16th Int. Symposium on Zirconium in the NuclearIndustry, May 13, 2010, Chengdu, China

Shadow Corrosion-InducedBow Of Zircaloy-2 Channels

S. T. Mahmood, P. E. Cantonwine, Y.P. Lin, D. C. CrawfordGlobal Nuclear Fuel

E. V. Mader, K. EdsingerElectric Power Research Institute

2

Outline

Background

Poolside data

Hotcell data

Discussion

Conclusions

3

Channels in a BWR and Potential forControl Blade Interference

• BWR fuel assemblies located insidechannel boxes.

• Four assemblies in each control cellwith a control blade (CB).

• CB-channel interference possible ifchannel deformation is excessive.

• Channel deformation from bulge andbow.

• Channel bow occurs due to lengthdifferential from:

– Neutron fluence gradient– Hydrogen differential from

shadow corrosion on CB side

4

Shadow Corrosion-Induced Channel Bow• Shadow Corrosion: enhanced in-reactor

corrosion on Zr-alloy when placed in closeproximity to a dissimilar metal.

• Extent of shadow corrosion depends on gapdistance.

• Enhanced corrosion on channel face exposedto stainless steel CB casing.

• Higher corrosion associated with higherhydrogen concentration.

• Higher hydrogen concentration results in largervolume increase.

• Channel bow from length differential acrosschannel.

•

90 in

90 in

90 in

))(( tHPUBow Dµ

5

Objective of Present Work• Previous work based on 3-cycle channels

(Proc. Int. LWRFP Meeting, San Francisco, 2007)– ~1500 – 2100 days– ~37 – 48 GWd/MTU

• Shadow bow correlated with hydrogen differential

• Main open issue– Kinetics of corrosion and hydriding– CB exposure early, channel bow late in life

• Present work– Broader exposure range by including 1-cycle and 4-cycle channels– Improved understanding of general and shadow corrosion and hydriding

from metallography and hydrogen measurements– Examine distance effect§ CB-channel interference depends on gap between channel and CB§ Relative gap for S-lattice = 0.74, C-lattice = 0.83 and D-lattice = 1.0

6

Terminology

Shadow Bow:Measured – predicted bow from fluence gradientBlade-side:Channel face adjacent to the control blade(sides 3 and 4).Non-blade-side:Channel face away from the control blade(sides 1 and 2).

ECBE:Effective Control Blade Exposure is theproduct of the control blade insertionlength (inches) and insertion time (days)for each insertion applied, weighted bytime of application, and summed.

0

20

40

60

80

100

120

140

160

0 500 1000 1500 2000 2500 3000

Residence time (days)

Inse

rtio

n D

ista

nce

(in)

Bundle 1: ECBE = 17795 inch-days

Bundle 2: ECBE = 712 inch-days

4.1 mm

ECBE712 in-day

ECBE~18,000in-day

Similar CB insertion, but verydifferent ECBE due to weighting

A B C D E F G H J

12

3

45

6

7

8

9

Side 4

Side1

Side3

Side 2

10

K

ControlBlade

7

Channel Bow Measurements

Bow measurements biased high (towards CB).

S-lattice plants tend to develop higher bow at lower exposure.

-500

-400

-300

-200

-100

0

100

200

300

400

500

20 25 30 35 40 45 50 55Exposure (GWd/MTU)

Mea

sure

d B

ow (m

il)S-Lattice ECBE > 10000 inch-daysD-lattice ECBE > 10000 inch-days

+- 2 Sigma Uncertaintyin Measured Bow Datawhen only FluenceGradient-Induced Bow isactive (I.e, ECBE ~ 0)

12.7 mm

-12.7 mm

Zircaloy-2 channels

S- and D-Lattice plants

Comparison of ECBE >10000 inch-days with noearly-life control

8

Lattice Type and Channel Thickness Effects

Shadow bow decrease with increasing gap.

Shadow bow increase with decreasing channel thickness.

