characterization of oxygen distribution in loca …€¦ · distribution in loca situations duriez...

CHARACTERIZATION OF OXYGENDISTRIBUTION IN LOCA SITUATIONS

Duriez C.1, Guilbert S.1 , Stern A.2, Grandjean C.1, Bělovský L.3, Desquines J.1

1 IRSN

² IRSN post-doctorate, now at CEA

3 ALIAS Cz

16th International Symposium on the Zirconium in the Nuclear Industry – Chengdu, China 2/ 22

Scope of the presentation•DIFFOX:

IRSN develops a simulation code aiming at modeling the oxygen profile within acladding submitted to a LOCA transient.

•Validation of High Temperature oxidation Process:

Oxidation kinetics and oxygen content profile compared to literature data

Two main areas for DIFFOX improvement have been studied:

•Low Temperature oxide dissolution process:

Influence from bare up to 70 microns thick LT oxide layer

•Influence of hydrogen on the Zy-O phase diagram:

Effect of 600 ppm hydrogen content on the a/a+b and a+b/b transus


DIFFOX: main modeling options of the code

b(O

)-Zr

a(O

)+b(

O)-

Zr

a(O

)-Zr

ZrO

2 • Single and two-sided oxidation,

• Thermal transients modeled,• Solving of diffusion equation with movingboundaries (1D, implicit finite differences),• LT and HT oxide layers modeled,

• Partial dissolution of LT oxide layer duringthe HT transient,

• ZrO2, a(O)-Zr, b(O)-Zr , a(O)+b(O)-Zr layersmodeled,

• Diffusion coefficients mainly derived from literature review,

• Influence of hydrogen on Zy-O phase diagram boundaries modeled:

latest DIFFOX version

Expectedinfluence of H

500

700

900

1100

1300

1500

0 1 2 3 4O (Wt %)

Tem

pera

ttur

e (°

C)

a Zr

b Zr a+b

Zr

5

[H]=0[H]≠0

LT-Z

rO2

HT-

ZrO

2


DIFFOX: Oxide scale modeling

The development and validation of DIFFOX are major issue for IRSNValidation on Leistikow 650-1400°C isothermal tests (1987)

t-ZrO2

oxide (total ZrO2 thickness)

oute

r oxi

de s

urfa

ce

tetr

agon

alsu

b-la

yer

(<12

00°C

)

bulk oxide pre-oxide

high-temperature oxide low-temperature oxide

Met

al

monoclinic (<850 °C)or

mon + tet (850-1200 °C)or

tetragonal (>1200 °C)


mon + tet (>1100 °C)

t-ZrO2

oxide (total ZrO2 thickness)

oute

r oxi

de s

urfa

ce

tetr

agon

alsu

b-la

yer

(<12

00°C

)

bulk oxide pre-oxide

high-temperature oxide low-temperature oxide

Met

al


mon + tet (850-1200 °C)or

tetragonal (>1200 °C)


mon + tet (>1100 °C)


Zirconium alloy:Stress Relieved Annealed (SRA) low-tin Zircaloy-4 tubes from CEZUS

Low Temperature (LT) corrosion:• Specimens were LT pre-oxidized at 500°C under pure oxygen to assess theinfluence of in-service corrosion

• 10 to 60 days exposure leading to 10 to 70 micrometers thick pre-oxide layers

Hydriding:• 600 wppm electrolytic hydriding of 50 cm long samples performed at EDF (160h)• Hot extraction at 4 different locations confirmed values ranging between 575-640ppm.• 24 h at 430°C heat treatment to achieve a quite uniform H distribution

Tested materials

100 µm

Example:60 days exposure

Polarized lightBright light


High Temperature oxidation: protocol validation on bare Zy-4

ArSteam generator

sample

Ar

Sample holderwith alumina

furnace

quenching bath(water)

Vertical furnace for HToxidation

The HT oxidation protocol gives oxidation kinetics consistent with literaturedata between 900 and 1200°C for a wide range of oxidation durations

