Atomistic Simulations of Damage in Silica Glass and Graphite Due to Irradiation

1. Atomistic Simulations of Damage in Silica Glass and Graphite Due to Irradiation Alison Kubota1, Maria-Jose Caturla1, Tomas Diaz de la Rubia1, Stephen A. Payne2, Susana Reyes3, Jeff Latkowski4 1 CMS, 2 LS&T, 3 PAT, 4 Eng., Lawrence Livermore National Laboratory Laser IFE meeting November 13-14, 2001

2. Introduction High neutron fluxes will reach both the first wall and the optics in a fusion reactor The damage produced by this radiation will change the mechanical, thermal and optical properties of these materials The purpose of this work is to understand the detailed atomistic mechanism of neutron irradiation damage and annealing in fused silica and graphite through atomistic simulations guided by experiments.

3. Neutron Fluence Neutron Fluence Neutron Flux Neutron fluxes in the Sombrero reactor Chamber wall Optics Neutron Flux We need to understand the effect of these fluxes in materials properties

4. Modeling Approach Molecular dynamics used to understand damage by recoils produced by neutron irradiation This approach has been successfully and widely used to study radiation damage in metals However, atomistic models of radiation damage in silica and graphite are very limited

5. Damage in Silica Glass: issues Neutron irradiation can induce obscuration of the optics through color centers Spectroscopic observations show increase in defect densities (NBOHC, ODC, E’) with MeV neutron irradiation. These defect concentrations are shown to decrease with annealing, though the annealing mechanism is not well understood. There are some suggestions that cascade overlap can also contribute to reduced defect densities Induced optical absorption in silica glasses from neutron and gamma irradiation Absorption spectra during annealing at 350°C C. D. Marshall, J. A. Speth, S. A. Payne, Non-Crystalline Solids, 212 (1997) 59

6. Introduction to Molecular Dynamics Modeling Molecular Dynamics for processes far-from-equilibrium, with atomic-scale detail. MD involves the integration of Newton’s Equation, dxi2/dt2 = -iV(r1,…,rn) with V(r1,…,rn) taken as modified Born-Mayer-Huggins potentials of Garofalini for Si-O systems, V2ij = Aij exp(-rij/ij) + ZiZj/rij erfc(rij/ij) + Splined Universal Potential (For High Energy Interactions) V3ijk = Si-O-Si and O-Si-O Bond-Angle-Dependent Term The Garofalini Potentials have been used in numerous studies examining the bulk, surface and interfacial properties of fused silica. Simulations run with MDCASK LLNL software on a 1024-processor IBM SP2 and a 512-processor Compaq cluster.

7. Melt-Quench Sequence for Fused Silica Initial Condition 7000K (25psec) 300K (25psec) 6000K (25psec) 1000K Increments 25 psec each increment 1000K (25psec) (100) b-cristobalite Fused Silica Neutron structure factor Bond Angle From Feuston and Garofalini Our model reproduces the structure of fused silica

8. Objectives of the MD simulations in Silica • Compute number of Oxygen Deficient Centers produced by recoils with energies on the order of keV • Understand mechanisms of defect production in silica • Defect evolution at high temperatures: how does defect annealing occur? • Study radiation at high doses: compute number of defects under cascade overlap Compare with experimental observation of radiation and annealing in silica

9. Undamaged Fused Silica Damaged Fused Silica Annealing (600K) Cascade Overlap Simulation Procedure Cascade Simulation Is there recovery? Cascade Simulation Temperature Bath • Questions: • What is the mechanism for defect annealing? • Is there recovery due to cascade overlap?

10. 1 keV PKA in Fused Silica Cascade tracks shown with color corresponding to particle energy. Replacements are those 4-fold coordinated Si whose O neighbors have changed. 0.08 ps 1.45 ps Primary Knock-On Atom Replacement 14.3nm Oxygen Deficient Center During the cascade, ODC defects are formed along the cascade tracks Many (not all) of the defects are annihilated after the full evolution of the cascade.

11. 2 keV PKA in Fused Silica Cascade tracks shown with color corresponding to particle energy. Oxygen deficient center (ODC) defects shown as red, while replacements are shown blue. 0.06 ps 0.78 ps Primary Knock-On Atom 14.3nm TRIM2000 estimates the maximum cascade extent to be ~16nm.

12. 0.10 ps 0.16 ps 2.67 ps 28.6 nm 5 keV PKA in Fused Silica Large production of ODC defects produced along the cascade tracks during the cascade. Residual defects observed after the cascade. TRIM2000 estimates the maximum cascade extent to be ~30nm.

