18th International Symposium

on Zirconium in the Nuclear Industry,

Hilton Head Island, SC , 15-19/05/2016

A. Kasperski, C. Duriez, M. Mermoux

Work performed in the frame of the DENOPI project, funded by the

French government as part of the “Investment for the Future”

Program reference ANR-11-RSNR-0006

Combined Raman imaging and 18O

tracer analysis for the study of

Zircaloy-4 high temperature oxidation

in spent fuel pool accidentINSTITUTE FOR

RADIOPROTECTION AND

NUCLEAR SAFETY

Context: spent fuel pool loss of cooling/loss of coolant accidents

Exposure of spent fuel assemblies to an environment containing air and steam

Heat released by the oxidation reactions leads to temperature escalation

Reactor pool Spent fuel storage pool at La Hague

the temperature range of interest is ~ 700-1000°C

2/22

In air, a kinetic transition occurs much earlier than in O2 or in steam

The post-transition acceleration is strong and is due to a catalytic effect of nitrogen

(formation of ZrN and its oxidation)

Effect of nitrogen on high temperature (HT) oxidation of Zr alloys

Material: bare Zy-4

Oxidation: Isothermal at 850°C

200 µm

13.6% ECR

3/22

ZrO2

Metal

Metal

air

O2

ZrN

H2O

Impact of low temperature oxide on high temperature oxidation in air

- lowers the HT oxidation rate

- delays the air attack

Low temperature oxidation (LTO) : simulates in-reactor corrosion of fuel cladding

4/22

depending on LT oxidation conditions (T, atmosphere, apparatus)

HT oxidation 850°C in air

The LT oxide:

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0 60 120 180 240 300

d(D

m/S

)/d

t (g

m-2

s-1

)

time (min)

Bare Zy4

Time (min)LT oxide thickness of ~ 30 µm

Oxid

ati

on r

ate

(g.m

-2.s

-1)

Study of the HT oxygen transport through a LT oxide

Bare Zy-4LT oxidized

Zy-4

LT + HT

oxidized Zy-4LT Oxidation HT Oxidation

425°C 16O2 + 15% H2

16O

850°C18O2 or 18O2 + N2

▌ To characterize the HT oxygen transport through a LT oxide

▌ To provide diffusion coefficients for oxidation codes

Aims:

Means:

Two-stage oxidation experiments with 18O tracer

Raman spectroscopy to quantify the 18O content

Experimental protocol:

5/22

1st stage: low temperature oxidation in 16O2 + H216O

500 mm

16O2 + H216O

After 250 days of LT oxidation:

- [H] = 350-400 wt ppm

- Oxide thickness = 30-33 µm

20 m

m

6/22

0

100

200

300

400

500

600

0

5

10

15

20

25

30

35

0 100 200 300

H c

onte

nt

(wt

ppm

)

ZrO

2th

ickness

(µm

)

Time (days)

425°C

Double-side oxidation

Optical microscopy of the low temperature oxide

Radial cross section

Outer-side

Inner-side

50 µm

50 µm

Axial cracks in the outer oxide only

Stratified microstructure

Axial

cracks

External surface

7/22

𝒆𝒛

(Oz) axis𝒆𝒓

𝒆𝜽

10 µm

-100

100

300

500

700

900

-2000

-1000

0

1000

2000

3000

4000

150 350 550 750

Inte

nsi

ty (

A.

U.)

Inte

nsi

ty (

A.

