post quench ductility of zirconium alloy cladding materials...three rings from one oxidation sample,...

Westinghouse Non-Proprietary Class 3 © 2016 Westinghouse Electric Company LLC. All Rights Reserved.

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Post Quench Ductility of Zirconium Alloy Cladding Materials

A. Mueller D. Mitchell J. Romero* A. Garde J. Partezana A. Atwood G. Pan

18th International Symposium on Zirconium in the Nuclear Industry Hilton Head, SC, May 15-20, 2016

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Background • Loss-of-Coolant Accident (LOCA) conditions

– Steam oxidation at high temperature – Followed by quenching in water – Minimum ductility must be retained

• Hydrogen has a significant influence

– Absorbed during normal operation

• In-reactor operation limits – Peak cladding temperature (PCT) and total oxidation – As a function of hydrogen content

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Background Oxide α(O) Prior-β

Inner Diameter

Outer Diameter

100 µm

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Objectives • Investigate post-quench ductility (PQD)

– Zircaloy-4, ZIRLO® and Optimized ZIRLO™ – As a function of hydrogen content – As-fabricated and hydrogen charged

• Investigate post-quench microstructure

– Evolution of α(O) and prior-β layers

ZIRLO and Optimized ZIRLO are trademarks or registered trademarks of Westinghouse Electric Company LLC in the United States and may be registered in other countries throughout the world. All rights reserved. Unauthorized use is strictly prohibited. Other names may be trademarks of their respective owners.

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Experimental • Zircaloy-4, ZIRLO and Optimized ZIRLO tested side-by-side • Hydrogen content from ~10 to 600 ppm • Peak cladding temperature (PCT) is a function of H content

– 1200°C for H ≤ 400 ppm and 1125°C for H ≥ 400 ppm

Alloy Nb Sn Fe Cr Zircaloy-4 - 1.3 0.20 0.10

ZIRLO 1.0 1.00 0.10 -

Optimized ZIRLO 1.0 0.67 0.10 -

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Experimental - Oxidation

1. Heating to PCT (1200°C or 1125°C) – Holding to achieve desired oxidation

2. Slow cooling (~2°C/s) to 800°C 3. Water quenching

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Experimental – Ring Compression Testing • Three rings from one oxidation sample, tested at 135°C • Offset strain calculated from load-displacement curve • Determination of ductile or brittle result

– Average measured offset strain versus threshold

ZIRLO 12.7% ECR 200 ppm H Avg. Offset Strain 1.6%

ZIRLO 19.1% ECR 10 ppm H

Avg. Offset Strain 13.4%

Brittle Ductile

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Results – Ductile-to-Brittle Transition

Hydrogen (ppm)

0 100 200 300 400 500 600 700 800

EC

R (%

)

0

2

4

6

8

10

12

14

16

18

20

22

Zircaloy-4

1200°C

Proposed Limit

1125°COpen = DuctileClosed = Brittle

Hydrogen (ppm)

0 100 200 300 400 500 600 700 800

ZIRLO

1200°C

Proposed Limit


Similar ductile-to brittle transition Transition occurs over a range

Equivalent Cladding Reacted (ECR) is defined as the percentage of the cladding thickness that would be oxidized if all the oxygen absorbed stayed in the oxide layer as ZrO2.

Brittle

Ductile

Brittle

Ductile

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Results – Ductile-to-Brittle Transition

Hydrogen (ppm)

0 100 200 300 400 500 600 700 800

EC

R (%

)

0

2

4

6

8

10

12

14

16

18

20

22

Zircaloy-4

1200°C

Proposed Limit


Hydrogen (ppm)

0 100 200 300 400 500 600 700 800

Optimized ZIRLO

1200°C

Proposed Limit


Brittle

Ductile

Brittle

Ductile

Similar ductile-to brittle transition Transition occurs over a range

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Post-Quench Microstructure

19.1% ECR and ~10 ppm H (both materials)

Zircaloy-4 ZIRLO

Protrusions of α(O) layer observed in ZIRLO and Optimized ZIRLO

Prior-β α(O) Oxide

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Post-Quench Microstructure – Layer Thickness

