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Microstructural and morphological aspects of plutonium hydride

Brierley, Martin

[Thesis]. Manchester, UK: The University of Manchester; 2016.

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Abstract

Plutonium is a hazardous radioactive material; the α-particles that are emitted are particularly damaging to health should contamination be inhaled or ingested into the body. During long term storage a number of conditions have been observed which can cause plutonium to corrode, which liberates particles from the surface. It is imperative to understand the processes involved in the corrosion of plutonium during long term storage to predict the likely state that metallic pieces may be found should subsequent handling be required. The growth mechanisms of plutonium hydride beyond the nucleation stage are not well understood. Detailed characterisation of the microstructural features associated with hydride reaction sites is required to develop a mechanistic understanding of the growth stage of hydrogen corrosion.Suitable processes and analysis methods were developed using cerium as an analogous material to δ-plutonium; during this stage, the knowledge of the corrosion of cerium by hydrogen was significantly improved using in situ gas dosing equipment, metallographic preparation, light microscopy, scanning electron microscopy (SEM), ion milling, secondary ion mass spectrometry (SIMS), atomic force microscopy (AFM) and vacuum nanoindentation. SEM and ion milling methods developed on cerium were subsequently used on the analysis on pre-formed and passivated hydride reaction sites on δ-plutonium .In situ exposure of electro-refined plutonium and a Pu 0.3 wt% Ga alloy were investigated without prior exposure to oxygen, revealing the as-formed microstructure of the hydride reaction product to be analysed. Subsequent metallographic preparation was used to confirm findings from the in situ analysis.The highest resolution analysis of the hydride product formed on cerium, delta plutonium and electro-refined plutonium has been obtained to date. Hydride reaction sites formed on cerium and δ-Pu were observed to be oblate, confirming growth anisotropy. A mechanism for the anisotropic growth was proposed where the stress fields introduced into the metal surrounding a lower density hydride play a significant role in further development of a hydride reaction site, causing failure of the surface oxide diffusion barrier surrounding a hydride reaction site.