The disposal of radioactive wastes underground

Proceedings of the Geologists’ Association 123 (2012) 46–63

Review paper

The disposal of radioactive wastes underground

Neil Chapman a,*, Alan Hooper b

a Department of Materials Science and Engineering, University of Sheffield, UK and MCM Consulting, Baden-Dattwil, Switzerlandb Department of Earth Science and Engineering, Imperial College, London and Alan Hooper Consulting Ltd., Kingswood, Wotton-u-Edge, UK

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

2. Radioactive wastes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

3. Geological disposal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

4. The role of the geological environment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

5. How geological disposal is being implemented . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

5.1. Addressing uncertainties and reaching a safe solution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

5.2. Regulatory targets for post-closure GDF safety . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

5.3. Non-technical issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58

5.4. The international perspective. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58

5.5. Current developments in the UK . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

6. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

A R T I C L E I N F O

Article history:

Received 20 August 2011

Received in revised form 12 October 2011

Accepted 17 October 2011

Available online 16 November 2011

Keywords:

Radioactive waste

Geological disposal

Safety function

A B S T R A C T

Nuclear power is strategically and quantitatively an important contributor to global electricity

generation capacity and produces a small amount of potentially highly hazardous wastes that require

careful management. The accepted solution for disposing of higher activity and longer-lived radioactive

wastes from the nuclear power industry and other sources is engineered emplacement in deep geological

disposal facilities (GDFs), situated many hundreds of metres underground. The first purpose-built GDFs

for the most active of these wastes (used nuclear fuel and high-level wastes) will be operational in about

ten years time in a few countries, with most other countries (including the UK) developing such facilities

during coming decades. This article reviews the conceptual basis for geological disposal, examines how

long-term safety is provided, considers the geological challenges to developing GDFs and the

uncertainties that have to be managed, and looks in more detail at some of the most advanced design

concepts. Because the issue of forecasting GDF evolution and behaviour over very long time periods lies at

the core of geological disposal, particular emphasis is placed on matching containment requirements

with diminishing hazard potential over many thousands of years. The article concludes with a

commentary on current developments in the UK geological disposal programme.

� 2011 The Geologists’ Association. Published by Elsevier Ltd. All rights reserved.

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Proceedings of the Geologists’ Association

jo ur n al ho m ep ag e: www .e ls evier . c om / lo cat e/p g eo la

1. Introduction

Nuclear power supplies about 14% of the world’s electricityrequirements today – in the European Union, about a third – andthere is an increasing demand for more nuclear power generation,partly in response to requirements for low-carbon energy sourcesand partly to improve the security of supply of electricity by

* Corresponding author.

E-mail address: [email protected] (N. Chapman).

0016-7878/$ – see front matter � 2011 The Geologists’ Association. Published by Else

doi:10.1016/j.pgeola.2011.10.001

diversification, especially in countries dependent on imported coal,oil or gas. Despite the enormous setback to the credibility ofnuclear power caused by the 2011 Fukushima disaster, theinexorable demand for energy suggests that this situation isunlikely to be affected significantly on a global scale, even though itwill slow or even halt nuclear developments in some countries.

We have used nuclear power to supply our electricity grids formore than 50 years and the industry has been producingradioactive wastes throughout this period. Like any source ofpower, the use of nuclear energy produces wastes – but inremarkably small quantities compared with the use of fossil fuels

vier Ltd. All rights reserved.

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N. Chapman, A. Hooper / Proceedings of the Geologists’ Association 123 (2012) 46–63 47

(0.05% of total power production waste volume in the EU, for 30% ofthe power generated). More specifically, generating 1 GW ofelectricity for a year (a common capacity for a medium-sizedpower station) produces about 25 tonnes of used nuclear fuel and afew hundred cubic metres of other wastes, compared to 6.5 milliontonnes of carbon dioxide, more than 300,000 tonnes of ash(containing around 400 tonnes of toxic heavy metals includingradioactive uranium and thorium) and, if not scrubbed out, over5000 tonnes of noxious gases from burning coal to produceelectricity. As we shall see later, some of the radioactive wastes areexceptionally hazardous materials and require the most carefulmanagement and disposal – the topic of this paper.

