MINES ParisTech CAS - Centre automatique et systèmes


Séance du jeudi 19 décembre 2013, 14h. La salle sera précisée plus tard
Thibault FANEY, University of California Berkeley - Nuclear Engineering Department

In fusion reactors, plasma facing components (PFC), and in particular,
the divertor will be irradiated with high fluxes (up to 1023 m-2s-1) of
low energy (~ 100 eV) helium and hydrogen ions. Tungsten is one of the
leading candidate materials for the divertor. However, the behaviour of
Tungsten under high doses of helium exposure remains to be fully
determined. Computational materials modeling has been used to
investigate the mechanisms controlling microstructural evolution in
Tungsten following high dose, high temperature helium exposure.
The aim of this talk is to present a framework to understand and predict
defect production and diffusion, clustering and interaction close to the
inner surface of the divertor due to high flux, low energy helium
irradiation. The framework presented is based on a multi-scale approach:
we present a spatially-dependent Cluster Dynamics (CD) model based on
reaction-diffusion rate theory to describe the evolution in space and
time of helium and its complexes. The processes modeled originate from
both Molecular Dynamics (MD) observations at the atomistic scale and
experimental results.

The use of cluster dynamics modeling suffers from physical as well as
computational challenges: physical challenges due to the large number of
physical quantities (diffusion coefficients and binding energies) to
determine for each chemical species, and the applied mathematics and
computational challenges due to the large number of species that are
modeled. The CD model is parameterized using MD simulation results, and
a solver was developed to efficiently deal with the large system of
non-linear partial differential equations describing the microstructure
evolution. The solver uses an algorithm designed to exploit the local
structure of the chemical reactions and makes use of parallel computing
to both speed-up computations and reduce the memory requirements per

We compare the model with results from molecular dynamics simulations of
helium cluster formation and evolution below the surface, results from
experiments performed using thermal desorption spectroscopy, and results
from experiments under fusion relevant conditions.