U)

Wavenumber (cm-1)

Raman spectroscopy of the low temperature oxide

Distorted t-ZrO2

Darker oxide close to the M/O interface correlated

with lower intensity of the Raman spectra

Crystallographic disorder

Substoechiometric oxide layer

Composition: mostly m-ZrO2+ distorted t-ZrO2 islands

at the M/O interface

8/22

Metal

ZrO2

Secondary vacuum pump

Mass spectrometer analysis

N2

18O2

2nd stage: high temperature exposure to 18O

Experimental device: thermobalance

Test Matrix

Temperature: 850°C

9/22

Atmosphere: 18O2 or 18O2 + N2 (O2/N2 = 4)

Duration: 30 – 60 – 120 – 360 min

Effect of 16O → 18O substitution on the m-ZrO2 spectrum:

- high downshift of the high frequency Raman lines

- the downshift is proportional to the 18O content

0

2000

4000

6000

8000

10000

12000

100 200 300 400 500 600 700 800

.… 100% 16O2

–– 40% 16O2 + 60% 18O2

Wavenumber (cm-1)

Co

un

ts

M. Guerain, M. Mermoux, C. Duriez, Corrosion Science 98 (2015) 140–149

m-ZrO2 : major phase after oxidation of Zr-alloys

The“475 cm-1” peak will be used to quantitatively map the 18O content in the

zirconia scales, with a probed volume of ~ 1 µm3.

Quantification of 18O in zirconia scale by Raman spectroscopy

10/22

200

300

400

500

600

700

800

0 20 40 60 80 100

Wav

enu

mb

er (

cm

-1)

18O/(

16O+

18O) (at%)

C18O = 18O /(16O + 18O) (at. %)

0 20 40 60 80

1. External surface

Cracks are preferred sites for 18O incorporation

Outer oxide characterization after exposure in 18O2 at 850°C

C18O (at. %)

30 min 60 min 120 min

100 µm

Laser

11/22

cracks

50 µm

Cracks =

favored paths for oxygen incorporation

and propagation through the scale

Outer oxide characterization after 120 min exposure in 18O2 + N2

2. Radial cross section

ZrN

12/22

0 8 16 24 32 40 48 56 46 72 80

Metal

ZrO2

C18O (at. % )

Nitrogen forms ZrN in the M/O interface

region

Good correlation between SIMS and Raman images

Raman imaging vs SIMS for 18O content mapping

C18O (at. % )

13/22

Raman spectroscopy

SIMS

14/22

10 µm 10 µm 10 µm

C18O (at. %)

ZrN

Minor reduction of the

oxide thickness

t ↗ => C18O in the scale ↗

N reaches the M/O

interface before 18O

Raman imaging of the inner oxide after exposure in 18O2 + N2 at 850°C

30 min 60 min 120 minMetal

ZrO2

Profiles of 18O content in the inner oxide scale

C18O distributions are similar after 18O2 exposure and 18O2 + N2 exposure

Surface Metal

15/22

Profiles of C18O :

120 min30 min 60 minX 360 min

Surface Metal

16/22

0

10

20

30

40

50

60

70

80

90

100

0 50 100 150 200

C18O

(su

rface)

(% a

t.)

t1/2 (s1/2)

18O2

18O2+N2

=> Gradient of C18O between the bulk and the periphery of the grains

Main diffusion path along the grain boundaries

C18O (surface layer) ∝ 𝒕

18O content in the inner oxide scale

18O2

18O2 + N2

In the scale near the surface,

FWHM (scale) > FWHM (powder)

0

10

20

30

40

50

60

70

80

90

100

400 425 450 475 500

Inte

nsi

ty (

A.U

.)

Wavenumber (cm-1)

m-ZrO2

Powder

In the

scale

C18O ~ 60%

non-homogeneous C18O within the

probed volume

Modeling of 18O profiles (1st attempt)

17/22

𝝏𝑪𝟏𝟖𝐎𝝏𝒕

= 𝑫𝟏𝟖↔𝟏𝟔

𝝏𝟐𝑪𝟏𝟖𝐎𝝏𝒙𝟐

+𝑫𝑶

𝑹𝑻

∆𝝁𝒐𝒙𝒆

𝝏𝑪𝟏𝟖𝐎𝝏𝒙

isotopic exchangeoxygen chemical

potential gradient

∆𝜇𝑜𝑥 = 𝜇𝑜𝑥𝑦𝑔𝑒𝑛 𝑂 𝑀) − 𝜇𝑜𝑥𝑦𝑔𝑒𝑛(𝐺 𝑂

= 𝑅𝑇 ln𝑐 )𝑜𝑥𝑦𝑔𝑒𝑛( 𝑂 𝑀

𝑐 )𝑜𝑥𝑦𝑔𝑒𝑛( 𝐺 𝑂

Modeling of 18O profiles (1st attempt)