Relative gap

S-lattice = 0.74C-lattice = 0.83D-lattice = 1.0

-100

-50

0

50

100

150

200

250

300

350

120T S-Lattice 120T C-Lattice 100T C-Lattice 100T D-Lattice

Channel Type

Mea

n Sh

adow

Bow

(mil)

8.9 mm

S vs. C C vs. D120 vs. 100

Channel Thickness

120T ~ 20% thickerthan 100T

9

Exposure Dependency of Shadow Bow

Shadow corrosion condition (CB insertion) generally occur early in life, butlarge shadow bow develop late in life. WHY?Did shadow oxide build up fast early in life and hydrogen pickup later, ordid oxide build up and hydrogen pickup both develop with exposure?

S-lattice

120T channels

ECBE > 20000 in-days

0

50

100

150

200

250

300

350

400

20.0 25.0 30.0 35.0 40.0 45.0 50.0

Exposure (GWd/MTU)

Shad

ow B

ow (m

il)

10.2 mm

10

Characteristics of Retrieved Coupons

11 channels, 2 sides, 4 elevationsPlants: 4Cycles: 1 to 4 two-year cycles Residence time: 680 – 2400 daysExposure: ~20 – 48 GWd/MTU ECBE: 0 – 34,000 inch-daysMeasured bow: up to ~370 mils (9.6 mm)

20 55 90 120

2 X X X X4 X X X X2 X X X X4 X X X X2 X X X X4 X X X X2 X X X X4 X X X X2 X X X X4 X X X X2 X4 X2 X X X X4 X X X X2 X X X X4 X X X X2 X X X X4 X X X X1 X X X X3 X X X X1 X X X X3 X X X X

D

LatticeType

S

S

C

679

2401

2401

1486

1882

1882

2079

ResidenceTime(days)

1486

1486

1486

24.6

376.2

3

3

27112

271122079

Zircaloy-2

Zircaloy-2

48.1

48.1

A

C7

8

B5

3

323

28

4

4

17795

712

1 34105 22

D10

11

Zircaloy-2

Zircaloy-2

43.9

42.9

A 9 Zircaloy-2 19.7S

10721

21916

16216

14025

Zircaloy-2

3

3

39.7

36.5

39.6

40.7

3

3

314

6 -30

Zircaloy-2

Zircaloy-2

48.2

47.9

3

3 0

Coupon Elevations (in)

1 61

2 119

ChannelSide

Zircaloy-2

Zircaloy-2

ECBE(in-days)

8133

292

4 336

PlantChannel

IDMax. Avg.Bow (mils)

ChannelMaterial

Exposure(GWd/MTU)

No. ofCycles

Zircaloy-2A B C D E F G H J

12

3

4

5

6

7

8

9

Side 4

Side1

Side3

Side 2

11

0

20

40

60

Oxi

de T

hick

ness

(mic

rons

)

1 2 3 4 5 6 7 8 9 10 11 20

5590 12

0

Channel ID

Elevation(in)

Blade

Outer surfaceBlade side

Oxide Thickness Overview

Outer surface on CB side has thickest oxide.

0102030405060

Oxi

de T

hick

ness

(mic

rons

)

1 2 3 4 5 6 7 8 9 10 11 20

5590 12

0

Channel ID

Elevation(in)

Outer surfaceNon-blade side

0102030405060

Oxi

de T

hick

ness

(mic

rons

)

1 2 3 4 5 6 7 8 9 10 11 20

5590 12

0

Channel ID

Elevation(in)

Blade

Inner surfaceBlade side

0102030405060

Oxi

de T

hick

ness

(mic

rons

)

1 2 3 4 5 6 7 8 9 10 11 20

5590 12

0

Channel ID

Elevation(in)

Control Blade

Inner surfaceNon-blade side

90 in Side 1 Side 3

Zircaloy-2

Outer Surface

Inner Surface

50 mm

12

4-Cycle Channel (~18000 in-day ECBE, D-lattice plant)

• Oxide thickness on blade side similar to other high bow 3-cycle channels.

• High hydrogen contents, especially on blade side.

• Lower ECBE 4-cycle channel had less blade side corrosion and hydrogen.

