Cabie, M.

Carlot, G.

Dumont, M.

Michel, A.

Onofri-Marroncle, C.

Sabathier-Devals, C.

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Helium precipitation study in UO$_2$ by Transmission Electron Microscopy

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Helium precipitation study in UO_2 by Transmission
Electron Microscopy
A. Michel, G. Carlot, C. Sabathier-Devals, C. Onofri-Marroncle, M. Dumont,
M. Cabie
To cite this version:
A. Michel, G. Carlot, C. Sabathier-Devals, C. Onofri-Marroncle, M. Dumont, et al.. Helium precip-
itation study in UO_2 by Transmission Electron Microscopy. CIMTEC 2018 - 8th Forum on New
Materials, Jun 2018, Perugia, Italy. ￿cea-02338635￿
PERUGIA, ITALY – 14TH JUNE 2018 
Helium precipitation study in UO2 
by Transmission Electron 
Microscopy 
CIMTEC 2018 
8th Forum on New Materials 
A. Michel1,*, G. Carlot1, C. Sabathier1, C. Onofri1, M. Dumont2 and 
M. Cabie3 
1. CEA / DEN / DEC, Saint Paul Lez Durance, France 
2. IM2NP – UMR CNRS 7334 – Aix-Marseille Université, Marseille, France 
3. CP2M –  Aix-Marseille Université, Marseille, France 
* adrien.michel2@cea.fr 
INTRODUCTION – SPENT NUCLEAR FUEL 
In-pile irradiation 
Reference scenario : 
Reprocessing 
Management of spent fuel 
Fission products 
Irradiation damages 
Bubbles of fission gases 
Microstructural changes 
Direct disposal of spent nuclear fuel in deep geological layer 
α-decay 
[ Ferry et al., 2004, CEA report DPC/SECR 04-032 ] 
Microstructure 
alteration 
and/or pellet 
damaging 
Optional scenario : 
Direct disposal 
 
 Impact of helium on the 
microstructure evolution of the 
spent fuel 
 
 To supply theoretical models of 
long time behaviour for spent fuel 
 
LITERATURE ON HELIUM PRECIPITATION IN FUEL 
Helium precipitation in UO2 and (U,Pu)O2 matrices 
Size and density characterization of 
helium bubbles populations in 
disposal conditions 
Separated effects study: 
Implantation - Irradiation – Temperature – 
Fine characterization  : TEM 
Implanted 
materials 
Self-irradiated 
materials 
[Sattonnay et al., J. Nucl. Mat. 355, 2006, 131 - 135] 
[Sabathier et al., F-BRIDGE, 2012] 
[Wiss et al., J. Nucl. Mat. 451, 2014, 198 - 206] 
[Talip et al., J. Nucl. Mat., 445, 2014, 117 - 127] 
Authors Fuel type Implantation 
Annealing 
conditions 
Bubble size 
(nm) 
Bubble density 
(b.m-3) 
Sattonnay et al. UO2 single crystal 
7 keV – 7.1015 
at.cm-2 - 1 at. % 
600°C ~ 25 / 
Sabathier et al. Polycristalline UO2 
7 keV - 7.1015 
at.cm-2 - 1 at. % 
 T° > 500°C No bubbles / 
Sabathier et al. Polycristalline UO2 
500 keV – 1.1016 – 
0.3 at. % 
900°C 0min 
1000°C 0min 
1000°C 30min 
1000°C 1h 
~ 1 
2.7 
1.5 - 2 
3 
/ 
4.1023 
1.1024 
1.1024 
Wiss  et al. (U0,9
238Pu0,1)O2 / > 600°C 1 / 
Talip et al. (U0,9
238Pu0,1)O2 / 
1130°C 1h 
1530°C 3h 
1 - 2 
5 – 6 
/ 
TEM observation of helium 
bubbles in 500 keV He-
implanted UO2 (10
16 at.cm-2) 
and annealed 30 min at 1000°C 
 Only few results on helium precipitation in 
oxide fuels and scattered results 
UO2 
EXPERIMENTAL APPROACH 
Helium implantation 
500 keV 
2 fluences 1.1016 and 3.1016 at.cm-2 
Depleted UO2 disks 
Blank UO2 
SEM micrography SRIM calculations 
1.1016 at.cm-2 
3.1016 at.cm-2 
Temperature (°C) Time (h) 
200 5,7 
400 10,3 
600 6,5 
Temperature (°C) Time (h) 
600 1 
600 6 
800 1 
In-situ annealing Ex-situ annealing 
FIB preparation 
Transmission Electron 
Microscopy (TEM) 
FIB cross section 
RT° 
FEI Tecnaï G2 
Spatial resolution : 0.27 nm 
 
