Molecular simulation on radiation behavior of Li 2 O Takuji Oda, Yasuhisa Oya, Satoru Tanaka...
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Molecular simulation Molecular simulation on radiation behavior on radiation behavior of Li of Li 2 2 O O Takuji Oda , Yasuhisa Oya, Satoru Tanaka Department of Quantum Engineering & Systems Science Department of Quantum Engineering & Systems Science The University of Tokyo The University of Tokyo
Molecular simulation on radiation behavior of Li 2 O Takuji Oda, Yasuhisa Oya, Satoru Tanaka Department of Quantum Engineering & Systems Science The University
Molecular simulation on radiation behavior of Li 2 O Takuji
Oda, Yasuhisa Oya, Satoru Tanaka Department of Quantum Engineering
& Systems Science The University of Tokyo
Slide 2
Background To establish a secure and efficient fuel cycle in a
fusion reactor, produced tritium must be recovered rapidly from the
breeding blanket. In the case of a solid breeding material (Li 2 O,
Li 2 TiO 3 etc), radiation defects created in the severe radiation
conditions affect the tritium behavior strongly. Hence, behaviors
of tritium and defects in Li 2 O have been extensively studied.
However, . The evaluated tritium diffusivities are scattered. The
concrete influence of each defect is not understood sufficiently. 6
Li + n 4 He (2.1 MeV) + T (2.7 MeV) Our aim is to model the
hydrogen isotope behavior precisely, based on the atomic-scale
understandings on the radiation effect.
Slide 3
Subjects T+T+ bulk V Li T+T+ (LiOT) n n Li (1) (2) Li + (4) O
Fig. 1. Tritium in Li 2 O (3) T-T- F V Li (1) Radiation behavior
(MD simulation) (2) Interaction with Li vac. (FT-IR exp. & DFT
calculation) (3) Interaction with F centers (DFT calculation) (4)
Influence of the dynamic Frenkel defects Frenkel defects (MD
simulation) surf.
Slide 4
Experimental ; FT-IR with an ion gun OD stretching vibrations
shows multiple peaks by interaction with a specific defect. Sample
Li 2 O s.c. 10mm, 1mm The behaviors of hydrogen isotopes in various
chemical states can be analyzed individually. Fig.2. IR absorption
experimental system
Slide 5
Calculation details-1 ; plane-wave pseudopotential DFT 2x2x2 Li
: O : Conventional cell (Li 8 O 4 ) 2x2x2 supercell (Li 64 O 32 )
Software: CASTEP code Functional: PBE K-point grid: 3x3x3 Energy
cutoff: 380 eV Calculation cost was reduced by use of plane-wave
basis and pseudopotential technique (O 1s).
Slide 6
Calculation details-2 ; classical molecular dynamics (MD) (i)
Coulombic interaction (ii) Short range interaction (10 cutoff) q 1
q 2 /r + A exp(-r/) - C/r 6 Fig. 2. Inter-ionic potential (Li-O)
Software: DL-POLY System: 5x5x5 or 7x7x7 supercell (Li 1000 O 500
or Li 2744 O 1372 ) Ensemble: NpT or NEV Time step: 1 fs or
variable step Simulation time: ~5 ns or ~4 ps In the classical MD,
electrons are not described explicitly. As a result, the
calculation cost is enough reduced to perform the dynamics
simulation. In the case of radiation simulation, the Buckingham
potential was connected to the ZBL potential by polynomial at
around 0.6-1 .
Slide 7
Subjects T+T+ bulk V Li T+T+ (LiOT) n n Li (1) (2) Li + (4) O
Fig. 1. Tritium in Li 2 O (3) T-T- F V Li (1) Radiation behavior
(MD simulation) (2) Interaction with Li vac. (FT-IR exp. & DFT
calculation) (3) Interaction with F centers (DFT calculation) (4)
Influence of the dynamic Frenkel defects Frenkel defects (MD
simulation) surf.
Slide 8
(2) Interaction with Li vac. ; FT-IR during 3keV D 2 +
irradiation Fig.3. O-D peaks during 3keV D 2 + irradiation Fig.4.
Intensity variation of each peak O-D is stabilized in the bulk by
interaction with a defect (2605 cm - 1 ) or by mutual aggregation
(LiOD phase: 2710 cm -1 ) 2710 cm -1 is LiOD phase. 2660 cm -1 is
mainly the surface O-D. 2605 cm -1 is not attributed.. [Low
fluence] Only the surface O-D. [High] The LiOD phase becomes
dominant. What is the defect ??
Slide 9
(2) Interaction with Li vac. ; FT-IR during heating after the D
2 + irr. increase decrease By the heating, the 2605 cm -1 peak
decreased, while the 2710 cm -1 peak increased. Fig.5. Variation in
O-D peaks during heating O-D aggregated each other: (LiO - -D + ) n
[2605 cm -1 ] LiOD phase [2710 cm -1 ] By the aggregation, (LiO -
-D + ) can be really stabilized ??