Because the amounts of waste from nuclear power generationare comparatively so small, they can be stored without incurringthe enormous management problem that would arise if otherpower generation wastes were to be stored, rather than simplydischarged to the environment. This has led to a situation wherethere has been no great urgency to dispose of the smaller volume,higher activity wastes. However, other factors colour this simplepicture and lead to a more complex story. Storing highly activeradioactive waste requires special, highly secure facilities and isexpensive; facilities for disposing of these wastes have not beenavailable and it has not been possible to develop them; there hasbeen considerable public opposition to siting radioactive wastefacilities away from the sites where wastes are stored; there is nowa demand to collect and dispose of carbon dioxide waste from fossilfuel power generation safely, in the same way that radioactivewastes need to be disposed.

This latter development has brought a new confluence inthinking about the use of the underground environment as adisposal location for power generation wastes. The nuclearindustry has been making extremely slow progress towardsgeological disposal of its more radioactive and longer-lived wastesfor the last 30 or 40 years; the fossil-fuel power generationindustry has only recently started to consider the same solution fordisposing of future carbon dioxide arisings. Whilst the nuclearindustry has effectively sat on its most difficult wastes for half acentury, waiting until it could achieve geological disposal facilitiesas a safe solution for its historical and future arisings, the fossil-fuelindustry has discharged all its carbon dioxide waste to theatmosphere and will now have to move quickly and on a massivescale towards geological disposal of its future arisings if it is to havean appreciable impact on climate change. The isolation, contain-ment and consequent safety that can be provided by disposal in thedeep underground environment is thus becoming a commontheme of relevance across the whole non-renewable energy supplysector – the sector that will supply almost all our energy needsworldwide for the foreseeable future (Chapman et al., 2011). Theimportance of this concept is thus notable and this paper looks atthe background to one of these areas – the geological disposal ofradioactive wastes: what it is, how it works, what is being donearound the world and what will happen in the UK over the nextdecades. In Europe, geological disposal of radioactive wastes isacknowledged by the European Commission (reflected in currentEU legislation: e.g. EC, 2011) as being widely accepted at thetechnical level as the most appropriate and sustainable solution,with a now mature underpinning of R&D stretching back to the1970s (EC, 2004).

2. Radioactive wastes

The bulk of the radioactive wastes generated by society comefrom nuclear power generation. We also produce wastes fromother industries, from research and development in universitiesand national laboratories, from hospitals, where radioactivematerials are used in diagnosis and treatment, and from numerous

smaller sources, including the smoke detectors in our own homes.With the exception of some mining wastes (e.g. from thephosphate fertiliser industry) these other wastes are either ofmuch smaller volume compared to those from nuclear powergeneration, or have much lower levels of radioactivity, or both.Most countries bring together all of the wastes with activitiesabove a certain level for common management – the lowestactivity wastes, especially if they have large volumes, tend to betreated like other industrial wastes and put back into the naturallyradioactive environment as mine spoil or to landfill.

In the nuclear industry in Europe, North America and otherdeveloped countries, even the lowest activity wastes have beenmanaged as specifically segregated radioactive materials. Thelower activity wastes (which generally also have short radioactivehalf-lives) have been disposed of in special facilities on, or justbelow, the land surface (trenches, vaults, pits and silos) with theintention of keeping the sites under institutional control afterclosure of the repositories for 100 years or more, in many casesuntil the overall radioactive hazard has decayed to levels belowradiological concern.