FEI TITAN 
Spatial resolution : 0.1 nm 
TEM OBSERVATIONS 
 Observation of a defects zone distinctive of the 
implantation ; only observable for the higher fluence 
so 3.1016 at.cm-2 
Under-focused image Over-focused image 
Over-focused image Under-focused image 
Condition of observation : white/black contrasts for under/over focused 
Bubbles 
Platelets 
1,2 µm 
Implantation 
zone 
Sample 
600°C and 3.1016 at.cm-2 implanted sample 
1 µm 
He 
1,2 µm 
1.1016 at.cm-2 
3.1016 at.cm-2 
He 
He 
He 
He 
He 
CONDITIONS FOR OBSERVATIONS 
Under-focused image Over-focused image 
Bubbles are observed in the implantation zone for 600°C 
implantations at 1.1016 and 3.1016 at.cm-2 
 
Images do not permit to know if there are bubbles or not for the 
other conditions 
1,2 µm 
Implantation 
zone 
Sample 
600°C and 3.1016 at.cm-2 implanted sample 
1 µm 
1,2 µm 
1.1016 at.cm-2 
3.1016 at.cm-2 
Bubbles 
Annealing Fluence (at.cm-2) Temperature (°C) Time (h) Bubbles 
In-situ 
1.1016 
200 4,4 No 
400 2,3 No 
600 1,8 Yes 
3.1016 
200 5,7 No 
400 10,3 No 
600 6,5 Yes 
He 
He 
He 
He 
He 
He 
Implantation 
conditions 
Observation 
depth (µm) 
 He 
concentration 
(at.%) 
Damages 
rate (dpa) 
Bubble density (b.m-3) 
For 100 nm thick 
Diameter 
(nm) 
Number of 
bubbles 
1.1016 – 600°C 0.1 0.0023 0.02 (7.5 ±  6.0) .1021 0.58 ± 0.58 4 
1.1016 – 600°C 0.8 0.072 0.12 (2.6 ± 1.5) .1022 0.91 ± 0.53 14 
3.1016 – 600°C 0.1 0.0069 0.06 (1.5 ±  1.0) .1022 0.79 ± 0.57 8 
3.1016 – 600°C 0.1 0.0069 0.06 (1.1 ± 0.8) .1022 0.85 ± 0.63 6 
3.1016 – 600°C 1.4 0.23 0.12 (4.5 ± 2.3) .1022 0.89 ± 0.47 24 
3.1016 – 600°C 1.2 0.92 0.81 (3.0 ± 1.9) .1022 0.86 ± 0.57 9 
3.1016 – 600°C 1.2 0.92 0.81 (2.4 ± 1.4) .1022 0.85 ± 0.52 13 
Important quantity of defects  Addition of parasite 
contrast  Insufficient image quality Maximum size of 1 nm 
Bubbles density of about 1022 b.m-3 
Very low statistics 
Need to improve the image analysis method 
ANALYSIS OF TEM IMAGES Bubbles 
Method : « simply sight count » 
Counts of all bubbles that appear in both images (under- and –over focused) 
 No evolution of the diameter and of the bubble 
density as a function of the depth or of the helium 
concentration 
Need to improve the observations 
 Complicated image analysis AND high measurement 
uncertainties 
3.1016 at.cm-2 – 600°C implanted sample after an 1h/400°C annealing on FIB cross section 
Thresholding of images and particle analysis 
Covering of 2 
images 
130 counted objects 
Density : (9.3 ± 3.6) .1022 b.m-3 
Mean diameter : 1.4 ± 0.86 nm 
ANALYSIS OF TEM IMAGES 
Development of a new method using 
ImageJ software : 
« Simply sight count » method : 
72 counted bubbles so (5.3 ± 2.2) .1022 b.m-3 for density and 
1.43 ± 0.90 nm for the mean diameter  
Correction of magnification as a function of focus 
Treatment and alignment of images  
Bubbles 
Observations with a TEM FEI TITAN 
- Comparison of both methods : 
Bubbles number Density (b.m-3) 
Mean diameter 
(nm) 
Simply sight count 
method 
72 (5.3 ± 2.2) .1022 1.43 ± 0.90 
Method using 
ImageJ 
130 (9.3 ± 3.6) .1022 1.40 ± 0.86  
CONCLUSION ON THE RESULTS Bubbles 
Difference on number of counted objects between the 2 
methods but not on the mean diameter 
Advantages 
 No dependence of the user on the results 
 Take into account the difference of alignment of 
the images 
 Treatment and thresholding steps enable to free 
itself from defects problems 
 Count rate more important 
 Implement automatic fonctions to detect the 
particles 
 Use of macros to automate the method 
Optimization 
Observations of the samples with the more powerful TEM 
Optimization step of the ImageJ method 
Use the ImageJ method to characterize helium bubbles improving values and 
measurements uncertainties 
CONDITIONS FOR OBSERVATION Platelets 
1,2 µm 
Implantation 
zone 
Sample 
600°C and 3.1016 at.cm-2 implanted sample 
1 µm 
1,2 µm 
1.1016 at.cm-2 
3.1016 at.cm-2 
Annealing Fluence (at.cm-2) Temperature (°C) Time (h) Platelets 
In-situ 
1.1016 200 / 400 / 600 / No 
3.1016 
200 5,7 No 
400 10,3 No 
600 6,5 Yes 
Ex-situ 
1.1016 800 1 No 
3.1016 
600 1 No 
600 6 Yes 
800 1 Yes 
He 
He 
He 
He 
He 
He 
On 200 nm centered at the depth of the implantation peak 
Over-focused image Under-focused image 
EELS ANALYSIS 
STEM HAADF Image 
EELS spectrum 
Increase observed at the 
level of the He K-edge 
UO2 plasmon peak 
For a diluted gas, the K threshold is 
equal to 21.218 eV [1] 
 