Similar layer thicknesses between alloys and along oxidation samples

High Oxidation ~10 ppm H

Low Oxidation 200 ppm H

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Summary – PQD Testing • Zircaloy-4, ZIRLO and Optimized ZIRLO tested

– High temperature oxidation and quenching – Evaluation of ductility via ring compression testing

• Variability within single oxidation samples – Similar ductile-to-brittle transition

• Transition occurs over a range – Protrusions of α(O) layer observed in Nb containing alloys – Similar average thickness of oxide and α(O) layers

• Prompts study of formation of post-quench microstructure

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Post-Quench Microstructure Optimized ZIRLO samples (~10 ppm H) subjected to standard cooling and direct quenching

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Post-Quench Microstructure – Prior-β Microhardness

Evolution of hardness (oxygen) during cooling

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Post-Quench Microstructure Standard Cooling

Direct Quench

200 µm

Optimized ZIRLO ~10ppm H ~19% ECR

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Post-Quench Microstructure – Formation of α(O) Optimized ZIRLO Direct Quench

Electron Image Oxygen

Indications of step change in O content within α(O) layer

100 µm

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Post-Quench Microstructure – Formation of α(O)

Oxygen

Optical Image

Electron Image

Profile 2

Profile 1

0 50 100 150 200

Oxy

gen

Microns

Profile 1Profile 2

Optimized ZIRLO Direct Quench

50 µm

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Post-Quench Microstructure – Formation of α(O) Optimized ZIRLO Standard Cooling

Optical (Polarized) Optical (Bright Field)

100 µm

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Post-Quench Microstructure – Formation of α(O) Optimized ZIRLO Standard Cooling

Iron Electron Image

Optical Image (Polarized)

Oxygen

Niobium

050100150O

xyge

n Microns

Profile 1Profile 2

Profile 1

Profile 2

50 µm

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Westinghouse Proprietary Class 2 © 2016 Westinghouse Electric Company LLC. All Rights Reserved.

Tem

pera

ture

Time

α β

β

α

Transformation temperatures unknown • Affected by O diffusion • Affected by kinetics

Microstructure Evolution

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Tem

pera

ture

Time

α β

β

α


Microstructure Evolution • Oxidation • α to β transformation • O ingress into β phase

– Stabilization of α(O) by diffusion

• β stabilizers in solution in β phase – Early stages of segregation

Oxide β-phase Oxygen rich α(O) α-phase β-stabilizer (slow) β-stabilizer (fast)

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Tem

pera

ture

Time

α β

β

α





• Oxide growth • α(O) growth • β grain growth • β stabilizers segregate

– Different diffusion kinetics


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Tem

pera

ture

Time

α β

β

α







• Thermal β to α transformation • Formation of α protrusions

– Direct quench: long protrusions – Standard cooling: uniform propagation

• β stabilizers segregate – Between α protrusions

• Oxygen segregates to new α – Standard cooling: O depletion in the

middle

Direct

Standard


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Tem

pera

ture

Time

α β

β

α







• Thermal β to α transformation • Formation of α protrusions

– Direct quench: long protrusions – Standard cooling: uniform propagation

• β stabilizers segregate – Between α protrusions

• Oxygen segregates to new α – Standard cooling: O depletion in the

middle

• β to α transformation complete • Formation of α colonies

– Morphology de pends on cooling

• Precipitation of β stabilizers – Between α lamellas

Direct

Standard

Standard Direct Oxide β-phase Oxygen rich α(O) α-phase β-stabilizer (slow) β-stabilizer (fast)

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Summary – Post-Quench Microstructure • Development of hard oxygen-enriched α(O) layer studied

– Probably formed in two stages • First stage at high temperature by O diffusion • Second stage during cooling

– Protrusions probably related to presence of slow β-stabilizers

• Formation α(O) layer is important for resulting ductility

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post quench ductility of zirconium alloy cladding materials...three rings from one oxidation sample,...

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