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R ADIATION D AMAGE IN C RISTALLINE W ASTEFORMS Maik Lang University of Tennessee Department of Nuclear Engineering Knoxville, TN, USA mlang2@utk.edu

University of Tennessee Knoxville, Tennessee Smokey Mountain National Park

Durable Materials for Radionuclide Immobilization Performance in extreme environments: Intergrowth of natural ⊳ intense radiation pyrochlore (Py) and ⊳ elevated temperature zirconolite (Z) ⊳ changing chemical composition G.R. Lumpkin, Elements (2006) ⊳ long-term disposal in changing environment Complex structural and chemical modifications: Ion track in Gd 2 Zr 2 O 7 (12-MeV C 60 ) ⊳ simple defects and defect clusters .. ⊳ order-disorder and crystalline-amorphous transformations ⊳ partial recrystallization of waste glasses ⊳ defect mobility and damage recovery at high temperature .. Crystalline Wasteforms: ⊳ chemical durable (very low leach rates) ⊳ compatibility for geological disposal J.M. Zhang et al ., J. Appl. Phys. (2010) ⊳ large intake of actinides ⊳ concern: radiation effects and crystalline-to-amorphous transformation .. Maik Lang – University of Tennessee 3

Crystalline Wasteforms for Radionuclide Immobilization ZrO 2 simple oxides: zirconia (Na,Ca,U) 2 (Nb,Ti,Ta) 2 O 6 complex oxides: pyrochlore (Na,Y) 4 (Zn,Fe) 3 (Ti,Nb) 6 O 18 (F,OH) 4 murataite CaZrTi 2 O 7 zirconolite CaTiO 3 perovskite ZrSiO 4 silicates: zircon* ThSiO 4 thorite* (Ca,Mg,Fe 2+ ) 3 (Al,Fe 3+ ,Cr 3+ ) 2 (SiO 4 ) garnet* (Ca,Ce) 5 (SiO 4 ) 3 (OH,F) britholite CaTiSiO 5 titanite LnPO 4 phosphates: monazite* Ca 4-x Ln 6+x (PO 4 ) y (O,F) 2 apatite* YPO 4 xenotime* *durable heavy minerals Maik Lang – University of Tennessee 4

Pyrochlores: Important Group of Materials Applications spin ice state  Exotic magnetic properties Ho 2 Ti 2 O 7  Fast ionic conductors  Thermal barrier coatings  Actinide immobilization Structure  A 2 B 2 O 6 O ’  2x2x2 supercell of fluorite  5 crystallographic sites Maik Lang – University of Tennessee 5

Disordering of Pyrochlore under Extreme Conditions pyrochlore structure defect-fluorite disordering structure A 2 B 2 O 7 amorphization r A /r B 30-MeV C 60 ions Gd 2 Zr 2 O 7 ⊳ cation and anion disorder ⊳ retaining crystallinity ⊳ loss of crystallinity Gd 2 Ti 2 O 7 Maik Lang – University of Tennessee 6

Radiation Effects in Actinide-Bearing Wasteforms Courtesy: Recoil Alpha Dr. William Weber Nucleus Particle (UT/ORNL) 239 Pu Alpha-Recoil Nucleus Alpha-Particle ➢ 70 - 100 keV ion 4.5 - 5.8 MeV ion ➢ 16 - 22 m m Range ➢ 30 - 40 nm Range ➢ ➢ Creates More Damage Creates Less Damage ➢ (~2000 Displaced Atoms) (~350 Displaced Atoms) Maik Lang – University of Tennessee 7

Radiation Effects in Actinide-Bearing Wasteforms Recoil Alpha Nucleus Particle Maik Lang – University of Tennessee 8

Radiation Effects in Nuclear Wasteforms Courtesy: Dr. William Weber (UT/ORNL) Ion Irradiation & Computer Simulation provide way to bridge Time Gap (Dose Rate Effects) between Laboratory Studies and Geologic Time Scales W.J. Weber et al., J. Mater. Research 13 (1998) 1434-1484 Maik Lang – University of Tennessee 9

Simulation of Alpha-Recoil Damage in Waste Forms Material Alpha Decay irradiated layer: 100 nm – 1 μ m ions energy release: ~5 MeV Ion-beam experiments: MeV energies  more realistic simulation of radiation effects (nuclear d E /d x )  small volume of modified material  many bulk characterization techniques Tandem Accelerator (E < 25 MeV) are not applicable available in many laboratories Maik Lang – University of Tennessee 10

Low-Energy Irradiation Effects in Pyrochlore Oxides Actinide decay in complex oxides  damage accumulation from self-irradiation R. Ewing, W.J. Weber, and J. Lian, J. Appl. Phys. 95 (2004) pyrochlore S.X. Wang et al ., J. Mater. Res. (1999) J. Lian et al ., Phys. Rev. Lett. (2001) B.D. Begg et al ., J. Nucl. Mater. (2001) Gd 2 Ti 2 O 7 Gd 2 Zr 2 O 7 1-MeV Kr 1-MeV Kr ions ions amorphous pyrochlore defect fluorite Maik Lang – University of Tennessee 11

Critical Temperature of Amorphization in Pyrochlore Gd 2 (Ti 1-x Zr x ) 2 O 7 10 x=1, crystalline 1x10 16 amorphization fluence(Ions/cm 2 ) x=0.5 amorphization dose (dpa) x=0.25 x=0.75, ~ 30% amorphous x=0 1.0 1x10 15 0.1 1x10 14 0 200 400 600 800 1000 1200 critical temperature T c (K) Maik Lang – University of Tennessee 12

