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Title: Simulating radiation damage in plutonium
Authors: Robinson, Marc
Issue Date: 2010
Publisher: © Marc Robinson
Abstract: Modelling radiation effects has become an important area of research due to the continual advances in nuclear technology. Plutonium, as a radioactive source, undergoes continual atomic level radiation events which over time change the mechanical properties and structural integrity of the material. Studying how damage accumulates and evolves is key to gaining an understanding of the ageing processes and the radiation tolerance of plutonium. This thesis presents findings based upon computer simulation using classical molecular dynamics (MD). This, combined with long time scale dynamics (LTSD) techniques enable low level investigations of collision cascades and studies of point defect diffusion. Using the Modified Embedded Atom Method (MEAM) to govern atomic interactions, pure δ-Pu, Ga stabilised δ-Pu and Pu-Ga-He systems were studied. The ballistic stages of the radiation event were found to be similar for both pure Pu and Ga stabilised Pu, with low impact energies producing a cascade structure comparable to other fcc metals. Isolated defects tended to be of type Pu, with displaced Ga atoms returning to lattice sites. A clear observation at high impact energies was the ease at which both Pu and Ga stabilised Pu amorphise, resulting in an increased recovery time. Over long time scales, calculations suggest dynamics will be dominated by interstitial diffusion, with vacancies more likely to migrate as clusters. Defect diffusion mechanisms were observed to avoid Ga-rich regions, with Ga atoms relatively immobile in the Pu matrix. Results suggest the inclusion of Ga does not effect defect production but strongly influences defect diffusion. Investigating the presence of He in Ga stabilised δ-Pu suggested He would tend to reside in a substitutional site and is then rendered immobile. Local vacancies induced He migration to a certain degree yet the mechanism for He diffusion which may lead to bubble growth is unclear. Calculations regarding a range of He bubble configurations indicated a 1:1 He:vacancy ratio concentration was favourable relative to isolated substitutional He atoms. Results also suggested there exists an additional stable bubble configuration of around 2:1 He:vacancy ratio with a diameter of more than 1.2 nm. Studying the interaction of radiation damage with a local He bubble indicated that cascade structure can be greatly effected, resulting in a significant increase in the number of residual defects.
Description: This thesis is confidential. A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University.
URI: https://dspace.lboro.ac.uk/2134/6254
Appears in Collections:Closed Access PhD Theses (Maths)

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