Here, we are concerned with higher activity wastes that mightbe destined for geological disposal, which begin with some of thewastes from reactor operations, decommissioning of old nuclearfacilities and recycling of used nuclear fuel, moving up the activityrange to separated materials from reprocessing and eventually toused fuel taken from nuclear power reactors when its efficiencyhas declined. As we move up this range, the volumes decreasesubstantially, but the radioactivity levels rise exponentially. Manyof these wastes also typically contain concentrations of long-livedradionuclides (with radioactive decay half-lives of thousands tomany millions of years) and some of the highest-level materials arecharacterised by significant heat output from the rapid decay ofhigh concentrations of short-lived radionuclides. Used (or ‘spent’)nuclear fuel continues to emit substantial amounts of heat forhundreds of years after it is removed from a reactor, in the longerterm as a result of the decay of the plutonium produced duringirradiation of the fuel.

Geological disposal has been selected as the best way ofhandling these wastes that combine, in various measure, higherlevels of radioactivity with concentrations of longer-lived radio-nuclides and, for some, significant but declining heat output. In theUK the current reference inventory of wastes assigned to geologicaldisposal is shown in Table 1 and contains both ‘legacy’ materialsthat have been produced and stored for many decades and, in the‘upper inventory’, materials that are likely to arise from a newgeneration of nuclear power reactors.

It can be seen from Table 1 that all of the wastes from a nuclearpower programme that will have been operational from the 1950sto the 2080s (providing perhaps 20% of our electricity over that 130year period) and which will be routed for geological disposal,amounts to about one million cubic metres – a heap of wastepackages less than the height of a house and 500 m square. Quiteclearly, however, these wastes cannot simply be piled up. Theyhave different characteristics, require different handling andseparation between packages, and need to be emplaced carefullyinto underground, engineered facilities (usually called ‘reposito-ries’; in the UK the term ‘geological disposal facility’ or GDF is used– here, we use both terms synonymously) using geometries andmaterials that respect these differences.

3. Geological disposal

The concept of geological disposal dates back to the late 1950s,when it was first advocated in the USA as the most appropriate wayto deal permanently with long-lived solid radioactive wastes (NRC,1957). In the UK, it was most recently defined in the MRWS White

Table 1The UK reference inventory (cubic metres of packaged volume) of radioactive wastes for geological disposal and an ‘upper’ inventory, which contains additional materials

from a future 60 years of operations of new nuclear power stations, the extended operation of existing power stations (and the reprocessing of all their existing spent fuel) and

various other assumptions (NDA, 2010b). LLW is low-level waste; ILW is intermediate-level waste; HLW is vitrified high-level waste from reprocessing spent fuel; LLWR is the

existing low-level waste repository near Drigg in Cumbria, which accepts much UK LLW for disposal in a surface (rather than geological) repository. Plutonium has been

separated from spent fuel by reprocessing, for re-use in nuclear fuel and the assumption here is that it will never be used (some is also surplus from the defence/weapons

sector). Uranium is a depleted by-product of fresh fuel manufacture or spent fuel reprocessing, with little use in current nuclear power generation technologies.

Material Reference inventory (m3 packaged volume) Upper inventory (m3 packaged volume)

LLW (not destined for the LLWR) 17,000 156,000

ILW 362,000 584,000

HLW 7500 23,000

Spent Fuel 10,400 22,300

Plutonium 7000 10,400

Uranium 94,500 175,000

Total 498,400 970,700

N. Chapman, A. Hooper / Proceedings of the Geologists’ Association 123 (2012) 46–6348

Paper (DEFRA et al., 2008) as ‘. . .burial underground (200–1000 m)

of radioactive wastes in a purpose built facility with no intention to

retrieve. . .. The overall aim of geological disposal, which isregarded as a permanent solution to management of the wastes,is to remove a hazardous material from the immediate human anddynamic, natural surface environment to a stable location where itwill remain, protected from disturbance by disruptive natural orhuman processes. The materials that we place underground willslowly degrade and even the most stable deep geologicalenvironments will eventually change with the passage ofgeological time, but the hazard potential of the wastes alsodecreases by natural radioactive decay, so the long-term safety of aGDF must be based on the balance of these processes.

