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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.

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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 thin film 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.

Preliminary results of UO2 + Nd film growth.

NERI-C PROJECT NO. 08-041

acknowledgements
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-C PROJECT NO. 08-041

slide3
Classical picture—transition state theory

yields jump frequency over saddle point

saddle point

Interstitial

self-diffusion--

dumbbell arrange-

Ment.

Diffusion—Microscopic point of view w/point defects

Vacancy self-diffusion

VSD

Diffusion processes at microscopic scale

coupled to point defects in crystalline solid

D(T)=Do exp(-Ea/kT)

NERI-C PROJECT NO. 08-041

slide4
Diffusion—Activation Energy of point defects

D(T)=Do exp(-Ea/kT)

Activation Energy, Ea Vacancy Interstitial

Ea = Ef+ Em ~1 eV ~2 eV

Ef –energy of formation ~0.2 eV ~2 eV

Em –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. 08-041

radiation damage process freely migrating defects fmds
Radiation Damage ProcessFreely-migrating defects, FMDs

Frenkel pair population

inside displacement cascade

Few point defects (FMDs) survive

displacement cascade quenching

Fast neutron

1st struck atom

(PKA)

FMDs—vacancies and

Interstitials in

~ equal numbers

  • Three phases
  • Formation
  • Recombination
  • Thermal spike

Displacement

Cascade—high

Density of Frenkel

Pairs (vac. + int.)

NERI-C PROJECT NO. 08-041

slide6
Radiation-Enhanced Diffusion (RED)—Combination of Elevated Point Defect Populations and Elevated Temperature

Thermal v/i population>>

Frenkel pairs

Thermal

VSD

T>1100K

Fate of FMDs

sink

sink

Sink-Limited

Kinetics

T~850-1050K

i

v

v+i recombination

T<800K

Recombination-

Limited Kinetics

v

i

Ballistic

Mixing

T~295K

Temperature

slide7
CeO2 and UO2 have same structure—Ceria often used as surrogate for Urania.

Fluorite Structure—anions red, cations white

Epitaxial relationship—

Fluorite structure:R-plane Sapphire

CeO2

Tm=2673 K

a=5.4114 A

UO2

Tm=3138 K

a=5.466 A

NERI-C PROJECT NO. 08-041

sample architecture w la impurity layer
Sample Architecture w/La Impurity Layer
  • Two ways to consider LaCeO2
  • Tracer or marker layer for cation diffusion
  • +3 dopant in CeO2
  • La is +3 actinide surrogate
  • (Am, for example) and high-yield
  • (A=139) fission product.

CeO2

370 Å

~3 Å

1ML LaCeO2

CeO2

Sapphire

NERI-C PROJECT NO. 08-041

experimental facilities at illinois
Experimental Facilities at Illinois
  • Microanalytical: AES, SIMS, RBS, XRD/XRR, TEM.
  • Implantation/Bombardment: Van de Graaff (0.5-2.3 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

NERI-C PROJECT NO. 08-041

ion bombardment trim results
Ion Bombardment—TRIM results

1.8 MeV Kr+ implantation into CeO2 on sapphire

1.8 MeV Kr+ Energy to Recoils—FD (need later)

CeO2

FD =115 eV/Å/ion

sapphire

FD

Kr

CeO2

Variable temperature, constant fluence bombardment: F = 1x1016 ions/cm2

 0.02 FIMA ~2% burnup

NERI-C PROJECT NO. 08-041

secondary ion mass spectroscopy sims
Secondary Ion Mass Spectroscopy (SIMS)

O or Cs sputter

beam rastered over

400 x 400 mm2 area

Residual positive charge

on sample surface after O sputter

beam raster

Au analytical beam

Beam rastered over

50 x 50 mm2 area

+

+

+

+

+

+

+

Sample surface

Positive-charged species liberated

by analytical beam & accelerated

across voltage bias—mass separated

by time-of-flight

CeO2

NERI-C PROJECT NO. 08-041

xrd analysis of mbe ceo 2 film
XRD Analysis of MBE CeO2 film

Specular Scan

Rocking Curve

In-plane f Scan

CeO2 is single crystal—no grain boundaries.

NERI-C PROJECT NO. 08-041

sims results rt
SIMS Results—RT

1.8 MeV Kr+ bombardment

Variable fluence; constant T

Ballistic mixing parameter

x= Dt /FFD

Relates to energy deposition

to RMS distance

La depth profiles

1-D Diffusion Geometry: s 2 ~ Dt

2Dt = (sirr)2 – (sref) 2

As grown: s~26Å

CeO2

  • = 4 Å5/eV in CeO2
  • = 120 Å5/eV in Au
  • ~ 1-5 Å5/eV in MgO

NERI-C PROJECT NO. 08-041

sims results elevated t
SIMS Results—Elevated T

1.8 MeV Kr+ bombardment

Variable T; constant fluence

Kinetic Rate Theory

Time rate of change = Production – Loss to sinks

- Loss via recombination

La depth profiles

RT irradiated: s~36Å

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

induced by bombardment

vo—thermal equil. vacancy fraction

ni—interstitial jump frequency

NERI-C PROJECT NO. 08-041

steady state solutions to kinetic rate theory
Steady-State Solutions to Kinetic Rate Theory

Total vacancy fraction

Total interstitial fraction

Diffusivities due to Frenkel defects

Total diffusivity

NERI-C PROJECT NO. 08-041

three temperature regimes
Three Temperature Regimes

Recombination

limited: v+i=0

Low T <800K

Sink limited:

v  dislocation

i  dislocation

Intermediate T

D’ ≠ f(T)

High T >1100K

VSD

NERI-C PROJECT NO. 08-041

diffusivity versus temperature
Diffusivity versus Temperature

D(T)=Do exp(-Ea/kT)

VSD

VSD

RED

NERI-C PROJECT NO. 08-041

discussion
Discussion
  • Cation vs. Anion diffusion.
  • +3 dopant-anion vacancy cluster.
  • No influence from grain boundaries.

NERI-C PROJECT NO. 08-041

slide19
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

magnetron sputtering system at illinois
Magnetron Sputtering System at Illinois

Targets: depleted U; Ce; Nd; Mo

Power Supply: 3 DC; 1 RF

Gas Supply: O2: 1x10-9 to 1x10-3 T

Ar: 1 to 100 sccm

Max. Ts=850 C

NERI-C PROJECT NO. 08-041

uo 2 single crystal film growth on ysz
UO2 Single Crystal Film Growth on YSZ

Strain free

UO2

RBS—UO2

Smooth surface

Single crystal

domain

NERI-C PROJECT NO. 08-041

sims on uo 2 nd
SIMS on UO2 + Nd

Nd isotopes

s~31 Å

U-235 region

U isotopes

NERI-C PROJECT NO. 08-041

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