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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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slide1

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

slide2

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.

slide3

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

slide4

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

slide5

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

slide6

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.

slide7

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

slide8

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

slide9

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?
slide10

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.

slide11

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.

slide12

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.

slide13

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.

slide14

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

slide15

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.

slide16

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

More cascade events and longer annealing times are necessary to improve statistics

slide17

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

slide18

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.

slide19

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
slide20

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

slide21

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)

slide22

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)

slide23

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

slide24

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
slide25

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