Special Issue "Geological Disposal of High Level Radioactive Waste - The Relationship between Engineered and Natural Barriers"

A special issue of Geosciences (ISSN 2076-3263).

Deadline for manuscript submissions: 31 March 2017

Special Issue Editors

Guest Editor
Prof. Rebecca Lunn

Head of Department of Civil and Environmental Engineering, University of Strathclyde, James Weir Building, 68 Montrose St, Glasgow, Glasgow City, UK
Website | E-Mail
Phone: +44 (0)141 548 2826
Interests: Ground engineering; Ground barrier technologies; Nuclear disposal; Nuclear decommissioning; Geomechanics; Geoscience and energy-geothermal, hydrocarbon production and exploitation, carbon storage, shale-gas etc.
Guest Editor
Prof. Simon Harley

Professor of Lower Crustal Processes, School of GeoSciences, University of Edinburgh, Grant institute, James Hutton Road, Edinburgh, EH9 3FE, UK
Website | E-Mail
Phone: (0044) (0)131 650 4839 | (+44) 7889584904
Interests: Magnetic monitoring of EBS evolution in GDFs; Bentonite behaviour in high pH and saline fluids; geochemistry of saline fluids in GDF systems; geochronology geochemistry; U-Th mineral behaviour
Guest Editor
Dr. Simon Norris

Senior Research Manager, Radioactive Waste Management Limited, Building 587, Curie Avenue, Harwell Science and Innovation Campus, Didcot, Oxon, OX11 0RH, UK
Website | E-Mail
Phone: +44 (0) 1925 802891 | +44 (0) 7887 775309
Interests: Geological disposal; Geological Disposal Facility; Radioactive waste; Safety case; Coupled processes (thermal hydrogeological mechanical chemical); Underground research laboratory / underground research facility; Waste inventory, radionuclides; Engineered barrier system and its evolution; Geosphere and its long term evolution (natural, and in response to GDF presence); Groundwater; Waste-derived gas; Disposal concept; Conceptual model; International precedence, regulatory requirements

Special Issue Information

Dear Colleagues,

The major objective of this Special Issue is to examine the interdependencies between the natural host rock barrier that surrounds a high level waste repository and the engineered barriers that protect the waste within it. A significant challenge in engineering barrier design is predicting the long-term behaviour of the bentonite, or other sealing material, at the interface between the barrier and the geosphere and at the interface between the barrier and the waste canister. The repository presents extreme environmental conditions alongside a requirement for understanding barrier material behaviour over very long, million-year, timescales. Over such times-scales, conditions at the interfaces between barriers will change. For example, freshwater ingress may occur due to glacial melting, high pH groundwater may result from cement leaching in remote parts of the repository, and microbial populations may evolve to utilise energy derived from the waste.

This Special Issue aims to provide an outlet for rapid, widely accessible publication of peer-reviewed studies that explore the behaviour of engineered barriers over time subject to changing environmental conditions within the host rock and waste packages. Such changes may include temperature variations, pH, fluid geochemistry, gas liberation, and biological mediation of these changes. Research submissions that comprise laboratory experiments, field trials and/or model development are welcomed.

Prof. Rebecca Lunn
Prof. Simon Harley
Dr. Simon Norris
Guest Editors

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All papers will be peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Geosciences is an international peer-reviewed open access quarterly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 350 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

  • groundwater
  • novel grouts
  • bentonite
  • hydraulic barriers
  • civil engineering
  • geomechanics
  • flow

Published Papers (5 papers)