Comparison to literature data

0

50

100

150

200

0 1000 2000 3000 4000 5000 6000

Durations (s)

Wei

ght g

ain

(g/m

²)900°C

1000°C

1100°C1200°C

Billone 2008 (US-NRC)

present study

Hozer 2008 (KFKI)Ozawa 2000 (NDC)Kawasaki 1978 (JAERI)Brachet 2001 (CEA)

Billone 2008 (US-NRC)

present study

Hozer 2008 (KFKI)Ozawa 2000 (NDC)Kawasaki 1978 (JAERI)Brachet 2001 (CEA)

Cathcart-Pawelcorrelation


HT dissolution of the LT oxide layer


HT dissolution of the LT oxide layer: main objectivesSafety concern:

The HT dissolution of the LT oxide layer potentially induces significant oxygencharging in the sample and can have a deleterious impact on the Post-Quench-Ductility.

DIFFOX:The DIFFOX code has the potential of simulating this LT oxide layer dissolution butrequires validation data.

Experiments performed for validation of DIFFOX LT oxide layer dissolution modeling:•HT vacuum annealing of pre-oxidized samples (maximizing the dissolution process),

•HT steam annealing of pre-oxidized samples (competition between HT oxidation andHT dissolution).

HT conditions (900°C-6000s):•At the present stage experimental conditions don’t address the LOCA safety issue butthe DIFFOX validation issue.

•The HT heat treatment is chosen to enhance dissolution of the LT oxide.


• Oxidation:

• Post-quench optical metallography:

• ZrO2 and aZr(O): layer thickness measurement at 8 angular locations (both:inner and outer layers): eZrO2 = 18.5 ± 0.7 mircons

eaZr(O)= 34.5 ± 1.0 microns

• EPMA radial O concentration profiles and O distribution maps (O Ka line)

• Average H content measurements in the sample by hot extraction : negligible

High Temperature oxidation of as-received samples

100 µm100 µm 100 µm100 µm

no LT oxtime

temperatureHT: 900°C – 6000 s

As received Zy-4 + HT oxidation

A similar procedure was systematically applied to all tested samples

steam


•Oxidation:

•Post-quench optical metallography:

•ZrO2 and aZr(O): layer thickness measurement at 8 angular locations (both:inner and outer layers): DeZrO2 = - 5 to -10 microns

eaZr(O) ~ 35.0 microns

500°C LT ox

time

temperatureHT: 900°C – 6000 s

Pre-oxidized + HT vacuum ann.vacuum

HT Dissolution of the LT oxide layer: HT vacuum annealing

Oxygen

50 mm

10 mm LT ZrO2 20 mm LT ZrO2 30 mm LT ZrO2 40 mm LT ZrO2 60 mm LT ZrO2 70 mm LT ZrO2


•Oxidation:

•Post-quench optical metallography:

•ZrO2 and aZr(O): layer thickness measurementDeZrO2 ~ -5 microns DeZrO2 = +20 micronseaZr(O) ~ 40.0 microns eaZr(O) ~ 30.0 microns

500°C LT ox

time

temperatureHT: 900°C – 6000 s Pre-oxidized + HT oxidation

steam

HT Dissolution of the LT oxide layer: HT steam oxidation

Oxygen

Example: 20 mm LT ZrO2

0.5 mm

25 mm

dark–brownregions

light–brownregions

HT oxide

LT oxideLT oxide

25 mm


HT Dissolution of the LT oxide layer: DIFFOX vs experiments20 mm LT ZrO2 followed by HT steam oxidation (900°C – 6000 s)

• DIFFOX predicts a protective LT ZrO2 layer• No HT oxide expected,• aZr(O) layer thickness over-predicted in regions with HT oxide

Oxy

gen

conc

entr

atio

n (w

t%)

