Radiation-Enhanced Diffusion of La in Ceria Summary  NERI-C collaboration to study actinide surrogate and fission gas behavior in thin film 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.  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).  CeO 2 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.  Preliminary results of UO 2 + Nd film growth. 5/9/2015 NERI-C PROJECT NO

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 5/9/2015NERI-C PROJECT NO

Diffusion—Microscopic point of view w/point defects Diffusion processes at microscopic scale coupled to point defects in crystalline solid Vacancy self-diffusion VSD Interstitial self- diffusion-- dumbbell arrange- Ment. Classical picture—transition state theory yields jump frequency over saddle point saddle point D(T)=D o exp(-E a /kT) NERI-C PROJECT NO

Diffusion—Activation Energy of point defects D(T)=D o exp(-E a /kT) Activation Energy, E a VacancyInterstitial E a = E f + E m ~1 eV~2 eV E f –energy of formation~0.2 eV~2 eV E m –energy of migration~1 eV~0.1 eV Interstitial defects more costly to make, but easier to move. As a consequence, VSD dominate mechanism for self-diffusion. NERI-C PROJECT NO

Radiation Damage Process Freely-migrating defects, FMDs 5/9/2015NERI-C PROJECT NO Fast neutron 1 st struck atom (PKA) Displacement Cascade—high Density of Frenkel Pairs (vac. + int.) Frenkel pair population inside displacement cascade Three phases i)Formation ii)Recombination iii)Thermal spike Few point defects (FMDs) survive displacement cascade quenching FMDs—vacancies and Interstitials in ~ equal numbers

Radiation-Enhanced Diffusion (RED)— Combination of Elevated Point Defect Populations and Elevated Temperature 5/9/20156 Ballistic Mixing Fate of FMDs Recombination- Limited Kinetics Sink-Limited Kinetics Thermal VSD Temperature v+i recombination v i sink Thermal v/i population>> Frenkel pairs T~295K T~ K T<800K T>1100K v i

5/9/2015NERI-C PROJECT NO CeO 2 and UO 2 have same structure—Ceria often used as surrogate for Urania. Fluorite Structure—anions red, cations white CeO2 Tm=2673 K a= A UO2 Tm=3138 K a=5.466 A Epitaxial relationship— Fluorite structure:R-plane Sapphire

Sample Architecture w/La Impurity Layer 5/9/2015NERI-C PROJECT NO Sapphire CeO 2 1ML LaCeO Å ~3 Å Two ways to consider LaCeO 2  Tracer or marker layer for cation diffusion  +3 dopant in CeO 2 La is +3 actinide surrogate (Am, for example) and high-yield (A=139) fission product.

Experimental Facilities at Illinois 5/9/2015NERI-C PROJECT NO  Microanalytical: AES, SIMS, RBS, XRD/XRR, TEM.  Implantation/Bombardment: Van de Graaff ( MeV; H, He, Xe, Kr, Ne; ~100 nA).  1.8 MeV Kr + ions ~100 nA; variable fluence; variable temperature. Physical Electronics PHI Trift III SIMS Instrument High Voltage Engineering Van de Graaff Accelerator

Ion Bombardment—TRIM results 5/9/ NERI-C PROJECT NO MeV Kr + implantation into CeO 2 on sapphire 1.8 MeV Kr + Energy to Recoils—F D (need later) F D =115 eV/Å/ion CeO 2 sapphire Kr FDFD Variable temperature, constant fluence bombardment: = 1x10 16 ions/cm 2  0.02 FIMA ~2% burnup

Secondary Ion Mass Spectroscopy (SIMS) 5/9/2015NERI-C PROJECT NO Sample surface O or Cs sputter beam rastered over 400 x 400 m 2 area Au analytical beam Beam rastered over 50 x 50 m 2 area Residual positive charge on sample surface after O sputter beam raster Positive-charged species liberated by analytical beam & accelerated across voltage bias—mass separated by time-of-flight CeO 2

XRD Analysis of MBE CeO 2 film 5/9/2015NERI-C PROJECT NO Specular Scan Rocking Curve In-plane  Scan CeO 2 is single crystal—no grain boundaries.

SIMS Results—RT 5/9/2015NERI-C PROJECT NO D Diffusion Geometry:  2 ~ Dt 2Dt = ( irr ) 2 – ( ref ) 2 La depth profiles Ballistic mixing parameter = Dt /F D Relates to energy deposition to RMS distance  = 4 Å 5 /eV in CeO 2  = 120 Å 5 /eV in Au  ~ 1-5 Å 5 /eV in MgO 1.8 MeV Kr + bombardment Variable fluence; constant T CeO 2 As grown: ~26Å

SIMS Results—Elevated T 5/9/2015 NERI-C PROJECT NO La depth profiles 1.8 MeV Kr + bombardment Variable T; constant fluence Kinetic Rate Theory K—Frenkel pair production rate K~0.02 1/s (heavy ion) K~ /s (fast neutron) K v,i —defect removal rates at sinks v, i —point defect fractions induced by bombardment v o —thermal equil. vacancy fraction i —interstitial jump frequency Time rate of change = Production – Loss to sinks - Loss via recombination RT irradiated: ~36Å

Steady-State Solutions to Kinetic Rate Theory 5/9/2015NERI-C PROJECT NO Total vacancy fraction Total interstitial fraction Diffusivities due to Frenkel defects Total diffusivity

Three Temperature Regimes 5/9/2015NERI-C PROJECT NO Low T <800K Intermediate T High T >1100K D’ ≠ f(T) Recombination limited: v+i=0 Sink limited: v  dislocation i  dislocation VSD

Diffusivity versus Temperature 5/9/2015NERI-C PROJECT NO VSD D(T)=D o exp(-E a /kT) RED

Discussion  Cation vs. Anion diffusion.  +3 dopant-anion vacancy cluster.  No influence from grain boundaries. 5/9/2015NERI-C PROJECT NO

Magnetron Sputtering System at Illinois 5/9/2015NERI-C PROJECT NO Targets: depleted U; Ce; Nd; Mo Power Supply: 3 DC; 1 RF Gas Supply: O 2 : 1x10 -9 to 1x10 -3 T Ar: 1 to 100 sccm Max. T s =850 C

UO 2 Single Crystal Film Growth on YSZ 5/9/2015NERI-C PROJECT NO Strain free UO 2 Smooth surface Single crystal domain RBS—UO 2

SIMS on UO 2 + Nd 5/9/2015NERI-C PROJECT NO Nd isotopes U isotopes U-235 region ~31 Å