Simulation of Radiation Effects with Swift Heavy Ions Material Spontaneous Fission irradiated layer: 10 μ m – 100 μ m Courtesy: William Weber (UT) ions energy release: ~200 MeV Ion-beam experiments: GeV energies  different ion-matter interactions (electronic d E /d x ) www.gsi.de  large volume of modified material  access to many bulk characterization techniques (e.g., X-ray and Linear and ring accelerators (E ~1 GeV) neutron scattering) available at large user facilities Maik Lang – University of Tennessee 13

High-Energy Irradiation Effects in Pyrochlore Oxides Actinide decay in complex oxides  damage accumulation from self-irradiation R. Ewing, W.J. Weber, and J. Lian, J. Appl. Phys. 95 (2004) pyrochlore Gd 2 Zr 2 O 7 Gd 2 Ti 2 O 7 Gd 2 TiZrO 7 30-MeV C 60 ions J. Zhang, M. Toulemonde, M. Lang, J.Costantini, S. Della-Negra, R. Ewing, J. Mater. Res. 30 (2015) Maik Lang – University of Tennessee 14

Radiation Effects: Synchrotron X-Ray Characterization M. Lang, et al ., Journal of Materials Research (2015). Sample chamber diameter: 100 μ m thickness: 50 μ m thickness: 12.5 μ m GSI Helmholtz Center (Germany) and Joint Institute for Nuclear Research (Russia) 197 Au (2.2 GeV) 132 Xe (167 MeV) Advanced Photon Source (Argonne National Lab.) Maik Lang – University of Tennessee 15

Radiation Effects in Complex Oxides: X-Ray Diffraction XRD peak deconvolution A 2 Sn 2 O 7 irradiated with 2.2 GeV 197 Au ⇨ amorphous fraction C.L. Tracy, et al ., PRB (2016) M. Lang, et al ., PRB (2009) Maik Lang – University of Tennessee 16

Transmission Electron Microscopy: Track Morphology changing composition Gd 2 Ti 2 O 7 Gd 2 Ti 1 O 5 2.2-GeV 197 Au 2.2-GeV 197 Au 40 keV/nm; RT 40 keV/nm; RT decreasing J. Zhang, M. Lang, M. Toulemonde, R. Devanathan, R.C. Ewing, W.J. Weber, J. Mater. Res. (2010). energy density decreasing temperature Gd 2 Ti 2 O 7 Gd 2 Ti 2 O 7 2.2-GeV 197 Au 101 Ru 1.1-GeV 40 keV/nm; 8 K 20 keV/nm ; RT 5 nm Maik Lang – University of Tennessee 17

Limitation of X-ray and Electron Probes Z-dependence of X-ray (electron) interactions: ⊳ X-rays (electrons) scatter off atomic electrons ⊳ very small scattering contributions from low-Z elements  oxygen sublattice basically inaccessible for oxides ⊳ elements with comparable Z contribute equally  atomic positions of similar cations indistinguishable Simulated ND pattern Simulated XRD pattern .. Maik Lang – University of Tennessee 18

Limitation of Diffraction Experiments Diffraction experiments: ⊳ access to long-range structure of crystalline materials ⊳ no information of medium-range and short-range order  no structural information from amorphous solids (e.g., wasteglass) ⊳ diffuse scattering discarded during structural refinement  local defect structure and disorder inaccessible .. PDF: short range Total Scattering: long range Fourier Transform 𝑅 𝑛𝑏𝑦 𝐻 𝑠 = 2 𝜌 න 𝑅 𝑇 𝑅 − 1 sin 𝑅𝑠 𝑒𝑅 𝑅 𝑛𝑗𝑜 Maik Lang – University of Tennessee 19

Neutron Total Scattering Experiments at ORNL Spallation Neutron Source Large Ion Accelerator Facility structural characterization simulation of radiation effects maximization of irradiated sample mass minimization of required sample mass intense neutron beam (10 8 cm -2 ·sec -1 ) swift heavy ions (large range) ~100 mg  investigation of radiation effects by neutron total scattering Maik Lang – University of Tennessee 20

Neutron Total Scattering Experiments at ORNL The Nanoscale-Ordered Materials Diffractometer (NOMAD) NOMAD detector  neutron wavelength: 0.1 – 3 Å 10 8 cm -2 ∙ sec -1  flux on sample:  large detector coverage  high-resolution pair distribution function (PDF)  defects and local disorder  sample mass: 100 mg Maik Lang – University of Tennessee 21

Pair Distribution Function (PDF) Analysis UO 2 ⊳ more intuitive real- space representation ⊳ pairwise interatomic distances 2.7 Å O O U ⊳ position = interatomic distance O 2.4 Å ⊳ intensity ∝ coordination number 3.9 Å U U ⊳ width = spread in interatomic distances Maik Lang – University of Tennessee 22

Neutron Diffraction: Order – Disorder Transformations A 2 Zr 2 O 7 disordered fluorite Ho 2 Zr 2 O 7 order-disorder transformation  antisite defects (cations) Ho 2 Ti 2 O 7  randomization of oxygen vacancies A 2 Ti 2 O 7 ordered pyrochlore Maik Lang – University of Tennessee 23

Neutron PDF: Order – Disorder Transformations Ho 2 Zr 2 O 7 (weberite) Ho 2 Ti 2 O 7 (pyrochlore) Ho 2 Zr 2 O 7 (fluorite) Fd-3m Fm-3m Ccmm J. Shamblin, et al ., Nature Materials, 15, 507-511 (2016). superstructure superstructure Maik Lang – University of Tennessee 24