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Research

Open AccessArticle Selection of Colloidal Silica Grouts with Respect to Gelling and Erosion Behaviour
Geosciences 2017, 7(1), 6; doi:10.3390/geosciences7010006
Received: 4 October 2016 / Revised: 16 January 2017 / Accepted: 22 January 2017 / Published: 6 February 2017
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Abstract
Cembinder, Eka EXP36, and MEYCO MP320 are three colloidal silica materials that have been proposed for post-excavation grouting of deep tunnels in a radioactive waste repository. In this study, samples of these colloidal silicas were tested for their particle size distribution, gel induction
[...] Read more.
Cembinder, Eka EXP36, and MEYCO MP320 are three colloidal silica materials that have been proposed for post-excavation grouting of deep tunnels in a radioactive waste repository. In this study, samples of these colloidal silicas were tested for their particle size distribution, gel induction time (tG), gel time (TG), and physical erosion, under mildly saline groundwater flow conditions. In order to achieve a desired gel time range, from 15 to 50 min, it is recommended that the colloidal silica is mixed with a NaCl accelerator at a 5:1 volume ratio. At 20 °C, the concentration range for the NaCl solution should be 1.5 to 1.7 M for MEYCO, 1.23 to 1.38 M for Eka EXP36, and 1.3 to 1.47 M for Cembinder. The physical erosion of the set silicas remained steady during a 10 h flow cell experiment, when grouts were subjected to 0.05 M NaCl at a superficial velocity of 2.2 × 10−5 m/s. For these test conditions, the results show that MEYCO has the highest average erosion rate (0.85 mg/h) of the three grout materials, as well as the greatest variability in this rate. Cembinder performed best with the lowest silica removal rate. Extrapolation of the measured erosion rates suggests that grout fracture dilation would not be significant under natural quiescent groundwater flow conditions, but would be high if there was hydraulic communication between the geosphere and the repository. Full article
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Open AccessArticle Assessing the Long-Term Behaviour of the Industrial Bentonites Employed in a Repository for Radioactive Wastes by Studying Natural Bentonites in the Field
Geosciences 2017, 7(1), 5; doi:10.3390/geosciences7010005
Received: 26 September 2016 / Revised: 29 November 2016 / Accepted: 20 December 2016 / Published: 18 January 2017
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Abstract
Bentonite makes an important contribution to the performance of the engineered barriers in most radioactive waste repository designs. The choice of bentonite results from its favourable properties for waste isolation and its stability in relevant geological environments. However, the longevity of bentonite (especially
[...] Read more.
Bentonite makes an important contribution to the performance of the engineered barriers in most radioactive waste repository designs. The choice of bentonite results from its favourable properties for waste isolation and its stability in relevant geological environments. However, the longevity of bentonite (especially the resistance to waste container sinking) has been little studied. Modelling results suggest significant bentonite deformation and associated canister sinking is unlikely and, here, long-term natural system data are used as a reality check on model predictions. Results indicate that bentonite from the investigated site shows no significant deviation in bulk physical parameters from repository bentonite. However, micro-scale shear planes can be seen throughout the sampled cores. The presence of multi-directional S- and C-type shears suggests they originate from loading from the overlying limestone, not gravitational tectonics. The plastic limits and angles of shearing resistance for natural and repository bentonites suggest both are susceptible to shearing. The impact of bentonite shear under load could be minimised by appropriate design, but existing lower activity waste container designs do not consider the potentially high external stresses from the bentonite backfill and this should be addressed in future. Full article
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Open AccessArticle Bentonite Permeability at Elevated Temperature
Geosciences 2017, 7(1), 3; doi:10.3390/geosciences7010003
Received: 19 October 2016 / Revised: 13 December 2016 / Accepted: 19 December 2016 / Published: 11 January 2017
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Abstract
Repository designs frequently favour geological disposal of radioactive waste with a backfill material occupying void space around the waste. The backfill material must tolerate the high temperatures produced by decaying radioactive waste to prevent its failure or degradation, leading to increased hydraulic conductivity
[...] Read more.
Repository designs frequently favour geological disposal of radioactive waste with a backfill material occupying void space around the waste. The backfill material must tolerate the high temperatures produced by decaying radioactive waste to prevent its failure or degradation, leading to increased hydraulic conductivity and reduced sealing performance. The results of four experiments investigating the effect of temperature on the permeability of a bentonite backfill are presented. Bentonite is a clay commonly proposed as the backfill in repository designs because of its high swelling capacity and very low permeability. The experiments were conducted in two sets of purpose-built, temperature controlled apparatus, designed to simulate isotropic pressure and constant volume conditions within the testing range of 4–6 MPa average effective stress. The response of bentonite during thermal loading at temperatures up to 200 °C was investigated, extending the previously considered temperature range. The results provide details of bentonite’s intrinsic permeability, total stress, swelling pressure and porewater pressure during thermal cycles. We find that bentonite’s hydraulic properties are sensitive to thermal loading and the type of imposed boundary condition. However, the permeability change is not large and can mostly be accounted for by water viscosity changes. Thus, under 150 °C, temperature has a minimal impact on bentonite’s hydraulic permeability. Full article
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Open AccessArticle Network Modelling of the Influence of Swelling on the Transport Behaviour of Bentonite
Geosciences 2016, 6(4), 55; doi:10.3390/geosciences6040055
Received: 27 September 2016 / Revised: 24 November 2016 / Accepted: 28 November 2016 / Published: 8 December 2016
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Abstract
Wetting of bentonite is a complex hydro-mechanical process that involves swelling and, if confined, significant structural changes in its void structure. A coupled structural transport network model is proposed to investigate the effect of wetting of bentonite on retention conductivity and swelling pressure
[...] Read more.
Wetting of bentonite is a complex hydro-mechanical process that involves swelling and, if confined, significant structural changes in its void structure. A coupled structural transport network model is proposed to investigate the effect of wetting of bentonite on retention conductivity and swelling pressure response. The transport network of spheres and pipes, representing voids and throats, respectively, relies on Laplace–Young’s equation to model the wetting process. The structural network uses a simple elasto-plastic approach without hardening to model the rearrangement of the fabric. Swelling is introduced in the form of an eigenstrain in the structural elements, which are adjacent to water filled spheres. For a constrained cell, swelling is shown to produce plastic strains, which result in a reduction of pipe and sphere spaces and, therefore, influence the conductivity and retention behaviour. Full article
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Open AccessArticle Magnetic Materials: Novel Monitors of Long-Term Evolution of Engineered Barrier Systems
Geosciences 2016, 6(4), 54; doi:10.3390/geosciences6040054
Received: 30 August 2016 / Revised: 11 November 2016 / Accepted: 24 November 2016 / Published: 7 December 2016
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Abstract
Most safety cases for the deep geological disposal of radioactive waste are reliant on the swelling of bentonite in the engineered barrier system as it saturates with groundwater. Assurance of safety therefore requires effective monitoring of bentonite saturation. The time- and fluid-dependent corrosion
[...] Read more.
Most safety cases for the deep geological disposal of radioactive waste are reliant on the swelling of bentonite in the engineered barrier system as it saturates with groundwater. Assurance of safety therefore requires effective monitoring of bentonite saturation. The time- and fluid-dependent corrosion of synthetic magnets embedded in bentonite is demonstrated here to provide a novel and passive means of monitoring saturation. Experiments have been conducted at 70 °C in which neo magnets, AlNiCo magnets, and ferrite magnets have been reacted with saline (NaCl, KCl, CaCl2) solutions and alkaline fluids (NaOH, KOH, Ca(OH)2 solutions; pH = 12) in the presence of bentonite. Nd-Fe-B magnets undergo extensive corrosion that results in a dramatic change from ferromagnetic to superparamagnetic behaviour concomitant with bentonite saturation. AlNiCo magnets in saline solutions show corrosion but only limited decreases in their magnetic intensities, and ferrite magnets are essentially unreactive on the experimental timescales, retaining their initial magnetic properties. For all magnets the impact of their corrosion on bentonite swelling is negligible; alteration of bentonite is essentially governed by the applied fluid composition. In principle, synthetic magnet arrays can, with further development, be designed and embedded in bentonite to monitor its fluid saturation without compromising the integrity of the engineered barrier system itself. Full article
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Planned Papers