Analytical solution :

Analytical solution fits well experimental profiles

18/22

𝝏𝑪𝟏𝟖𝐎𝝏𝒕

= 𝑫𝟏𝟖↔𝟏𝟔

𝝏𝟐𝑪𝟏𝟖𝐎𝝏𝒙𝟐

+𝑫𝑶

𝑹𝑻

∆𝝁𝒐𝒙𝒆

𝝏𝑪𝟏𝟖𝐎𝝏𝒙

isotopic exchange oxygen chemical

potential gradient

120 min

30 min60 minX

360 min

Surface Metal Surface Metal

𝐶18O(𝑥, 𝑡) = 𝑐𝑠erfc𝑥 +

𝑫𝑶∆𝜇𝑜𝑥𝑅𝑇𝑒 𝑡

2 𝑫𝟏𝟖↔𝟏𝟔𝑡

19/22

1.E-15

1.E-14

1.E-13

1.E-12

1.E-11

1.E-10

1.E-09

1.E-08

1.E-07

1.E-06

1.E-05

6 7 8 9 10 11 12 13 14 15

D (

cm

2/s

)

104/T(K)

Oxygen diffusion coefficients

DO

D18<->16

10-5

10-6

10-7

10-8

10-9

10-10

10-11

10-12

10-13

10-14

10-15

900 800 700 6001000 T (°C)

𝜕𝐶18O𝜕𝑡

= 𝑫𝟏𝟖↔𝟏𝟔

𝜕2𝐶18O𝜕𝑥2

+𝑫𝑶

𝑅𝑇

∆𝜇𝑜𝑥𝑒

𝜕𝐶18O𝜕𝑥

isotopic exchange oxygen chemical potential gradient

DO ~ D(GB)

20/22

1.E-15

1.E-14

1.E-13

1.E-12

1.E-11

1.E-10

1.E-09

1.E-08

1.E-07

1.E-06

1.E-05

6 7 8 9 10 11 12 13 14 15

D (

cm

2/s

)

104/T(K)

Oxygen diffusion coefficients

DO

D18<->16

10-5

10-6

10-7

10-8

10-9

10-10

10-11

10-12

10-13

10-14

10-15

900 800 700 6001000 T (°C)

from 18O tracer

experiments

from HT oxide

growth rates

Oxygen diffusion coefficients

Literature data:

DO/D16<->18 ~103 => the isotopic exchange is limited by the bulk diffusion

DO (this study) ~ DO ( oxyde HT)

Combination of 18O tracer experiments and Raman spectroscopy for the study of oxygen

transport through a LT scale which is protective against HT oxidation

Summary

Raman spectroscopy is a valuable tool to qualify 18O content in zirconia scales:

The quantification of 18O was confirmed by SIMS analyses

Raman gives indication on the inhomogeneity of the 18O distribution in the probed volume

On the transport of O and N through a low temperature oxide:

Outer-side:

cracks = preferred paths for the air ingress

Inner-side:

• Oxygen: 1. solid state diffusion mechanism

• Nitrogen:

has no influence on HT oxygen diffusion through the oxide scale

but reaches the M/O interface more rapidly than oxygen

2. grain boundaries = main diffusion paths for oxygen

3. LT oxide ≈ HT oxide regarding HT oxygen transport

21/22

Thanks for your attention

- D. Drouan, IRSN (TAG, Metallography)

- P. Lacôte, IRSN (Metallography)

- F. Jomard, GEMAC (SIMS)

- E. Bachelet, LCOGT (Data imaging)

- F. Jacq, IRSN (Modeling)

- DENOPI Consortium, IRSN/LVEEM/LEPMI/Mines de St-Etienne (Discussions)

Thanks for discussions and technical assistance to :

22/22

Acknowledgments