The conceptual basis of geological disposal has been firmlyestablished internationally for the last 30 years as being basedupon the ‘multi-barrier system’, whereby a series of engineeredand natural barriers act in concert to isolate the wastes and containthe radionuclides associated with the wastes. The relativestrengths of the various barriers at different times after closureof a disposal facility and the way that they interact with each otherdepend upon the design of the disposal system, which itself isheavily dependent on the geological environment in which thefacility is to be constructed. Consequently, the multi-barriersystem can work in different ways at different times in differentdisposal concepts. The typical components found in a multi-barriersystem are shown in Fig. 1.

The multi-barrier concept of disposal addresses two principalgoals with respect to providing safety – the isolation of the wastes

Fig. 1. Schematic illustration of the multi-barrier concept for geological disposal,

showing the solid waste enclosed in a series of engineered barrier (the engineered

barrier system: EBS) located in a host rock formation lying within its broader

geological environment (the natural barrier system). The relative strength of each

barrier, the functions they perform and the way that they interact and support each

other, all evolve with time. The multibarrier system does not work by simple

‘defence in depth’, ‘multiple redundancy, ‘sequential failure’ or ‘fail-safe’.

and the containment of the radionuclides associated with thewastes:

� Isolation: safely removes the wastes from direct interactionwith people and the environment – to achieve this meansfinding locations and geological environments for a disposalfacility that are deep, inaccessible and stable over long periods(for example, where rapid uplift, erosion and exposure of thewaste will not occur) and which are unlikely to be drilled intoor deeply excavated in a search for natural resources in thefuture.� Containment: means retaining the radionuclides within various

parts of the multibarrier system until natural processes ofradioactive decay have reduced the hazard potential consider-ably – for many radionuclides, disposal concepts can providetotal containment until they decay to insignificant levels ofradioactivity within the immediate environs of the wastepackage. Nevertheless, the engineered barriers in a disposalfacility will degrade progressively over hundreds and thou-sands of years and lose their ability to provide completecontainment. Because some radionuclides decay extremelyslowly and/or are mobile in deep groundwaters, their completecontainment is not possible. Assessing the safety of geologicaldisposal involves evaluating the fate and impact of theseextremely low concentrations of radioactivity that mighteventually reach people and the surface environment, eventhough this may not happen until many thousands of years intothe future.

Both of these key functions are especially important in theearly years after closure of the disposal facility, when the hazardpotential of the wastes is highest. Fig. 2 shows the decliningradiotoxicity of SF and HLW as a function of time. It can be seenthat this declines by factors of thousands over a period of somehundreds to a few thousand years. Consequently, providingisolation and containment over this ‘early’ period of extremelyhigh hazard potential is paramount and is a key objective whensiting and designing a GDF. The figure also shows that theradioactivity of the wastes eventually declines to levels similar tonatural uranium ore formations over periods from a fewthousands to around a hundred thousand years, depending onthe waste type. By this time, the enormous reduction in hazardpotential that has occurred means that the primary isolation andcontainment functions of geological disposal have largely beenachieved, but we still need to consider the possible impacts of theresidual radionuclides on people and the environment, out toaround a million years. Safety analyses thus continue to calculaterisks to people for a long period after isolation and containmenthave done their main work and have kept the vast proportion ofthe radionuclides deep within the rock until they have decayedaway.

Fig. 2. The declining radiotoxicity of spent fuel and HLW as function of time (after

the fuel has been taken out of the reactor or, for HLW, after reprocessing), relative to

the radiotoxicity of the uranium ore that was originally used to make the fuel

(horizontal line). The crossover time when spent fuel has a similar level of

radiotoxicity to the original ore is in the order of a hundred thousand years. HLW

has an equivalent crossover time of only about 3000 years. The decline in

radioactivity and heat production with time follows a very similar profile.


The various components of the multi-barrier system contributeto fulfilling the high-level safety objectives of isolation andcontainment in different ways and over different timescales.Practice in many national geological disposal programmes is todefine safety functions for each component, which set out whateach specific barrier component contributes towards post-closuresafety (NEA, 2009). As noted above, for any given barrier, thesefunctions vary from concept to concept, from time to time andbetween geological environments. In keeping with the concept ofthe multi-barrier system, the overall safety of a disposal systemdoes not depend upon any one of these functions alone, but uponhow the functions interact with each other as a function of time asthe closed disposal facility slowly evolves. Table 2 provides acomprehensive list of safety functions that is generic with respectto these variables – a simple examination of the table shows thatnot all the safety functions can be achieved at any one time or forevery disposal concept.

