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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
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Alison Kubota1, Maria-Jose Caturla1,
Tomas Diaz de la Rubia1,
Stephen A. Payne2, Susana Reyes3,
1 CMS, 2 LS&T, 3 PAT, 4 Eng., Lawrence Livermore National Laboratory
Laser IFE meeting
November 13-14, 2001
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.
Neutron fluxes in the Sombrero reactor
We need to understand the effect of these fluxes in materials properties
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
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
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.
25 psec each
Neutron structure factor
From Feuston and Garofalini
Our model reproduces the structure of fused silica
Compare with experimental observation of radiation and annealing in silica
Is there recovery?
Cascade tracks shown with color corresponding to particle energy. Replacements are those 4-fold coordinated Si whose O neighbors have changed.
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.
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
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.
Displacements from 5 keV PKA in Fused Silica
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.
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
Primary Knock-On Atom
Effect of cascade overlap
Multiple cascades show that the number of defects does not increase linearly with additional overlapped cascades.
Number of ODCs produced by single and multiple recoils and after annealing
More cascade events and longer annealing times are necessary to improve statistics
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
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.
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.
Tritium Diffusion and Tritium Retention
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
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)
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)
and Behavior of Defects Produced during
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
Calculations of defect structures and energetics will have to be validated with first principles calculations and compared to previous models
A. Romano, S. Yip and Ju Li (MIT) and M. J. Caturla and B. D. Wirth (LLNL)
12.5 % Si FPs
25 % Si FPs
(W.J. Weber, Nucl. Inst. Meth. Phys.B166-167 (2000),98)
25 % Si FPs
12.50 % Si FPs
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