The below list represents only planned manuscripts. Some of these manuscripts have not been received by the Editorial Office yet. Papers submitted to MDPI journals are subject to peer-review.

Title: Assessing the long-term behaviour of the industrial bentonites employed in a repository for radioactive wastes by studying natural bentonites
Authors: W. R. Alexander, H. M. Reijonen, H. Takano, K. Futakuchi, T. Kiijima,
A. Siathas, A. E. Milodowski, G. MacKinnon and A. F. Pitty

Title: Network modelling of the influence of swelling on the transport behaviour of bentonite
Authors: I. Athanasiadis, P. Grassl, S. Wheeler

Title: Geomechanical and mineralogical characteristics of the engineered barrier after short and long-term Thermo-Hygro-Chemico exposure
Authors: Christopher William Davies, Colin Davie and Alasdair Charles

Title: Bentonite permeability; the impact of elevated temperature
Authors: Jon Harrington, Katherine Daniels and Andrew Wiseall.

Title: The gelling, grouting and erosion of a range of colloidal silica gels for post-excavation grouting
Authors: Pingqian Shen, Nick Hankins and Stephan Jefferis.

Title: The role of temperature effects in the safety case for the disposal of high-level radioactive waste and spent fuel: A comparison of thermal design concepts.
Author: Joachim Heierli et al.
Abstract: The disposal of heat-generating radioactive waste in deep underground facilities requires favourable temperature conditions over the project lifetime and beyond. In the Swiss nuclear waste management programme, the national cooperative for the disposal of radioactive waste follows the strategy of capping the temperature in barrier materials to fixed boundaries. The specifications of the repository and its components are determined subsequently. Alternatively, part of the specifications can be determined ex ante by minimising the contribution of remote waste batches to temperature increase in barrier materials. In that approach, the temperature boundaries are the result of the optimisation procedure rather than its parameter. From an engineering viewpoint, the two approaches lead to different results and different project workflows. This contribution examines obstacles in substantiating the safety case for a final repository following either approach, considering legal requirements, demonstration of safety, representativeness of pilot disposal areas and retrievability of the waste during the operational lifetime of the facility. It is concluded that the safety case tends to be more difficult to substantiate under temperature capping than under temperature optimisation. This does not imply that thermal optimisation is the safer option, but that research should be conducted to a point where a decision can be drawn regarding which approach to prefer in the future.

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