An essential aspect of geological disposal is that a GDF providesprotection and safety in a completely passive manner once it hasbeen closed – no further actions are required from people tomanage the facility and the wastes, and, over immensely longtimes, the facility and the wastes become part of the deep, naturalenvironment.

It is expected that the operational life of a typical GDF would bemany decades, even over 100 years in some countries, dependingon how much ‘backlog’ waste exists and how much is to beproduced in the future, after the repository becomes available. Inall cases, the intention is that, upon completion of disposaloperations, the GDF will be partially or totally backfilled and theaccess works will be completely sealed. After a repository has beenclosed, conditions in the rocks surrounding the repository at depthwill return slowly to those of the natural, undisturbed environ-ment before the GDF was constructed. As discussed in the nextsection, key features of the deep geological environment are stableover very long periods of time and natural hydrochemicalprocesses are slow. The engineered barrier system (EBS) willprovide a very long period of containment, depending on theenvironment and the materials used, during which much of theradioactivity of the waste will decay. It is inevitable that,eventually, the engineered barriers will degrade by interactionwith groundwaters and porewaters in the rock. This may takemany thousands or tens of thousands of years and, in some

environments or for some of the EBS materials envisaged, evenlonger. Once water contacts the waste, some radionuclides willdissolve and be mobilised into the porewater or groundwatersystem, but the partially degraded engineered barriers willcontinue to hinder the mobilisation of these small amounts ofradioactivity for hundreds of thousands of years. Any radionuclidesthat migrate into the groundwater system around the repositorywill be in minute amounts and will be dispersed during slowmovement through the geological environment. The objective ofgeological disposal is to ensure that, even in the distant future(many thousands of years hence) the presence of any suchradioactivity in the groundwater system does not cause unaccept-able health risks to future generations.

Over the last 35 years a range of generic, but host formation-specific, GDF designs has been developed around the world and arange of materials proposed for various components of the EBS.Both the design and the materials selected depend upon thecategory of waste to be disposed of and the geological environmentunder consideration. In some countries, including the UK, there is apreference to have a single GDF for all wastes that would requiregeological disposal, which means that a disposal facility willincorporate sections that have, perhaps, two or three differentbasic designs to accommodate the different wastes. There aremany further design considerations, of course, when it comes tofitting a generic concept to a specific site: the ability to be flexibleand adapt design, depth and geometry to local conditions byexploiting the best volumes of rock or avoiding certain geologicalfeatures; optimising operational procedures and costs; accommo-dating local community requirements; minimising environmentalimpacts of construction, surface facilities and GDF operation – allthese and more must be taken into account.

When developing its approach to disposing of the wide rangeof legacy wastes arising from more than 50 years of the UKnuclear programme, the Nuclear Decommissioning Authority(NDA) took a fresh look at GDF designs and developed a set ofgeneric concepts that could be appropriate to UK geologicalenvironments. Unlike many other countries, because the UKwaste management programme has adopted a voluntaryapproach to inviting potentially interested host communitiesto participate in the siting process, the geological environmentcannot be stipulated in advance. The UK has almost all of thecommonly considered host-rock formations being consideredworldwide and any of them might emerge from the voluntaryprocess. The set of concepts developed by the NDA took the bestexperience from all other relevant national programmes world-wide. It comprised twelve that would be suitable for HLW, spentfuel and plutonium (Chapman et al., 2009a; Baldwin et al., 2008)and four that would be suitable for all the other (generally ILW)wastes (Hicks et al., 2008) in the reference and upper inventorydescribed in Table 1. A selection of these concepts for HLW andspent fuel is shown in Fig. 3, along with comments on the hostrock formations and geological environments for which the basicconcepts were originally developed. Section 5 looks at examplesof how some of these concepts have been developed intodetailed, site-specific or formation-specific designs in othercountries.

