ABSTRACT
IAEA-F4-TM-42581
IAEA Technical Meeting on Atomic, Molecular and Plasma Material Interaction Data for
Fusion Science and Technology
Influence of hydrogen-vacancy interaction on the mobility of
hydrogen and vacancy in bcc-metal
Deqiong Zhu, Takuji Oda
Department of Nuclear Engineering, Seoul National University, Seoul, Korea
E-mail address of main author: [email protected]
The retention of tritium in plasma facing component is a key issue related to the safety and economics
of fusion reactors. Understanding the interaction between hydrogen isotopes and irradiation defects is
indispensable to evaluate the tritium inventory in plasma facing component. In the present work, the
influence of hydrogen-vacancy interaction (H-V interaction) on the mobility of hydrogen and vacancy
in bcc-metal is studied by molecular statics (MS) and molecular dynamics (MD) simulations. α-iron is
chosen as the modeling material because (1) both α-iron and tungsten have bcc structures and similar
characteristics to hydrogen and (2) more reliable potential models are available for α-iron regarding the
interaction with hydrogen. In addition, as one of the constitutive component of structural component,
the study of hydrogen-vacancy interaction in α-iron itself attracts great interest. In simulations, a
reported Fe-H potential model [1] of the embedded atom method (EAM) is employed. MS and MD
simulations are conducted by using the LAMMPS code. The three-dimensional periodic boundary
conditions (PBC) are imposed to a supercell in order to model a bcc-Fe crystal.
The diffusivities of hydrogen and vacancy in the systems with and without vacancy are evaluated and
compared. It has been found that the mobility of hydrogen is clearly decreased due to the binding
interaction between hydrogen and vacancy when they co-exist in α-iron bulk, and that the decrease trend
is more significant at lower temperatures. On the other hand, the influence of H-V interaction on the
mobility of vacancy depends on the configuration of vacancies. The mobility is decreased for the isolated
vacancies, while it is increased for the vacancy cluster. No influence from the vacancy configuration on
the mobility of hydrogen is observed.
Furthermore, the diffusivities of hydrogen in systems with varied H/V ratios (V1-H1, V1-H3, V1-H6,
V1-H12, V8-H8) are checked. The results indicate that the diffusivity of hydrogen increases with the
H/V ratio increasing. A simple model to evaluate the effective diffusivity of hydrogen (Deff) in the system
with vacancy co-existing is proposed by summating the diffusivity of hydrogen atoms that are trapped
in vacancy (Dtrap) and the diffusivity of hydrogen atoms that are not trapped in vacancy (Dnon-trap),
𝐷𝑒𝑓𝑓 = 𝐷𝑡𝑟𝑎𝑝 × 𝑓𝑡𝑟𝑎𝑝 + 𝐷𝑛𝑜𝑛−𝑡𝑟𝑎𝑝 × 𝑓𝑛𝑜𝑛−𝑡𝑟𝑎𝑝
and the effect of H/V ratio on the diffusivity of hydrogen is discussed by exploring the overall fraction
of the trapped hydrogen atoms (ftrap) and the non-trapped hydrogen atoms (fnon-trap). In comparison
between hydrogen diffusivities given by the model equation and those determined directly with MD
simulation, it is revealed that the effective diffusivity of hydrogen is mainly contributed by the hydrogen
atoms that are not trapped in vacancy.
[1] Ashwin Ramasubramaniam, et al., Phys. Rev. B 79 (2009) 174101.
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