A B C D E F G H J

1234

56

7

8

9

Side 4

Side1

Side3

Side 2

55 in

90 in Side 1

277 ppm

196 ppm

33 mm total

29 mm total

250 mm250 mmSide 3

629 ppm

672 ppm

60 mm total

61 mm totaloxide

250 mm250 mm

13

Single-Cycle Channel (~34000 in-day ECBE, S-lattice plant)

• Very high ECBE from plant with small inter-channel gap.

• Difference in oxide and hydrogen between blade and non-blade sides.

• Blade side oxide and hydrogen much less than in 3- and 4-cycle channels.

120 in

90 in

Side 2

13 ppm

11 ppm

Side 4

49 ppm

48 ppm

14 mm total21 mm total

28 mm total 12 mm total

250 mm250 mm250 mm250 mm

14

Hydriding Kinetics

• H concentration increases with number of cycles.• H concentration higher on blade side.• HPU (%) increase with number of cycles.• Similar HPU (%) on blade/non-blade side of multi-cycle channelsbut higher on blade side of 1-cycle channel.

CB

Sid

e, M

etal

Non

-CB

Sid

e, M

etal

CB

Sid

e, O

xide

Non

-CB

Sid

e, O

xide 9

12

34

1110

0

50

100

150

200

250

300

350

400

Hyd

roge

n C

once

ntra

tion

(ppm

)

Channel ID

1 Cycle

4 Cycles3 Cycles

9 1 7 8 2 4 3 5 11 10Non-CB Side

CB Side

0

10

20

30

40

50

60

Hyd

roge

n Pi

ck U

p (%

)

Channel ID

4 Cycles

3 Cycles

1 Cycle

15

Measured vs. Estimated Total Channel Bow

usingtheoreticalcorrelation

usingexperimentalcorrelation

usingtheoreticalcorrelation

usingexperimentalcorrelation

1 74 56 2 76 58 612 190 144 56 246 200 1193 295 225 8 303 233 2924 376 286 19 395 305 336

B 5 346 263 3 349 266 3147 -139 -106 146 7 40 258 291 221 149 440 370 376

A 9 48 37 -10 38 27 2210 436 332 96 532 428 32311 98 75 -1 97 74 28

Total Estimated Bow(mils)

EstimatedBow due toIrradiation

Growth(mils)

MeasuredBow (mils)Ch ID

A

D

C

Estimated Bow due to HDifferential (mils)

Plant

0

100

200

300

400

500

600

0 100 200 300 400 500 600

Measured Bow (mils)

Tota

l Est

imat

ed B

ow (m

ils)

0.25% lin. changeper 1000 ppm H

0.33% lin. changeper 1000 ppm H

• Linear correlation between measured and estimated channel bow.

• Further validation of bow due to length increase from hydride precipitation.

• 0.25% better fit than theoretical 0.33% linear change per 1000 ppm H.

16

ECBE Effect on Corrosion

• ECBE correlations for multiple-cycle channels.

• Oxide thickness increase with ECBE if shadow corrosion was present.

• Both channel exposure and ECBE affect blade side oxide thickness.

05

101520253035404550

0 5000 10000 15000 20000 25000 30000 35000

ECBE (in-days)

Out

er S

urfa

ce O

xide

Thi

ckne

ss(m

icro

ns)

Non-CB sideCB side

Channel 9Single Cycle

Exhibitedshadowcorrosion

4C

4C

4C: 4-cycle channels

17

Shadow and General Corrosion Kinetics

• Blade vs. non-blade side differential in oxide thickness setup after 1st cycleexposure to CB.

• Shadow oxide on blade side with significant ECBE grow at a different ratethan non-shadow oxide.

• Zry-2 develop thinner oxide than Zry-4.

05

101520253035404550

10 15 20 25 30 35 40 45 50

Channel Exposure (GWd/MTU)

Out

er S

urfa

ce O

xide

Thi

ckne

ss(m

icro

ns)

Non-CB sideCB side

Zircaloy-4

))(( tHPUBow Dµ

18

Hydrogen Pickup Kinetics

0

10

20

30

40

50

60

70

500 1000 1500 2000 2500

Channel Residence Time (days)

Hydr

ogen

Pic

ku U

p (%

) Non-CB sideCB side

Zircaloy-4

• Higher HPU% on blade side of 1-cycle channel with shadow corrosion.