For some types of waste, an alternative to disposal in a GDFcould be emplacement at great depth (several kilometres) in deepboreholes. The concept of deep borehole disposal (DBD) of spentfuel, HLW and other radioactive wastes has been discussed activelyfor many decades but is significantly less developed than GDFdisposal and would require further assessment and technologydevelopment and testing before it could be implemented. DBDinvolves emplacement of waste packages in the bottom sections ofdeep boreholes constructed to depths of several kilometres, withthe upper kilometres of the holes being backfilled and sealed,

Table 2Post-closure safety functions of the principal barriers in the multibarrier system.

Barrier component Safety function

Wasteform: the solid waste, which might be

‘conditioned’ in some way: e.g. materials in fragments

might be encased in a cement grout matrix

� Provide a stable, low-solubility matrix that limits the rate of release of the majority of

radionuclides by dissolving slowly in groundwaters that come into contact with it

Waste container: generally metal (typically steel) or

concrete: for higher activity wastes such as HLW and

spent fuel, the container might have an outer metal

overpack (e.g. massive iron, copper or titanium

‘overpacks’)

� Protect the wasteform from physical disruption (e.g. by movement in the bedrock)

� Prevent groundwaters from reaching the wasteform for a period of time

� Act as a partial barrier limiting the movement of water in and around the wasteform after

corrosion has breached the container

� Control the redox conditions in the vicinity of the wasteform by corrosion reactions, thus

controlling the solubility of some radionuclides

� Allow the passage of gas from the wasteform out into the surrounding engineered barrier

system

Buffer or backfill around the waste container, separating

the package from the rock. In many designs, a natural

clay buffer (bentonite) is used which takes up water

from the rock and expands to seal the packages into

the rock.

� Protect the waste container from physical disruption (e.g. by movement in the bedrock)

� Control the rate at which groundwaters can move to and around the waste container (e.g. by

preventing flow)

� Control the rate at which chemical corrodants in groundwaters can move to the waste

container

� Condition the chemical characteristics of groundwater and pore water in contact with the

container and the wasteform so as to reduce corrosion rate and/or solubility of

radionuclides

� Control the rate at which dissolved radionuclides can move from the wasteform out, into

the surrounding rock

� Control or prevent the movement of radionuclide-containing colloids from the wasteform

into the rock

� Suppress microbial activity in the vicinity of the waste

� Permit the passage of gas from the waste and the corroding container out into the rock (see

Section 5.1 for more details)

Mass backfill to fill all the excavated openings in the

GDF. Different natural materials and cements will be

used in different parts of the GDF and will be chosen to

match the geological environment

� Restore mechanical continuity and stability to the rock and engineered barrier region of the

facility so that the other engineered barriers are not physically disrupted (e.g. as a clay

buffer takes up water and expands)

� Close voids that could otherwise act as groundwater flow pathways within the facility

� Prevent easy access of people to the waste packages

Sealing systems emplaced locally in tunnels and shafts

at key points in the system

� Cut off potential fast groundwater flow pathways within the backfilled facility (e.g. at the

interface between mass backfill and rock)

� Prevent access of people into the backfilled facility

Natural geological barrier: the host rock in which the

waste emplacement tunnels and caverns are

constructed and all the overlying geological

formations, which may be different to the host

formation

� Isolate the waste from people and the natural surface environment by providing a massive

radiation shield

� Protect and buffer the engineered barrier system from dynamic human and natural

processes and events occurring at the surface and in the upper region of the cover rocks (e.g.

major changes in climate, such as glaciation)

