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

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A special issue of Geosciences (ISSN 2076-3263). This special issue belongs to the section "Natural Hazards".

Deadline for manuscript submissions: closed (31 March 2017) | Viewed by 57823

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Prof. Rebecca Lunn

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Guest Editor

Head of Department of Civil and Environmental Engineering, University of Strathclyde, James Weir Building, 68 Montrose St, Glasgow, Glasgow City, UK
Interests: ground engineering; ground barrier technologies; nuclear disposal; nuclear decommissioning; geomechanics; geoscience and energy-geothermal; hydrocarbon production and exploitation; carbon storage; shale-gas etc.

Prof. Dr. Simon Harley

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Guest Editor

Professor of Lower Crustal Processes, School of GeoSciences, University of Edinburgh, Grant institute, James Hutton Road, Edinburgh EH9 3FE, UK
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

Dr. Simon Norris

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Guest Editor

Senior Research Manager, Radioactive Waste Management Limited, Building 587, Curie Avenue, Harwell Science and Innovation Campus, Didcot, Oxon OX11 0RH, UK
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

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Keywords

groundwater
novel grouts
bentonite
hydraulic barriers
civil engineering
geomechanics
flow

Published Papers (11 papers)

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Research

7897 KiB

Open AccessArticle

Physicochemical and Geotechnical Alterations to MX-80 Bentonite at the Waste Canister Interface in an Engineered Barrier System

by Christopher W. Davies, Colin T. Davie, Charles A. Edward and Maggie L. White

Geosciences 2017, 7(3), 69; https://doi.org/10.3390/geosciences7030069 - 10 Aug 2017

Cited by 7 | Viewed by 3893

Abstract

The study investigated the basic geomechanical and mineralogical evolution of the bentonite barrier under various experimental boundary conditions which replicated the near-field Thermo-Hydro-Chemico (THC) conditions in a repository. The relationships between the physicochemical alterations and changes in the geotechnical properties have seldom been studied, especially on a consistent dataset. This paper attempts to link the physicochemical properties of Na-bentonite (MX-80) to the macro-scale engineering functionality of the bentonite post THC exposure. Experiments investigated the impact of THC variables on the engineering and physicochemical functionality of the bentonite with respect to its application within a High-Level Waste (HLW) engineered barrier system. Intrinsic alterations to the MX-80 bentonite under relatively short-term exposure to hydrothermal and chemical conditions were measured. Additionally, two long-term tests were conducted under ambient conditions to consider the impact of exposure duration. The intrinsic measurements were then related to the overall performance of the bentonite as a candidate barrier material for application in a UK geological disposal facility. Findings indicate that exposure to thermo-saline-corrosion conditions (i.e., corrosion products derived from structural grade 275 carbon steel) inhibits the free swell capacity and plasticity of the bentonite. However, the measured values remained above the design limits set out for the Swedish multi-barrier concept, from which the UK concept may take a lead. Corrosion alone does not appear to significantly affect the geotechnical measurements compared with the influence of thermal loading and high saline pore water after relatively short-term exposure. Thermal and corrosion exposure displayed no impact on the intrinsic swelling of the smectite component, indicating that no significant structural alteration had occurred. However, when exploring more complex saline solutions i.e., mixed Na, K and Ca, rather than the reference NaCl, divalent cation replacement was observed within the interlayer exchange site. This was accelerated in higher thermal loading conditions. Full article

(This article belongs to the Special Issue Geological Disposal of High Level Radioactive Waste - The Relationship between Engineered and Natural Barriers)

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2756 KiB

Open AccessArticle

About the Possibility of Disposal of HLRW in Deep Boreholes in Germany

by Guido Bracke, Frank Charlier, Axel Liebscher, Frank R. Schilling and Thomas Röckel

Geosciences 2017, 7(3), 58; https://doi.org/10.3390/geosciences7030058 - 18 Jul 2017

Cited by 10 | Viewed by 5542

Abstract

Using deep boreholes for the final disposal of high-level radioactive waste (HLRW) can take advantage of multiple geologic barriers as safety features and aims for the safe containment of radionuclides by containment-providing rock zones (CPRZ). The great depth efficiently prolongs or hinders radionuclide transport and also impedes proliferation. Finally, there may be a time benefit with regard to technical implementation and costs. Due to the phase-out from nuclear energy in Germany the number of boreholes could be less than 100. A simplified, generic safety concept, and the requirements for the diameter of boreholes and containers are derived in this paper. Furthermore, the operational safety of emplacement, the retrieval of waste and sealing of the boreholes is discussed. It is outlined that boreholes can be sealed quickly and over long distances with proven technologies, for example, using the creep properties of salt rock formations. This concept is assessed for its compliance with the safety requirements of the German Federal Ministry for the Environment, Nature Conservation, Building and Nuclear Safety (BMUB), and the requirements and criteria for site selection defined by the German commission on “Storage of high-level radioactive waste”. The retrievability of HLRW is assessed to be technically feasible based on today´s knowledge, but recoverability after closure cannot be guaranteed for long time spans. Further developments in details of the concept of deep borehole disposal (DBD), a demonstration of its technical feasibility and an assessment of operational and long-term safety are still necessary to make DBD an approved option. Full article

