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A mechanism for bentonite buffer erosion in a fracture with a naturally varying aperture

Published online by Cambridge University Press:  02 January 2018

Christopher Reid ,
Rebecca Lunn ,
Gráinne El Mountassir  and
Alessandro Tarantino
Christopher Reid*
Affiliation:
Department of Civil and Environmental Engineering, University of Strathclyde, Glasgow G1 1XQ, UK
Rebecca Lunn
Affiliation:
Department of Civil and Environmental Engineering, University of Strathclyde, Glasgow G1 1XQ, UK
Gráinne El Mountassir
Affiliation:
Department of Civil and Environmental Engineering, University of Strathclyde, Glasgow G1 1XQ, UK
Alessandro Tarantino
Affiliation:
Department of Civil and Environmental Engineering, University of Strathclyde, Glasgow G1 1XQ, UK
*
*E-mail: [email protected]
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Abstract

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In the deep geological disposal of nuclear waste in crystalline rock, erosion of the bentonite buffer may occur during periods of glaciation. Previous researchers have examined the mechanism and rates of extrusion and erosion for purified montmorillonite samples in smooth planar fractures. In this paper, we investigate the influence of using MX-80 material (as delivered, i.e. including accessory minerals) and a naturally varying aperture on bentonite erosion. A bespoke fracture flow cell was constructed for this purpose and flow through conducted with deionized water. Throughout the experiment, gravimetric analysis was undertaken on the effluent and the swelling pressure of the bentonite monitored. Quantitative image analysis of the extrusion process was also undertaken. When the swelling pressure data were analysed, alongside both the oscillations in erosion rate and the area of the accessory-mineral ring, a two-stage mechanism governing the erosion process became apparent. Once an accessory-mineral ring had formed at the edge of the extruded material, further increases in swelling pressure resulted in a breach in the accessory-mineral ring, triggering an erosive period during which, the mineral ring was supplemented with additional minerals. The cycle repeated until the ring was sufficiently strong that it remained intact. This observed process results in erosion rates one order of magnitude less than those currently used in long-term safetycase calculations.

Keywords

bentonitemontmorillonitebuffererosionnuclear-waste disposal
Type
Research Article
Information
Mineralogical Magazine , Volume 79 , Issue 6 , November 2015 , pp. 1485 - 1494
Creative Commons
Creative Common License - CCCreative Common License - BY
Copyright © The Mineralogical Society of Great Britain and Ireland 2015. This is an open access article, distributed under the terms of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2015

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