� Protect the engineered barrier system by providing a stable mechanical and chemical

environment at depth that does not change quickly with the passage of time and can thus be

forecast with confidence

� Provide rock properties and a weakly dynamic hydrogeological environment that controls

the rate at which deep groundwaters can move to, through and from the backfilled and

sealed facility, or completely prevent flow

� Ensure that chemical, mechanical and hydrogeological evolution of the deep system is slow

and can be forecast with confidence

� Provide properties that retard the movement of any radionuclides in groundwater – these

include sorption onto mineral surfaces and properties that promote hydraulic dispersion

and dilution of radionuclide concentrations

� Allow the conduction of heat generated by the waste away from the engineered barrier

system so as to prevent unacceptable temperature rises

� Disperse gases produced in the facility so as to prevent mechanical disruption of the

engineered barrier system


possibly with the uppermost sections being obliterated to makerelocation difficult and re-access problematic. Waste might beemplaced in sections of a borehole at depths from 3 to 5 km, withthe safety concept being based principally on the considerableisolation provided by the great depth. DBD can be seen as anessentially irreversible option and, for this reason, it has beensuggested as an especially appropriate solution for disposal offissile materials such as separated Pu, where nuclear safeguardswill be of central concern. The volume of even a very deep boreholeis restricted, so it is not seen as a solution for large volume wastestreams and all work has concentrated on its use for disposal ofHLW and SF (plus conditioned Pu waste forms). The mostcomprehensive recent evaluations of DBD have taken place in

the USA (Sapiie and Driscoll, 2009; Brady et al., 2009), withdescriptions of the various methods currently under considerationbeing presented by Arnold et al. (2010) and Gibb (2010).

4. The role of the geological environment

As discussed above, we use the term ‘geological environment’here to include the rock formation that hosts the GDF disposaltunnels or caverns and any other formations that surround oroverlie the host formation and whose properties affect thebehaviour of the repository. For example, the hydrogeologicaland chemical behaviour of pore waters in a ‘host clay formation arelikely to be affected by the properties of the underlying and

Fig. 3. Schematic illustrations of genericised GDF concepts for disposal of HLW and SF, incorporating almost all those being considered worldwide and currently being

assessed by the Nuclear Decommissioning Authority for use in the UK. From left to right, examples of their intended use are: top row: the Swedish/Finnish KBS-3 concept for

use in hard rocks, an early Belgian concept for use in clay from which the current French concept is a development, Swiss and German concepts suitable for all host rocks and

two concepts with a ‘supercontainer’ – the first with long horizontal boreholes for use in hard rocks, the second being the current Belgian concept for clay: bottom row: a

similar supercontainer concept suitable in particular for hard rocks, a large cavern concept for multi-purpose transport/disposal casks; a similar concept using concrete casks

and cement backfill; a matrix of boreholes in hard rocks or salt and a ‘hydraulic cage’ concept for hard rocks. From Chapman et al. (2009a).


overlying formations (e.g. sandstones, limestones and other clays)in a layered sequence of sediments. Topography and surfacefeatures (such as proximity to the present-day coastline) alsocontribute to the behaviour of the deep geological environment by

Hard rocks, such as g ranite

Argillacesedimenformati

Heat conduction

Goo d Var iab

Host r ock hydrauli c conductivity

Goo d Very Go

Stable nea r field hydrochemistry

Variable Very Go

Low-flux geologica l environmen t

Variable Ex tremely

Intrus ion potential

Goo d Var iab

Construction flexibi lity

Extremely Good Var iab

Gas dispers abilit y

Extremely Good Variable to

Fig. 4. Qualitative and schematic indication of the strengths and weaknesses of typical hos

containment and isolation properties of the geological barrier (left hand column). Note t

lead to ‘scoring’ a specific host formation (or site) quite differently – and note the caveat i

group of ‘unsaturated tuffs’, which was a geological environment uniquely evaluated in th

metres depth and the repository would lie in hydrogeologically unsaturated hard rocks w

worldwide are in saturated formations where chemical conditions are generally reduc

imposing hydraulic gradients on the groundwater system ofhydrochemical interfaces between bodies of deep water ofdifferent compositions. Consequently, understanding how a GDFwill evolve and perform with respect to long-term safety requires

ous tary

ons Evapo rites

Unsatura ted volcanic tuff s

le Ex tremely Good Goo d

od Extremely Good Good

od Ex tremely Good Variable

Good Ex tremely Good Goo d

le Poo r Very Goo d

le Goo d Very Good

Po or Low Relevan ce Extremely

Good

t rock formations that have been considered for disposal, with respect to the generic

hat there can be considerable variability between formations in a group that would

n the text about there being no ‘best rock’. The right-hand column identifies a fourth

e USA at Yucca Mountain in Nevada. Here, the water table lies at several hundreds of

ith consequently oxidising conditions. All other sites being evaluated or developed

ing.