Slide 10
(2) Interaction with Li vac. ; stabilization by aggregation
(DFT) Li : O : H : A: 1 isolated (LiO - - H + ) B: 2 isolated (LiO
- - H + ) C: (LiO - - H + ) 2 Stabilization by aggregation is
confirmed ! Fig.6. Electronic density
Slide 11
(3) Interaction with F centers ; locally stable positions near
F centers (DFT) Li:, O:, H:, F centers: *By controlling the system
charge, O vac., F +, and F 0 are modeled. Fig. 7. H + neighboring F
center in Li 2 O
Slide 12
(3) Interaction with F centers ; stability around F centers
(DFT) Fig. 8. Stability of H near F center F centers trap H
strongly, and reduce it to H -.
Slide 13
(4) Influence of the dynamic Frenkel defect ; what is the
superionics in Li 2 O ? (MD) O Li 1600 K (superionics) 2600 K
(liquid) 1000 K (solid) Fig. 9. Projected ionic densities on (100)
plane Just Li behaves like liquid even below the melting point
>> the superionics. Most Li migrates along [100] (~90%),
assisted by the dynamic Frenkel defects. Li O Vacant site Fig. 10.
Li 2 O crystal
Slide 14
(4) Influence of the dynamic Frenkel defect ; what is the
dynamic Frenkel defect ? (MD) (a) Extrinsic region (by a Li
vacancy) >> 0.25 eV (b) Below the critical temp. (by the
dynamic defect)>> 1.9 eV (c) Above the critical temp.
>> 0.62 eV (d) Liquid state >> 0.40 eV Fig. 11.
Variation of Li diffusion coefficients f: correlation factor
>> ~ 0.653 in theory d: distance in a jump >> ~0.25 nm
along [Freq.]: vibration frequency >> ~ 3x10 13 s -1 from MD
E d : diffusion barrier N defect / N atom : defect density
Slide 15
(4) Influence of the dynamic Frenkel defect ; the dynamic
defect, a defect cluster, etc (MD) Fig. 12. Contribution of the
dynamic Frenkel defect to Li diffusivities Even in the highly
Li-burnup conditions, the contribution of the dynamic Frenkel
defect in the Li diffusivity reaches 50 % above 1200 K. The dynamic
defects may also affect T + behavior, due to the similarity. The
participation of the dynamic defect is significant above 1200
K.
Slide 16
(1) Radiation behavior of Li 2 O ; 102.9 eV Li PKA along (MD)
Movie 1. Li PKA along [110] (PKA energy: 102.9 eV, NEV with 0K
initial temp.)
Slide 17
(1) Radiation behavior of Li 2 O ; threshold displacement
energy (MD) Li O Vacant Fig. 13. Li 2 O crystal [555] [550] [500]
[505] ( 0 eV80 eV ) O displacement Li displacement (left: vac.,
right: O) Fig. 14. Threshold displacement energies Angle dependence
of the threshold displacement energy was obtained: angular
resolution of 6x6=36 for each under NEV ensemble (0 K initial
temp.) O requires much more high energy for displacement than Li.
The threshold energy can be ordered as [111] [110] [100].
Slide 18
(1) Radiation behavior of Li 2 O ; key points for the modeling
(MD) Number of stable defects are sensitively dependent on the PKA
energy. (due to the self-annealing effect, etc) Fig. 15. Number of
Li vac. survived after 4 ps Fig. 16. Variation of the maximum
energy The threshold energy is not enough to describe the radiation
event. The PKA energy is immediately spread into the system. This
behavior could be related to the self-annealing effect, the
radiation induced diffusion, etc.
Slide 19
Summary T+T+ bulk V Li T+T+ (LiOT) n n Li (1) (2) Li + (4) O
Fig. 1. Tritium in Li 2 O (3) T-T- F V Li (2) Interaction with Li
vac. (FT-IR exp. & DFT calculation) (3) Interaction with F
centers (DFT calculation) surf. Li vac. heightens the stability of
T + (formation of subs. T + ). (LiO - - T + ) becomes more stable
by aggregation. (4) Influence of the dynamic Frenkel defects
Frenkel defects (MD simulation) (1) Radiation behavior (MD
simulation) F centers trap T + strongly and reduce it to T -.
Capturing force depends on the charge state of F centers: F 0 >
F + > O vac. The dynamic defect assists Li diffusion strongly,
over 1200 K.. The dynamic defect may also affect T + behavior. O
requires much higher energy for displacement than Li. The threshold
energy: [111] [110] [100]. The PKA energy is rapidly spread into
the system.
Slide 20
Future works (1) Radiation behavior How about electron
excitation >> ?? How about model dependences >>
checking by other models (2) Interaction with Li vac. How to
aggregate each other >> classical MD >> modeling T + in
Li 2 O (3) Interaction with F centers How to detrap >>
ab-initio MD >> FT-IR & UV absorption experiment (4) The
dynamic Frenkel defect How to interact with T + >> classical
MD >> modeling T + in Li 2 O
Slide 21
Acknowledgements We are very grateful to Dr. R. Devanathan, Dr.
F. Gao, Dr. W.J. Weber and Dr. L.R. Corrales for help and support
during the present research. This research was performed in part
using the MSCF in EMSL, a national scientific user facility
sponsored by the U.S. DOE, OBER and located at PNNL. We are also
grateful to for financial support on the present research. the 21
st Century COE Program, Mechanical Systems Innovation, by the
Ministry of Education, Culture, Sports, Science and Technology the
Tokyo Denryoku Zaidan the Atomic Energy Society of Japan