Radiation-Enhanced Diffusion of La in Ceria Summary NERI-C collaboration to study actinide...
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Radiation-Enhanced Diffusion of La in Ceria
Summary NERI-C collaboration to study actinide surrogate and fission gas behavior in UO2.
Started with CeO2—development of UO2 fabrication facilities required time. Use of thin film samples with controlled microstructure and impurity content. Behaviors of interest: diffusion, segregation, bubble formation; influence of grain
boundaries. Techniques: Experimental—SIMS, XAS, XPS, RBS, TEM. Computational—kMC, DFT, MD.
Outline Introduction to thermal diffusion and radiation-enhanced diffusion (RED). CeO2 system—cation vs. anion sublattice, film characterization Experimental results—SIMS profiles, analysis to determine diffusivities. Discussion of results—diffusivity vs. temperature, three temperature regimes, influence of
vacancies on oxygen anion sublattice. Results of UO2 + Nd.
04/19/23 NERI PROJECT NO. 08-041
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04/19/23 NERI PROJECT NO. 08-041 2
Acknowledgements University of Illinois
J. Stubbins, R. Averback. P. Bellon, J. Eckstein H. Pappas, M. Strehle, H. Ju, M. El-Bakhshwan,
X. Han, D. Heuser. T. Spilla, D. Jeffers, S. Burdin
Funding DOE NEUP/NERI-C program UIUC MRL and DOE
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Diffusion—Microscopic point of view
Diffusion processes at microscopic scalecoupled to lattice defects in crystalline solid
thermal vacancy
Vacancy self-diffusionVSD
Interstitial self-diffusion--these arrangements are calledcrowdions
Classical picture—transition state theoryyields jump frequency over saddle point
saddle point
D(T)=Do exp(-Ea/kT)
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Radiation-Enhanced Diffusion
Radiation damage elevates point defect population above thermal equilibrium.
Thermally-driven transport of point defects to sinks leads to segregation, phase separation, bubble formation.
RED—combination of point defect formation under bombardment and thermally-driven transport (i.e., diffusion).
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Crystal Structure
Fluorite Structure—anions red, cations white
CeO2Tm=2673 Ka=5.4114 A
UO2Tm=3138 Ka=5.466 A
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04/19/23 NERI PROJECT NO. 08-041 6
Molecular Beam EpitaxyR-plane sapphire + CeO2 or UO2Lattice mismatch: CeO2 <2% UO2 <1%
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CM2 TP2
TP1
Forelinepump
FV1 FV2
GV1
GV2Sample Trans. Arm
PrimaryChamber
Load-lock
S1 S2 S3
CG1
IG1
CM1
ThicknessMonitor
O2AirAr1 Ar2
CG2 IG2
VV1
VV2
MassSpec.
SV5
SV2 SV3SV1
MFC2
SV6SV4
MFC1
TCG
TP—turbo pumpGV—gate valveFV—foreline valveVV—vent valveSV—solenoid valveRV—relief valveVLV—variable leak valve
CG—convectron gaugeIG—ion gaugeTCG—thermocouple gaugePG—Pirani gaugeCM—capacitance manometerMFC—mass flow controllerS—sputter gun
SPUTTER DEPOSITION FACILITY SCHEMATIC
CM3
RV
PG
APC
VLV
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04/19/23 NERI PROJECT NO. 08-041 8
Magnetron Sputtering System at Illinois
Targets: depleted U; Ce; NdPower Supply: 3 DC; 1 RFGas Supply: O2: 0 to 10 sccm Ar: 1 to 100 sccmMax. Ts=850 C
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XRD Analysis of MBE CeO2 film
Specular Scan Rocking Curve In-plane Scan
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Experimental Facilities at Illinois
Microanalytical: AES, SIMS, RBS, XRD/XRR, TEM, SEM, AFM. Implantation/Bombardment: tandem van de Graaff (0.5-2.3
MeV; H, He, Xe, Kr, Ne; ~100 nA)
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SIMS Results—RT
2Dt = (irr)2 – (ref) 2
La depth profiles
Ballistic mixing parameter
= Dt /FD = 4 Å5/eV
1.8 MeV Kr+ bombardmentVariable fluence; constant T
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SIMS Results—Elevated T
La depth profiles
1.8 MeV Kr+ bombardmentVariable T; constant fluence
[3a]
Kinetic Rate Theory
K—Frenkel pair production rateK~0.02 1/s (heavy ion)K~10-10 1/s (fast neutron)Kv,i—defect removal rates at sinksv,i—point defect fractions under bombardmentvo—thermal equil. vacancy fraction
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Steady-State Solutions to Kinetic Rate Theory
Total vacancy fraction
Total interstitial fraction
Diffusivities due to Frenkel defects
Total diffusivity
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Three Temperature Regimes
Low T <800K
Intermediate T
High T >1100K
D’≠T
Recombinationlimited: v+i=0
Sink limited: v dislocationi dislocation
VSD
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Diffusivity versus Temperature
VSDRED
VSD
D(T)=Do exp(-Ea/kT)
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Discussion
Cation vs. Anion diffusion. +3 dopant-anion vacancy cluster. No influence from grain boundaries.
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UO2 Single Crystal Film Growth on YSZ
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SIMS on UO2 + Nd