Oxy

gen

conc

entr

atio

n (

Oxy

gen

conc

entr

atio

n (

0

5

10

15

20

25

30

0 50 100 150 200

DIFFOX simulation

MeasuredO-profile:20 mm LT ZrO2 + HT steam oxidized

DIFFOX simulation


DIFFOX simulation


Distance to outer surface (µm) Distance to outer surface (µm)O

xyge

n co

ncen

trat

ion

(wt%

)0 50 100 150 200

DIFFOX simulation


DIFFOX simulation


0

5

10

15

20

25

30Region with no HT oxide Region with HT oxide


HT Oxidation of as-received samples: DIFFOX vs experimentsAs-received cladding HT steam oxidation (900°C – 6000 s)

Very good code-to-data agreementPolishing reveals only part of the aZr(O) region

0

5

10

15

20

25

30

0 50 100 150 200

Distance to outer surface (µm)

Oxy

gen

conc

entr

atio

n (w

t%) DIFFOX simulation

Measured O- profile:as- received + HT steam oxidized

DIFFOX simulation

Measured O- profile:as- received + HT steam oxidized


HT Dissolution of the LT oxide layer: DIFFOX vs experiments20 mm LT ZrO2 followed by HT vacuum anneal (900°C – 6000 s)

Very good code-to-data agreement

0

5

10

15

20

25

30

0 50 100 150 200

Distance to outer surface (µm)

Oxy

gen

conc

entr

atio

n (w

t%) DIFFOX simulation

Measured O- profile:20 mm LT ZrO2 + HT vacuum annealed

DIFFOX simulation

Measured O- profile:20 mm LT ZrO2 + HT vacuum annealed


Investigating the Influence of Hon Zy-O phase diagram


Expected influence of Hydrogen on the oxygen distribution

•Hydrogen shifts the a/(a+b) and b/(a+b) boundaries to higher Oconcentrations:

O solubility in the bZr phase is increased•Few experiments addressing influence of hydrogen,•It was thus decided to improve the description of this effect in DIFFOX code.

500

700

900

1100

1300

1500

0 1 2 3 4

O (Wt %)

Tem

pera

tture

(°C)

a Zr(O)

b Zr a+

b Zr

5

[H]=0

[H]≠0

O content

Distance to the outer surface

ZrO

2 aZr(

O)

bZr

[H]➷[H]➹


Strategy to evaluate the influence of 600 ppm H

• (a+b) phase tests• a phase tests• 3 temperatures

21 samples tested

500

600

700

800

900

1000

1100

1200

1300

1400

1500

0 1 2 3 4O (wt%)

Tem

pera

ture

( °C)

Expected influenceof 600 wt ppm H

[H]=

0a

bZra+b Zr

51.3 2.0 5.0

a+b Zr expecteda Zr expected

500

600

700

800

900

1000

1100

1200

1300

1400

1500

0 1 2 3 4O (wt%)

Tem

pera

ture

( °C)

Expected influenceof 600 wt ppm H

[H]=

0aZr(O)

bZra+b Zr

51.3 2.0 5.0




Testing procedure

time

temperature

steam

1050°C - 5 min

H h

omog

eniz

atio

n+

Zr te

xtur

e re

mov

al

O c

harg

ing

bysa

mpl

e ox

idat

ion

dura

tion

adju

sted

(Cat

h.Pa

w.)

3 temp. tested: 1000, 1100 and 1200 °C

Elec

trol

ytic

al H

Cha

rgin

g at

ED

F

H,O

mea

sure

men

t (H

E)O

mea

sure

men

t by

wei

ght g

ain

O d

isso

lutio

n an

dho

mog

eniz

atio

n

H m

easu

rem

ent (

HE)

Melting pointof ZrO2 too hot

10 hr at 1000°C3h at 1100 and 1200°C

(CEA procedure)

Wat

er q

uenc

h(a

+bZr

sam

ples

onl

y)

