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.

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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

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)

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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Molecular Beam EpitaxyR-plane sapphire + CeO2 or UO2Lattice mismatch: CeO2 <2% UO2 <1%

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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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