• HPU% differential between blade and non-blade side diminishes withtime/exposure.

• Zry-4 shows low HPU% compared with Zry-2.

9 1 7 8 2 4 3 5 11 10Non-CB Side

CB Side

0

10

20

30

40

50

60

Hyd

roge

n Pi

ck U

p (%

)

Channel ID

4 Cycles

3 Cycles

1 Cycle

9 1 7 8 2 4 3 5 11 10Non-CB Side

CB Side

0

10

20

30

40

50

60

Hyd

roge

n Pi

ck U

p (%

)

Channel ID

4 Cycles

3 Cycles

1 Cycle

9 1 7 8 2 4 3 5 11 10Non-CB Side

CB Side

0

10

20

30

40

50

60

Hyd

roge

n Pi

ck U

p (%

)

Channel ID

4 Cycles

3 Cycles

1 Cycle

))(( tHPUBow Dµ

19

Hydriding Kinetics

0

50

100

150

200250

300

350

400

450

10 15 20 25 30 35 40 45 50

Channel Exposure (GWd/MTU)Hy

drog

en C

once

ntra

tion

(ppm

)

Non-CB sideCB side

Zircaloy-4

• Blade vs. non-blade side differential in hydrogen setup after 1st cycleexposure to CB.

• Hydrogen on blade side with significant ECBE accumulate at a differentrate than non-shadow oxide, driven by corrosion kinetics.

• Zry-2 develop more hydrogen than Zry-4.

0

100

200

300

400

500

600

0 100 200 300 400 500 600

Measured Bow (mils)

Tota

l Est

imat

ed B

ow (m

ils)

0.25% lin.change per1000 ppm H

0.33% lin.change per1000 ppm H

))(( tHPUBow Dµ

20

Discussion

Large H differential causing bow is the combined result of increasing oxidedifferential and increasing HPU.

Nature of oxide is an important part of shadow corrosion mechanism.

Different shadow corrosion kinetics (after removal of shadow condition)implies a permanent change to the oxide.

HPU for Zry-2 increases with exposure; however, similarity in HPU for agiven multi-cycle channel (affected or not by shadow corrosion) impliessignificance of metallurgical condition affected by exposure/time.

Dissolution of Zr(Fe,Cr)2 and Zr2(Fe,Ni) are the key microstructuralchanges. Zry-4 without Zr2(Fe,Ni) does not develop high HPU.Incorporation of dissolved Ni into barrier oxide layer may be a factor forhigh HPU for Zry-2.

21

Summary and Conclusion - 1Shadow bow observations• Shadow bow develop at high exposure but conditions for shadow

corrosion occurred early in life.• Shadow corrosion-induced bow greatest in plants with the smallest

channel-to-blade gap.

Shadow Bow Correlation• Correlates with hydrogen differential (based on S-, C- and D-lattices

plants and 1- to 4-cycle channels).• Linear dimensional change of 0.25% per 1000 ppm hydrogen, less than

expected from theoretical isotropic considerations.

22

Summary and Conclusion - 2Kinetics of corrosion and hydriding• Different for the channel side with shadow corrosion vs. the side

unaffected by shadow corrosion.• Small differential in oxide thickness and hydrogen content established

after low exposure• Differentials increase with exposure/time, driven by higher rate of

corrosion on channel side affected by shadow corrosion• HPU% following initial shadow corrosion is higher• HPU% increases with exposure/time; little effect due to shadow corrosion• Zry-4 shows low HPU% to high exposure

Implication• HPU% variation with time and fluence likely associated with dissolution of

SPPs• Release of Ni from the dissolution of Zr2(Fe,Ni) SPPs may be the key to

high HPU% in Zry-2

16th Int. Symposium on Zirconium in the NuclearIndustry, May 9-13, 2010, Chengdu, China

Shadow Corrosion-InducedBow Of Zircaloy-2 Channels

S. T. Mahmood, P. E. Cantonwine, Y.P. Lin, D. C. CrawfordGlobal Nuclear Fuel

E. V. Mader, K. EdsingerElectric Power Research Institute