Ana

lysi

s pe

rfor

med

[H] and [O] contents are used to check that the vials keep tight during HTsteps

H,O

mea

sure

men

t (H

E)O

mea

sure

men

t by

wei

ght g

ain

Perturbatedby matr’l loss

not reliable

•H,O

mea

sure

men

t(H

E),

•Wei

ght g

ain

(not

acc

urat

e),

•EPM

A m

easu

rem

ents

(hom

ogen

eity

+ d

istr

ibut

ion)

Vacuum(quartz vial)

Vacuum(quartz vial)


Oxide aZr(O) Prior-bZr

0

5

10

15

20

25

-50 0 50 100

Oxy

gen

conc

entr

atio

n (w

t%)

Measured O profile (#26 sample):steam oxidation 60 s at 1200°C

DIFFOX simulation

a/(a+b)

(a+b)/b

Oxide aZr(O) Prior-bZr

0

5

10

15

20

25

-50 0 50 100Distance to metal / oxide interface (mm)

Oxy

gen

conc

entr

atio

n (w

t%)

Measured O profile (#26 sample):steam oxidation 60 s at 1200°C

DIFFOX simulation

a/(a+b)

(a+b)/b

Determination of phase boundaries: After steam oxidation

•Assuming slow diffusion of oxygen at RT, phase boundaries can be derivedfrom EPMA profile after steam oxidation (before homogenization process),•Low accuracy because EPMA measurement is not accurate at the interface•Some improvements are still needed in the DIFFOX code.

time

temperature

steam

1050°C - 5 min3 temp. tested: 1000, 1100 and 1200°C

10 hr at 1000°C3h at 1100 and 1200°C

(CEA procedure)

Wat

er q

uenc

h(a

+bZr

sam

ples

onl

y)

Vacuum(quartz vial)

Vacuum(quartz vial)


Determination of phase boundaries: after homogenization

•At equilibrium: direct measurement of O contents in both aZr(O) and prior bZr•Fe and Cr segregates to the b-phase during HT treatment

9/119/119/11

Distance to the external surface (mm)O c

once

ntra

tion

(wt %

)

Cr

Fe

O

200 µm

Cr

Fe

O

200 µm

0

1

2

3

0 50 100 150 200 250 300

Cr

Fe

O

200 µm

aZr

(O)

prio

r-bZr

Sample # 20

time

temperature

steam

1050°C -5 min3 temp. tested: 1000, 1100 and 1200°C

10 hr at 1000°C3h at 1100 and 1200°C

(CEA procedure)

Wat

er q

uenc

h(a

+bZrs

ampl

es o

nly)

Vacuum(quartz vial)

Vacuum(quartz vial)


Determination of phase boundaries: Results Summary

•The results are consistent with expectations•Some scatter in the data

This work, on oxidizedThis work, on homogenizedErickson ~600 ppmH (1964)Brachet 600 ppm (2001)

a/(a+b)

b/(a+b)

Chung & Kassner, 0 ppm H (1979)

500

600

700

800

900

1000

1100

1200

1300

1400

0 1 2 3 4O concentration (wt%)

T( °

C)

a Zr(O)

b Zr

a+b Zr


Summary•Overall agreement on the key role played by O-distribution during a LOCAtransient•DIFFOX is the IRSN calculation code dedicated to this assessment

•A set of experiments dedicated to the influence of LT oxide layer dissolutionwas performed showing that:

- the LT oxide layer can be both protective or not,- DIFFOX calculations are consistent with a protective layer

•The influence of 600 ppm H on the Zy-O phase diagram was studied and newmodeling data is provided.

•Further studies are required to provide validation data in the duration range of aLOCA (including shorter steam exposure and wider temperature range).

•Detailed influence of hydrogen on Zy-O phase diagram (300 and 1000 wppm)

•Transient oxidation-cooling experiments are planned to increase the validationfield of DIFFOX

Further studies

characterization of oxygen distribution in loca …€¦ · distribution in loca situations duriez...

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