Atomistic Simulations of Damage in Silica Glass and Graphite Due to Irradiation Alison Kubota 1, Maria-Jose Caturla 1, Tomas Diaz de la Rubia 1, Stephen A. Payne 2, Susana Reyes 3, Jeff Latkowski 4 1 CMS, 2 LS&T, 3 PAT, 4 Eng., Lawrence Livermore National Laboratory Laser IFE meeting November 13-14, 2001

Introduction 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. 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

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

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

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 Damage in Silica Glass: issues 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

Introduction to Molecular Dynamics Modeling Molecular Dynamics for processes far-from-equilibrium, with atomic- scale detail. MD involves the integration of Newton’s Equation, dx i 2 /dt 2 = -  i V(r 1,…,r n ) with V(r 1,…,r n ) taken as modified Born-Mayer-Huggins potentials of Garofalini for Si-O systems, V 2 ij = A ij exp(-r ij /  ij ) + Z i Z j /r ij erfc(r ij /  ij ) + Splined Universal Potential (For High Energy Interactions) V 3 ijk = 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.

Melt-Quench Sequence for Fused Silica Initial Condition  -cristobalite Fused Silica 6000K (25psec) 7000K (25psec) 300K (25psec) 1000K (25psec) 1000K Increments 25 psec each increment Bond Angle Neutron structure factor From Feuston and Garofalini Our model reproduces the structure of fused silica

Objectives of the MD simulations in Silica 1.Compute number of Oxygen Deficient Centers produced by recoils with energies on the order of keV 2.Understand mechanisms of defect production in silica 3.Defect evolution at high temperatures: how does defect annealing occur? 4.Study radiation at high doses: compute number of defects under cascade overlap Compare with experimental observation of radiation and annealing in silica

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

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. 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. Primary Knock-On Atom 14.3nm Replacement Oxygen Deficient Center 0.08 ps1.45 ps

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. Primary Knock-On Atom 14.3nm 0.06 ps0.78 ps TRIM2000 estimates the maximum cascade extent to be ~16nm.

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 nm 2.67 ps0.10 ps0.16 ps

Displacements from 5 keV PKA in Fused Silica 2.67 ps Displaced atoms are mostly oxygen. 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.

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

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

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

0.07 ps0.20 ps1.36 ps 2 keV Recoil in Fused Silica (0.4% OH Content) ODC NBOHC Replacement Cascade Track 2 keV 5 eV 200 eV 20 eV Structural Defects During the cascade, ODC and NBO defects are produced along the cascade tracks. Most of the structural defects recombine and change partners. The remaining residual defects are precursors to electronic defects. We are starting to study damage in the presence of OH Replacements NBO Defects 2 keV PKA in Fused Silica ODC Defects

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, H 2 O) on defect formation and transport. Understand the effectiveness of cascade overlap on defect annihilation in fused silica. Direction of the Model Development for Optics Damage

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

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

Interatomic potential Brenner’s Reactive Bond-Order Formalism Multibody Bond-Order Potential to model C/H and C/H/O systems. Stabilizes sp 2 and sp 3 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)

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

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

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

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