Abstract
A detailed understanding of the mechanisms and effects of radiation damage in phyllosilicate minerals is a necessary component of the evaluation of the safety case for a deep geological disposal facility (GDF) for radioactive waste. Structural and chemical changes induced by α-particle damage will affect the performance of these minerals as reactive barrier materials (both in the near and far-field) over time scales relevant to GDF integrity. In this study, two examples of chlorite group minerals have been irradiated at α-particle doses comparable to those predicted to be experienced by the clay buffer material surrounding high-level radioactive waste canisters. Crystallographic aberrations induced by the focused 4He2+ ion beam are revealed via high-resolution, microfocus X-ray diffraction mapping. Interlayer collapse by up to 0.5 A is prevalent across both macrocrystalline and microcrystalline samples, with the macrocrystalline specimen displaying a breakdown of the phyllosilicate structure into loosely ...
Highlights
Phyllosilicate minerals will have a key role in the safe geological isolation of nuclear waste[1,2] and their behavior under repository conditions must be fully understood
Two examples of chlorite group minerals were prepared for α-particle radiation damage analysis, differing in chemistry and crystal morphology
The effect of high fluence α-particle bombardment on chlorite minerals is the spectacular breakdown of the original structure and the generation of domains of new layered structure, in a manner similar to that observed in biotite
Summary
Phyllosilicate minerals will have a key role in the safe geological isolation of nuclear waste[1,2] and their behavior under repository conditions must be fully understood. Under the globally favored strategies to construct a deep geological disposal facility (GDF), bentonite (the collective term for naturally occurring clay consisting, dominantly, of montmorillonite (Na0.2Ca0.1Al2Si4O10(OH)2(H2O)10)) directly surrounding the waste canister will be one of the primary engineered “barriers” to limit radionuclide release into the environment.[3,4] In the far-field of a GDF, phyllosilicates (micas and clays) will be the most reactive components of the host rock, acting as “sinks” for radionuclides following eventual canister breakdown and release.[5] Chlorite (Mg,Fe,Al,Mn)6-. As a breakdown product of biotite mica, it is ubiquitous in the rock mass of altered granitic rocks and mineralized fractures,[6] as well as altered igneous rocks in general It will be present in mature iron-bearing mudstones and low-grade metamorphic terraines. A radiation damage investigation of chlorite is both highly relevant to far-field performance in many candidate rock types
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