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Radiation-Enhanced Diffusion of La in Ceria

Radiation-Enhanced Diffusion of La in Ceria. Summary NERI-C collaboration to study actinide surrogate and fission gas behavior in UO 2 . Started with CeO 2 —development of UO 2 fabrication facilities required time. Use of thin film samples with controlled microstructure and impurity content.

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Radiation-Enhanced Diffusion of La in Ceria

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  1. 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. 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 NERI PROJECT NO. 08-041

  3. Vacancy self-diffusion VSD thermal vacancy Classical picture—transition state theory yields jump frequency over saddle point saddle point Interstitial self-diffusion-- these arrangements are called crowdions Diffusion—Microscopic point of view Diffusion processes at microscopic scale coupled to lattice defects in crystalline solid D(T)=Do exp(-Ea/kT)

  4. 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). NERI PROJECT NO. 08-041

  5. Crystal Structure Fluorite Structure—anions red, cations white CeO2 Tm=2673 K a=5.4114 A UO2 Tm=3138 K a=5.466 A NERI PROJECT NO. 08-041

  6. Molecular Beam Epitaxy R-plane sapphire + CeO2 or UO2 Lattice mismatch: CeO2 <2% UO2 <1% NERI PROJECT NO. 08-041

  7. Ar2 Air Ar1 O2 Sputter Deposition Facility Schematic Foreline pump MFC1 MFC2 FV1 VV1 FV2 SV4 SV6 TP1 PG TCG Mass Spec. GV1 APC RV CG1 CM2 TP2 VV2 IG1 CM1 GV2 Sample Trans. Arm CM3 Primary Chamber Load- lock Thickness Monitor VLV CG2 IG2 SV5 TP—turbo pump GV—gate valve FV—foreline valve VV—vent valve SV—solenoid valve RV—relief valve VLV—variable leak valve CG—convectron gauge IG—ion gauge TCG—thermocouple gauge PG—Pirani gauge CM—capacitance manometer MFC—mass flow controller S—sputter gun S1 S2 S3 SV2 SV1 SV3

  8. Magnetron Sputtering System at Illinois Targets: depleted U; Ce; Nd Power Supply: 3 DC; 1 RF Gas Supply: O2: 0 to 10 sccm Ar: 1 to 100 sccm Max. Ts=850 C NERI PROJECT NO. 08-041

  9. XRD Analysis of MBE CeO2 film Specular Scan Rocking Curve In-plane f Scan NERI PROJECT NO. 08-041

  10. 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) NERI PROJECT NO. 08-041

  11. SIMS Results—RT 1.8 MeV Kr+ bombardment Variable fluence; constant T Ballistic mixing parameter 2Dt = (sirr)2 – (sref) 2 La depth profiles = Dt /FFD = 4 Å5/eV NERI PROJECT NO. 08-041

  12. SIMS Results—Elevated T [3a] 1.8 MeV Kr+ bombardment Variable T; constant fluence Kinetic Rate Theory La depth profiles K—Frenkel pair production rate K~0.02 1/s (heavy ion) K~10-10 1/s (fast neutron) Kv,i—defect removal rates at sinks v,i—point defect fractions under bombardment vo—thermal equil. vacancy fraction NERI PROJECT NO. 08-041

  13. Steady-State Solutions to Kinetic Rate Theory Total vacancy fraction Total interstitial fraction Diffusivities due to Frenkel defects Total diffusivity NERI PROJECT NO. 08-041

  14. Three Temperature Regimes Recombination limited: v+i=0 Low T <800K Sink limited: v  dislocation i  dislocation Intermediate T D’≠T High T >1100K VSD NERI PROJECT NO. 08-041

  15. Diffusivity versus Temperature D(T)=Do exp(-Ea/kT) VSD RED VSD NERI PROJECT NO. 08-041

  16. Discussion • Cation vs. Anion diffusion. • +3 dopant-anion vacancy cluster. • No influence from grain boundaries. NERI PROJECT NO. 08-041

  17. UO2 Single Crystal Film Growth on YSZ NERI PROJECT NO. 08-041

  18. SIMS on UO2 + Nd NERI PROJECT NO. 08-041

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