monitor all Units to assess affected population, growth rate, and

morphology of flaws starting with Unit 2 in Fall 2010

• Develop ET sizing capability and inspection strategy - pull

additional tubes, as needed, to support ET sizing

• Continue “causal factors” assessment activities:

– CANDU Owners Group (COG) R&D

– Monitor OPEX (operational experience) of other utilities that may

encounter similar flaws

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