(This article belongs to the Special Issue Geological Disposal of High Level Radioactive Waste - The Relationship between Engineered and Natural Barriers)

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2996 KiB

Open AccessArticle

The Impact of Biofilms upon Surfaces Relevant to an Intermediate Level Radioactive Waste Geological Disposal Facility under Simulated Near-Field Conditions

by Christopher J. Charles, Simon P. Rout, Andrew P. Laws, Brian R. Jackson, Sally A. Boxall and Paul N. Humphreys

Geosciences 2017, 7(3), 57; https://doi.org/10.3390/geosciences7030057 - 12 Jul 2017

Cited by 4 | Viewed by 4259

Abstract

The ability of biofilms to form on a range of materials (cementious backfill (Nirex Reference Vault Backfill (NRVB)), graphite, and stainless steel) relevant to potential UK intermediate level radioactive waste (ILW) disposal concepts was investigated by exposing these surfaces to alkaliphilic flocs generated by mature biofilm communities. Flocs are aggregates of biofilm material that are able to act as a transport vector for the propagation of biofilms. In systems where biofilm formation was observed there was also a decrease in the sorption of isosaccharinic acids to the NRVB. The biofilms were composed of cells, extracellular DNA (eDNA), proteins, and lipids with a smaller polysaccharide fraction, which was biased towards mannopyranosyl linked carbohydrates. The same trend was seen with the graphite and stainless steel surfaces at these pH values, but in this case the biofilms associated with the stainless steel surfaces had a distinct eDNA basal layer that anchored the biofilm to the surface. At pH 13, no structured biofilm was observed, rather all the surfaces accumulated an indistinct organic layer composed of biofilm materials. This was particularly the case for the stainless steel coupons which accumulated relatively large quantities of eDNA. The results demonstrate that there is the potential for biofilm formation in an ILW-GDF provided an initiation source for the microbial biofilm is present. They also suggest that even when conditions are too harsh for biofilm formation, exposed surfaces may accumulate organic material such as eDNA. Full article

(This article belongs to the Special Issue Geological Disposal of High Level Radioactive Waste - The Relationship between Engineered and Natural Barriers)

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3211 KiB

Open AccessArticle

Response of Compacted Bentonites to Thermal and Thermo-Hydraulic Loadings at High Temperatures

by Snehasis Tripathy, Hywel Rhys Thomas and Panagiotis Stratos

Geosciences 2017, 7(3), 53; https://doi.org/10.3390/geosciences7030053 - 07 Jul 2017

Cited by 10 | Viewed by 4393

Abstract

The final disposal of high-level nuclear waste in many countries is preferred to be in deep geological repositories. Compacted bentonites are proposed for use as the buffer surrounding the waste canisters which may be subjected to both thermal and hydraulic loadings. A significant increase in the temperature is anticipated within the buffer, particularly during the early phase of the repository lifetime. In this study, several non-isothermal and non-isothermal hydraulic tests were carried on compacted MX80 bentonite. Compacted bentonite specimens (water content = 15.2%, dry density = 1.65 Mg/m³) were subjected to a temperature of either 85 or 150 °C at one end, whereas the temperature at the opposite end was maintained at 25 °C. During the non-isothermal hydraulic tests, water was supplied from the opposite end of the heat source. The temperature and relative humidity were monitored along predetermined depths of the specimens. The profiles of water content, dry density, and degree of saturation were established after termination of the tests. The test results showed that thermal gradients caused redistribution of the water content, whereas thermo-hydraulic gradients caused both redistribution and an increase in the water content within compacted bentonites, both leading to development of axial stress of various magnitudes. The applied water injection pressures (5 and 600 kPa) and temperature gradients appeared to have very minimal impact on the magnitude of axial stress developed. The thickness of thermal insulation layer surrounding the testing devices was found to influence the temperature and relative humidity profiles thereby impacting the redistribution of water content within compacted bentonites. Under the influence of both the applied thermal and thermo-hydraulic gradients, the dry density of the bentonite specimens increased near the heat source, whereas it decreased at the opposite end. The test results emphasized the influence of elevated temperatures (up to 150 °C) on the thermo-hydro-mechanical response of compacted bentonites in the nuclear waste repository settings. Full article

(This article belongs to the Special Issue Geological Disposal of High Level Radioactive Waste - The Relationship between Engineered and Natural Barriers)

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1134 KiB

Open AccessArticle

The Role of Temperature in the Safety Case for High-Level Radioactive Waste Disposal: A Comparison of Design Concepts

by Joachim Heierli and Oliver Genoni

Geosciences 2017, 7(2), 42; https://doi.org/10.3390/geosciences7020042 - 13 Jun 2017

