combined raman imaging and o tracer analysis for …...optical microscopy of the low temperature...
TRANSCRIPT
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