It can be shifted to the high value (blue 
shift) when He density increases [2] 
[1] Kuhn, Atomic spectra (1962) 132 
[2] Fréchard et al. JNM 393 (2009) 102 
Platelets 
3.1016 at.cm-2 and 600°C implanted sample 
eV 
eV 
15 45 75 
15 45 75 
a
.u
. 
a
.u
. 
No 
increase 
He K-edge 
Step 1 : Length and 
thickness measurements 
Length 
Intensity profile 
ANALYSIS TEM IMAGES 
Implantation 
conditions 
(°C – at.cm-2) 
Bulk annealing 
Platelet length Platelet thickness 
Number of 
platelets 
Surface density 
of platelets 
(platelets.m-2) 
On the layer of 200 nm 
thick 
Minimum 
length (nm) 
Maximum length 
(nm) 
Minimum 
thickness (nm) 
Maximum 
thickness (nm) 
600 – 3.1016 No 17 ± 3 76 ± 15 0.4 ± 0.1 1.1 ± 0.5 12 (9.3 ± 2.7) .1013 
600 – 3.1016 No 5 ± 1 49 ± 8 0.6 ± 0.1 3.9 ± 0.8 25 (8.4 ± 1.7) .1013 
RT – 3.1016 800°C / 1 h  16 ± 3 77 ± 15 0.5 ± 0.1 2.1 ± 0.4 17 (3.7 ± 0.9) .1013 
RT – 3.1016 600°C / 6 h 6 ± 1 93 ± 18 0.4 ± 0.1 0.9 ± 0.2 11 (1.4 ± 0.4) .1014 
Length included between a few nanometers 
and around a hundred of nanometers 
Thickness of about a 
few nanometers 
Density of about 
1013/1014 platelets.m-2 
Platelets 
In
te
n
s
it
y
 
Length (nm) 
Step 2 : Diffraction 
patterns indexation 
Step 3 : Search of habit 
planes 
ANALYSIS TEM IMAGES 
Implantation 
conditions 
(°C – at.cm-2) 
Annealing 
Platelets 
number 
Habit plane 
600 – 3.1016 No 25 
{1 1 1} 
{1 1 1} 
RT – 3.1016 800°C / 1 h  17 {1 1 1} 
RT – 3.1016 600°C / 6 h 11 {1 1 1} 
 It seems that platelets preferentially appear in {1 1 1} 
type habit plans. 
Platelets 
(1 1 1) (-1 1 1) 
(1 -1 1) (-1 -1 1) 
4 possible variants of 
the {1 1 1} habit plan 
in UO2 : 
CONCLUSION ON HELIUM PRECIPITATION IN UO2 
 Presence of helium bubbles for 600°C 
implantations for two fluences 1.1016 and 
3.1016 at.cm-2 : maximum size of 1 nm and a 
density of about 1022 b.m-3  
Platelets 
 Presence of helium platelets for an 
implantation in temperature or a post-
implantation annealing at 600°C or 800°C for 
the higher fluence 3.1016 at.cm-2 
Bubbles 
Complementary 
TEM observations  
To characterize helium bubbles 
populations with improving 
values and uncertainties 
Influence of 
irradiation damages 
Influence of fission 
gases bubbles 
TEM characterization on MOX 
containing radiogenic He 
 Difficulties to highlight the influence of 
temperature and helium concentration on their 
features 
 Effect of annealing time on the presence or not 
of platelets. 
 Helium platelets preferentially form in {1 1 1} 
type habit plans 
To obtain complementary 
characterizations on helium 
bubbles features 
Development 
optimized method  
of TEM images 
SAXS 
measurements 
To study the intergranular 
behaviour of platelets 
To determine precisely habit 
planes with different variants 
He precipitation in UO2 He precipitation in (U,Pu)O2 
Outlook 
Contact : 
adrien.michel2@cea.fr 
15 
Thank you for your attention ! 


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Helium precipitation study in UO2 by Transmission Electron Microscopy

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