Cited by 6 | Viewed by 4510

Abstract

The disposal of heat-generating radioactive waste in deep underground facilities requires a sparing use of spatial resources on the one side and favorable temperature conditions over the project lifetime on the other side. Under heat-sensitive conditions, these goals run in opposite directions and therefore a balance of some kind must be found. Often the elected strategy is to determine the size of the repository by capping the temperatures in the near-field, thus setting an upper limit to the deterioration of barrier materials. Alternatively, the spatial resources available in the siting area can be used to further reduce temperatures as long as supplementary benefits are returned from doing so. Using analytical modeling of the heat flow in the circumambient rock of a repository for high-level waste and spent fuel, this contribution examines possible obstacles in substantiating the safety case, namely the retrievability of waste during the operational lifetime of the facility, the representativeness of pilot disposal areas for monitoring, and the effect of thermal anomalies underground. The results indicate that there are, amongst the visited criteria, several benefits to the temperature-optimizing strategy over the prevailing space-optimizing concepts. The right balance between saving spatial resources and obtaining optimal temperature conditions is yet to be found. Full article

(This article belongs to the Special Issue Geological Disposal of High Level Radioactive Waste - The Relationship between Engineered and Natural Barriers)

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3404 KiB

Open AccessArticle

Laboratory Testing of a MEMS Sensor System for In-Situ Monitoring of the Engineered Barrier in a Geological Disposal Facility

by Wenbin Yang, Rebecca J. Lunn, Alessandro Tarantino and Gráinne El Mountassir

Geosciences 2017, 7(2), 38; https://doi.org/10.3390/geosciences7020038 - 20 May 2017

Viewed by 4186

Abstract

Geological disposal facilities for radioactive waste pose significant challenges for robust monitoring of environmental conditions within the engineered barriers that surround the waste canister. Temperatures are elevated, due to the presence of heat generating waste, relative humidity varies from 20% to 100%, and swelling pressures within the bentonite barrier can typically be 2–10 MPa. Here, we test the robustness of a bespoke design MEMS sensor-based monitoring system, which we encapsulate in polyurethane resin. We place the sensor within an oedometer cell and show that despite a rise in swelling pressure to 2 MPa, our relative humidity (RH) measurements are unaffected. We then test the sensing system against a traditional RH sensor, using saturated bentonite with a range of RH values between 50% and 100%. Measurements differ, on average, by 2.87% RH, and are particularly far apart for values of RH greater than 98%. However, bespoke calibration of the MEMS sensing system using saturated solutions of known RH, reduces the measurement difference to an average of 1.97% RH, greatly increasing the accuracy for RH values close to 100%. Full article

(This article belongs to the Special Issue Geological Disposal of High Level Radioactive Waste - The Relationship between Engineered and Natural Barriers)

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3032 KiB

Open AccessArticle

Selection of Colloidal Silica Grouts with Respect to Gelling and Erosion Behaviour

by Pingqian Shen, Nicholas Hankins and Stephan Jefferis

Geosciences 2017, 7(1), 6; https://doi.org/10.3390/geosciences7010006 - 06 Feb 2017

Cited by 5 | Viewed by 6000

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 time (t_G), gel time (T_G), 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

(This article belongs to the Special Issue Geological Disposal of High Level Radioactive Waste - The Relationship between Engineered and Natural Barriers)

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6024 KiB

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

by W. Russell Alexander, Heini M. Reijonen, Gillian MacKinnon, Antoni E. Milodowski, Alistair F. Pitty and Andreas Siathas

Geosciences 2017, 7(1), 5; https://doi.org/10.3390/geosciences7010005 - 18 Jan 2017

Cited by 4 | Viewed by 6112

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 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

(This article belongs to the Special Issue Geological Disposal of High Level Radioactive Waste - The Relationship between Engineered and Natural Barriers)

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6078 KiB

Open AccessArticle

Bentonite Permeability at Elevated Temperature

by Katherine A. Daniels, Jon F. Harrington, Stephanie G. Zihms and Andrew C. Wiseall

Geosciences 2017, 7(1), 3; https://doi.org/10.3390/geosciences7010003 - 11 Jan 2017

Cited by 28 | Viewed by 7597

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 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

(This article belongs to the Special Issue Geological Disposal of High Level Radioactive Waste - The Relationship between Engineered and Natural Barriers)

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1576 KiB

Open AccessArticle

Network Modelling of the Influence of Swelling on the Transport Behaviour of Bentonite

by Ignatios Athanasiadis, Simon Wheeler and Peter Grassl

Geosciences 2016, 6(4), 55; https://doi.org/10.3390/geosciences6040055 - 08 Dec 2016

Cited by 1 | Viewed by 4990

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 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

(This article belongs to the Special Issue Geological Disposal of High Level Radioactive Waste - The Relationship between Engineered and Natural Barriers)

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5543 KiB

Open AccessArticle

Magnetic Materials: Novel Monitors of Long-Term Evolution of Engineered Barrier Systems

by Simon L. Harley, Nicola Rigonat and Ian B. Butler

Geosciences 2016, 6(4), 54; https://doi.org/10.3390/geosciences6040054 - 07 Dec 2016

Cited by 1 | Viewed by 4695

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 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

(This article belongs to the Special Issue Geological Disposal of High Level Radioactive Waste - The Relationship between Engineered and Natural Barriers)

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