Radiation damage in ceramic wasteforms

• Main Author: Archer, Adam
• Published: University of Bristol 2015
• Subjects: 621.48
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.702125

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.