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

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.

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.

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. φ10mm, 1mm The behaviors of hydrogen isotopes in various chemical states can be analyzed individually. Fig.2. IR absorption experimental system

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).

Calculation details-2 ; classical molecular dynamics (MD) (i) Coulombic interaction (ii) Short range interaction (10 Å cutoff) q 1 q 2 /r + A × exp(-r/ρ) - 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 Å.

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.

(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” ??

(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 ??

(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

(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

(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 -.

(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

(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 >> ~ 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

(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.

(1) Radiation behavior of Li 2 O ; eV Li PKA along (MD) Movie 1. Li PKA along [110] (PKA energy: eV, NEV with 0K initial temp.)

(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].

(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.

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.

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”

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