Molecular simulation on radiation behavior of Li2OTakuji Oda, Yasuhisa Oya, Satoru TanakaDepartment of Quantum Engineering & Systems Science The University of Tokyo

BackgroundTo 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 (Li2O, Li2TiO3 etc), radiation defects created in the severe radiation conditions affect the tritium behavior strongly.Hence, behaviors of tritium and defects in Li2O have been extensively studied. However, . The evaluated tritium diffusivities are scattered. The concrete influence of each defect is not understood sufficiently. 6Li + n 4He (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.

SubjectsT+bulkVLi T+(LiOT)nnLi(1)(2)Li+(4)OFig. 1. Tritium in Li2O(3)T-F VLi (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 (MD simulation) surf.

Experimental ; FT-IR with an ion gun OD stretching vibrations shows multiple peaks by interaction with a specific defect.SampleLi2O s.c.10mm, 1mmThe behaviors of hydrogen isotopes in various chemical states can be analyzed individually.Fig.2. IR absorption experimental system

Calculation details-1 ; plane-wave pseudopotential DFT2x2x2Conventional cell (Li8O4) 2x2x2 supercell (Li64O32)Software: CASTEP codeFunctional: PBEK-point grid: 3x3x3Energy cutoff: 380 eVCalculation 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 cutoff)q1q2/r + Aexp(-r/) - C/r6< Buckingham pair potential model>Fig. 2. Inter-ionic potential (Li-O)Software: DL-POLY System: 5x5x5 or 7x7x7 supercell (Li1000O500 or Li2744O1372)Ensemble: NpT or NEV Time step: 1 fs or variable stepSimulation time: ~5 ns or ~4 psIn 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 .

SubjectsT+bulkVLi T+(LiOT)nnLi(1)(2)Li+(4)OFig. 1. Tritium in Li2O(3)T-F VLi (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 (MD simulation) surf.

(2) Interaction with Li vac. ; FT-IR during 3keV D2+ irradiation Fig.3. O-D peaks during 3keV D2+ irradiationFig.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 D2+ irr.increasedecreaseBy the heating, the 2605 cm-1 peak decreased, while the 2710 cm-1 peak increased.Fig.5. Variation in O-D peaks during heatingO-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)A: 1 isolated (LiO- - H+)

B: 2 isolated (LiO- - H+)C: (LiO- - H+)2Stabilization 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 F0 are modeled.Fig. 7. H+ neighboring F center in Li2O

(3) Interaction with F centers ; stability around F centers (DFT)

Fig. 8. Stability of H near F centerF centers trap H strongly, and reduce it to H-.

(4) Influence of the dynamic Frenkel defect ; what is the superionics in Li2O ? (MD)OLi1600 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.LiOVacant siteFig. 10. Li2O 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 coefficientsf: correlation factor >> ~ 0.653 in theoryd: distance in a jump >> ~0.25 nm along [Freq.]: vibration frequency >> ~ 3x1013 s-1 from MDEd: diffusion barrierNdefect / Natom: 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 Li2O ; 102.9 eV Li PKA along (MD)Movie 1. Li PKA along [110] (PKA energy: 102.9 eV, NEV with 0K initial temp.)

(1) Radiation behavior of Li2O ; threshold displacement energy (MD)LiOVacant Fig. 13. Li2O crystal[555][550][500][505]( 0 eV80 eV )O displacementLi displacement (left: vac., right: O)Fig. 14. Threshold displacement energiesAngle 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 Li2O ; 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 psFig. 16. Variation of the maximum energyThe 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.

SummaryT+bulkVLi T+(LiOT)nnLi(1)(2)Li+(4)OFig. 1. Tritium in Li2O(3)T-F VLi (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 (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: F0 > 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 Li2O(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 Li2O

AcknowledgementsWe 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 21st 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