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 meetingNovember 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

Neu

tron

Flu

x

1014

1015

1016

1017

1018

1019

1020

10-7 10-6 10-5 10-4

Neutron flux vs. time at the final optic (n/cm2-s)

Neutron flux (n/cm2-s)

Neutron flux (n/cm2-s)

Time (s)

Neutron fluxes in the Sombrero reactor

108

109

1010

1011

10-4 10-3 10-2 10-1 100 101 102

Neutron fluence vs. energy at the final optic

Neutron fluence (n/cm2)

Neutron fluence (n/cm2)

Energy (meV)

Neu

tron

Flu

ence

1017

1018

1019

1020

1021

1022

10-7 10-6 10-5 10-4

Neutron flux vs. time at the chamber wall

flux (n/cm2-s)

Neutron flux (n/cm2-s)

Time (seconds)

Neu

tron

Flu

x

1010

1011

1012

1013

10-4 10-3 10-2 10-1 100 101 102

Neutron fluence vs. energy at the first wall

Neutron fluence (n/cm2)Neutron fluence (n/cm2)

Energy (MeV)

Neu

tron

Flu

ence

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,

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.

Melt-Quench Sequence for Fused Silica Initial Condition

-cristobalite Fused Silica

6000K(25psec)

7000K(25psec)

300K(25psec)

1000K(25psec)

Model Exp.Si-O-Si Peak 158° 144°-156°

Si-O-Si FWHM 32° 38°O-Si-O Peak 109° 109°

O-Si-O FWHM 12° 14°

1000K Increments25 psec each

increment

Bond AngleNeutron 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 silica3. 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

CascadeOverlap

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

14.3

nm

Replacement

Oxygen Deficient Center

0.08 ps 1.45 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

14.3

nm

0.06 ps 0.78 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.28

.6 n

m

2.67 ps0.10 ps 0.16 ps

Displacements from 5 keV PKA in Fused Silica

2.67 ps 2.67 ps

Displaced atoms are mostly oxygen.

Red segments are Si DisplacementsBlue 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

Replacements Oxygen 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

2nd Cascade

1st 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

Initial After PKA After 2nd

PKAAfter 600K

Annealfor 15 ps

2 keV PKA 8 15(+7)

17(+9, +2 after

1st PKA)

16(+8/+1 after

1st PKA)5 keV PKA 16 35

(+19)48

(+32, +13after 1st PKA)

31(+15/-4 after

1st PKA)

Number of ODCs produced by single and multiple recoils and after annealing

0.07 ps 0.20 ps 1.36 ps

2 keV Recoil in Fused Silica (0.4% OH Content)

ODC

NBOHC

ReplacementCascade 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, H2O) 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 potentialBrenner’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

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 Formationand 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 antisitesAmorphization is heterogeneous

SiC

Radiation induced amorphization in SiCA. Romano, S. Yip and Ju Li (MIT) and M. J. Caturla and B. D. Wirth (LLNL)

12.50 % Si FPs 25 % Si FPs

12.5 % Si FPs 25 % Si FPsNo antisites

50% antisites (W.J. Weber, Nucl. Inst. Meth. Phys. B166-167 (2000),98)

0.0

0.2

0.4

0.6

0.8

1.0

50% Antisites

Defected Fraction, f

d

Fraction of Si Interstitials0.00 0.05 0.10 0.15 0.20 0.25

1 - exp[-k(x/Φ)m]

NoAntisites

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