Title:
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Radiation damage in ceramic wasteforms
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As nuclear waste is being considered for geological deposition, a safe and durable method for
sequestering it from the environment is needed. The radiation tolerance of various ceramic
compounds was analysed using alpha-decay event molecular dynamics simulations. As the methods
traditionally used for damage identification tend to over-predict the amount of damage under nonequilibrium
conditions, a method novel for this type of simulation was developed. Using spherical
harmonics, the Steinhardt method uses the angles between atoms to build up a fingerprint of the local
structure which is translationally and rotationally invariant. This can then be compared to reference
structures in order to quantify the amount of each structure type within the simulation cell. Both single
and multiple decay event simulations were used to probe the pyrochlore series Gd₂(TixZr₁-x)₂0₇,where
05 x 51, in order to investigate the underlying mechanisms of damage and healing as well as how
these mechanisms change with the varying of x. The single decay event simulations outlined the
importance of cascade overlap in the accumulation of damage and so high pressure multiple decay
event simulations were run. Changes in volume and total energy were compared to structural data
from the Steinhardt order parameters in order to gain insight into the atomistic mechanisms
responsible for damage and healing. Atmospheric pressure simulations were also run and structural
changes consistent with ion bombardment experiments were observed. The results showed that the
radiation tolerance of the solid solution increases with increasing Zr content due to the enhanced
healing that high ion mobility brings with it. Proposed models for amorphisation kinetics were tested
against the structural data from these simulations and a new model was also suggested.
Multiple alpha decay simulations were also run on PU02 in order to investigate its tolerance
of alpha radiation. The local structure surrounding each atom was analysed using Steinhardt order
parameters in order to quantify the damage caused by the alpha decay events. The results of these
simulations show that like Gd₂Zr₂0₇, PU0₂ is extremely tolerant of alpha radiation due to its high ion
mobility.
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