Abstract
AbstractIn 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.
Highlights
THIS study focuses on the KBS-3V concept for the deep geological disposal of high level nuclear waste in crystalline rock, the current reference disposal concept of SKB and Posiva
The aim of the study was to evaluate the level of conservatism in current models, such as that used in the KBS-3V safety case, by quantifying the effect of accessory minerals in reducing buffer erosion in natural fractures
This study has demonstrated that the accumulation of a ring of accessory minerals does provide a mitigating effect against erosion of MX-80 bentonite extruded into fractures
Summary
THIS study focuses on the KBS-3V concept for the deep geological disposal of high level nuclear waste in crystalline rock, the current reference disposal concept of SKB and Posiva. The bentonite buffer plays an integral role in the multi-barrier system for the isolation of high level nuclear waste and is required to perform a number of functions. The buffer must generate a swelling pressure of between 2 and 10 MPa to suppress any detrimental. The buffer material subsequently swells within the confines of the deposition hole. The buffer is not fully confined in the deposition hole, as it may continue to swell into any transmissive cross-cutting rock fractures
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