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.  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.  Results of UO 2 + Nd. 9/19/2015 NERI PROJECT NO

9/19/2015NERI 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

Diffusion—Microscopic point of view Diffusion processes at microscopic scale coupled to lattice defects in crystalline solid thermal vacancy Vacancy self-diffusion VSD Interstitial self-diffusion-- these arrangements are called crowdions Classical picture—transition state theory yields jump frequency over saddle point saddle point D(T)=D o exp(-E a /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). 9/19/2015NERI PROJECT NO

9/19/2015NERI PROJECT NO Crystal Structure Fluorite Structure—anions red, cations white CeO2 Tm=2673 K a= A UO2 Tm=3138 K a=5.466 A

9/19/2015NERI PROJECT NO Molecular Beam Epitaxy R-plane sapphire + CeO2 or UO2 Lattice mismatch: CeO2 <2% UO2 <1%

9/19/2015NERI PROJECT NO Magnetron Sputtering System at Illinois Targets: depleted U; Ce; Nd Power Supply: 3 DC; 1 RF Gas Supply: O 2 : 0 to 10 sccm Ar: 1 to 100 sccm Max. T s =850 C

9/19/2015NERI PROJECT NO XRD Analysis of MBE CeO 2 film Specular Scan Rocking Curve In-plane  Scan

9/19/2015NERI PROJECT NO Experimental Facilities at Illinois  Microanalytical: AES, SIMS, RBS, XRD/XRR, TEM, SEM, AFM.  Implantation/Bombardment: tandem van de Graaff ( MeV; H, He, Xe, Kr, Ne; ~100 nA)

9/19/2015NERI PROJECT NO SIMS Results—RT 2Dt = ( irr ) 2 – ( ref ) 2 La depth profiles Ballistic mixing parameter  = Dt /F D = 4 Å 5 /eV 1.8 MeV Kr + bombardment Variable fluence; constant T

9/19/2015NERI PROJECT NO SIMS Results—Elevated T La depth profiles 1.8 MeV Kr + bombardment Variable T; constant fluence [3a] 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 under bombardment v o —thermal equil. vacancy fraction

9/19/2015NERI PROJECT NO Steady-State Solutions to Kinetic Rate Theory Total vacancy fraction Total interstitial fraction Diffusivities due to Frenkel defects Total diffusivity

9/19/2015NERI PROJECT NO Three Temperature Regimes Low T <800K Intermediate T High T >1100K D’≠T Recombination limited: v+i=0 Sink limited: v  dislocation i  dislocation VSD

9/19/2015NERI PROJECT NO Diffusivity versus Temperature VSD RED VSD D(T)=D o exp(-E a /kT)

9/19/2015NERI PROJECT NO Discussion  Cation vs. Anion diffusion.  +3 dopant-anion vacancy cluster.  No influence from grain boundaries.

9/19/2015NERI PROJECT NO UO 2 Single Crystal Film Growth on YSZ

9/19/2015NERI PROJECT NO SIMS on UO 2 + Nd