Investigating α-particle radiation damage in phyllosilicates using synchrotron microfocus-XRD/XAS: implications for geological disposal of nuclear waste

Abstract:

The response of mineral phases to the radiation fields that will be experienced in a geological disposal facility (GDF) for nuclear waste is poorly understood. Phyllosilicates are critical phases in a GDF with bentonite clay as the backfill of choice surrounding high level wastes in the engineered barrier, and clays and micas forming the most important reactive component of potential host rocks. It is essential that we understand changes in mineral properties and behaviour as a result of damage from both α and γ radiation over long timescales. Radiation damage has been demonstrated to affect the physical integrity and oxidation state¹ of minerals which will also influence their ability to react with radionuclides. Using the University of Manchester’s newly commissioned particle accelerator at the Dalton Cumbrian Facility, UK, model phyllosilicate minerals (e.g. biotite, chlorite) were irradiated with high energy (5MeV) alpha particles at controlled dose rates. This has been compared alongside radiation damage found in naturally formed ‘radiohalos’ – spherical areas of discolouration in minerals surrounding radioactive inclusions, resulting from alpha particle penetration, providing a natural analogue to study lattice damage under long term bombardment^1,2. Both natural and artificially irradiated samples have been analysed using microfocus X-ray absorption spectroscopy and high resolution X-ray diffraction mapping on Beamline I18 at Diamond Light Source; samples were probed for redox changes and long/short range disorder. This was combined with lattice scale imaging of damage using HR-TEM (Titan^TM Transmission Electron Microscope). The results show aberrations in lattice parameters as a result of irradiation, with multiple damage-induced ‘domains’ surrounded by amorphous regions. In the naturally damaged samples, neo-formed phyllosilicate phases are shown to be breakdown products of highly damaged regions. A clear reduction of the Fe(III) component has been demonstrated in iron-bearing phyllosilicates in both naturally and artificially damaged samples. Alterations in mineral structure and chemistry will have implications for the phases’ efficiency as a barrier material.

1. Pattrick, R A D et al., (2013) Min. Mag., 77, 2867–2882. 2. Bower et al., unpubl.