13. 2.67 ps Displacements from 5 keV PKA in Fused Silica 2.67 ps Red segments are Si Displacements Blue segments are O Displacements Displaced atoms are those whose position has moved further than 2Å from its initial position. Displaced atoms are mostly oxygen.

14. Replacements Oxygen Deficient Centers ( a ) 1 keV PKA ( b ) 2 keV PKA ( c ) 5 keV PKA Replacements vs. ODCs The defects produced during the cascade are accommodated back into the network through replacements

15. 2 keV PKA in Fused Silica: Damage Overlap Primary Knock-On Atom 2nd Cascade 1st Cascade Effect of cascade overlap Replacements ODC Defects Multiple cascades show that the number of defects does not increase linearly with additional overlapped cascades.

16. Number of ODCs produced by single and multiple recoils and after annealing More cascade events and longer annealing times are necessary to improve statistics

17. 0.07 ps 0.20 ps 1.36 ps Cascade Track Structural Defects Replacement 2 keV ODC 200 eV NBOHC 20 eV 5 eV We are starting to study damage in the presence of OH 2 keV Recoil in Fused Silica (0.4% OH Content) 2 keV PKA in Fused Silica Replacements During the cascade, ODC and NBO defects are produced along the cascade tracks. NBO Defects Most of the structural defects recombine and change partners. The remaining residual defects are precursors to electronic defects. ODC Defects

18. Direction of the Model Development for Optics Damage Self-healing properties demonstrated in simulations at very short time scales Determine the detailed mechanism of self-healing, such as defect transport models, ring contraction models, and viscous flow models. Examine the effect of hydrogen (OH, H2O) on defect formation and transport. Understand the effectiveness of cascade overlap on defect annihilation in fused silica.

19. Damage in Carbon materials (Graphite): issues • Defects produced by neutron irradiation can induce: • Dimensional Changes: swelling • Changes in Thermal Conductivity • Production of traps for Tritium

20. We are Modeling Radiation Damage in Graphite, Tritium Diffusion and Tritium Retention Simulation model Molecular dynamics simulations to study the defects produced during irradiation in graphite We have implemented a bond-order potential for Carbon-Hydrogen systems in our parallel molecular dynamics code. This is the most accurate empirical potential for Graphite to this date. Goal of the simulations Understand defect formation in graphite at the atomistic level and quantify number of defects with energy of recoils Understand Tritium diffusion in the presence of defects generated during irradiation Combine results of defect production with detailed neutron flux calculations at the first wall and understand the effects of pulse irradiation in final microstructure

21. Interatomic potential Brenner’s Reactive Bond-Order Formalism Multibody Bond-Order Potential to model C/H and C/H/O systems. Stabilizes sp2 and sp3 carbon based on local bonding environment. Used in studies of particle impacts with graphite (Beardmore and Smith, 1995) and polymers (Smith, 1996) O(n) scalable, comparable to Tersoff potential in complexity Parallel code for Bond-Order potentials implemented at LLNL (ASCI Blue, TC2K)

22. Modeling of Tritium Retention in Neutron-Irradiated Graphite requires of Diffusion Coefficients as input parameters Models to understand H/D/T inventories in graphite. Are the models and the fitted parameters reasonable? Taken from Haasz et al. (1995)

23. Atomistic Modeling Provides Details into the Formation and Behavior of Defects Produced during Neutron Irradiation Vacant sites Damage produced by a 200 eV C recoil along the c-direction in graphite Radiation produces vacant sites in the lattice that could act as trapping sites for Tritium Our calculations show a strong binding between a single vacancy and H ~ 3.8 eV Interstitials Calculations of defect structures and energetics will have to be validated with first principles calculations and compared to previous models

24. Radiation induced amorphization in SiC A. Romano, S. Yip and Ju Li (MIT) and M. J. Caturla and B. D. Wirth (LLNL) No antisites 12.5 % Si FPs 25 % Si FPs Si C 50% antisites (W.J. Weber, Nucl. Inst. Meth. Phys.B166-167 (2000),98) 25 % Si FPs 12.50 % Si FPs • MD simulations show that amorphization of SiC requires of the formation of antisites • Amorphization is heterogeneous

25. Direction of the Model Development for Damage in Graphite We have developed the computational capability to study radiation damage in C/H systems at the atomistic level with large scale MD simulations Compute number of defects produced in graphite during irradiation with energies of ~ keV Study the atomistic mechanisms for Tritium diffusion in graphite Study the binding of Tritium to different Vacancy complexes produced during irradiation The computed activation energies are input parameters for continuum models for defect diffusion The work was performed under the auspices of the U.S. Department of Energy by University of California Lawrence Livermore National Laboratory under contract No. W-7405-Eng-48