knowledge of the geological and geographical setting, as well asdetailed understanding of the host rock.

Table 2 showed the safety functions and consequent geologicalcharacteristics that are sought in a geological environment thatwould be appropriate for disposal. In principle, these character-istics (e.g. low flow, stable geochemistry) could be found in manydifferent rock formations. Nevertheless, over the last 35 yearsduring which geological disposal has developed from a concept toreality, most countries have focussed their attention on threebroad groups of rocks as host formations:

� Hard ‘crystalline’ rocks such as granite, gneiss and othermetamorphic or plutonic rocks. Much work has been carriedout worldwide on granitic rocks of varying compositions andages, and on Pre-Cambrian shield rocks (in Canada, Sweden andFinland). These rocks display highly varying degrees offracturing and homogeneity, with some locations that havebeen investigated showing very low open fracture frequenciesand consequent small fluxes of groundwater. In the more highlyfractured formations, groundwater movement in intercon-nected fracture networks can be significant and this tends tobe a feature of shallower regions of hard rock (the upper fewhundreds of metres), especially in terranes that have beenglaciated. Hard rocks can provide a strong and stable hostformation that can be both laterally and vertically extensive,and is easy to construct in, with volumes of rock that can bevery dry.� Argillaceous sedimentary rocks such as clays, mudstones and

marls. Originally identified as potential host formations as theywere considered ‘impermeable’ (in the engineering sense of thewater or oil industry) and possessed of a high capacity to sorbany radionuclides that might be leached from the waste, theserocks are now recognised to provide an extremely high level ofphysical containment because their porewaters can be shown tobe essentially immobile, with no likelihood of flow systemsdeveloping within even relatively thin (tens of metres) forma-tions on the timescales of interest for GDF safety. Thischaracteristic has been demonstrated in the Jurassic andPaleogene clay formations being targeted in France, Switzerlandand Belgium, using environmental isotopic and chemicalcompositional profiles of their porewaters (Mazurek et al.,2008). As a consequence, any radionuclides that enter porewaters can only move by diffusion, at rates of only a few metresin tens or hundreds of thousands of years. These rocks have awide range of mechanical strength properties from plastic topartially self-supporting, that spans a variety of tunnel engi-neering requirements – the weaker rocks require massive linersif the tunnels are to remain open for some decades.� Evaporite formations; principally dome and bedded halites.

These formations, although they can be structurally andcompositionally complex in the case of dome salts, are oftencited as providing ideal containment properties. In homogeneousregions of either bedded or dome formations, there is essentiallyno fluid that is sufficiently mobile to transport radionuclides tothe surrounding rock formations. The rocks are plastic, such thatany openings are self-sealing over relatively short time periods,and salt displays a high thermal conductivity, to remove heatfrom HLW or spent fuel. Crushed salt can be used to backfill andseal regions of the GDF. These formations were the first to beidentified as potential hosts for radioactive waste disposal aslong ago as 1950 (NRC, 1957) and have been studied in manycountries, including Germany, Italy, the Netherlands and theUSA. The first purpose-built GDF to enter operation (in 2000) is ina bedded halite formation in New Mexico, USA (the WIPP facility:WIPP, 2011) and Germany has used two old salt mines in domehalite formations to dispose of radioactive wastes (Morsleben

and Asse), with a third under development as a site for disposingof highly active wastes (Gorleben). In addition, Germany hasused old salt mines for disposing of toxic and hazardousindustrial wastes since the 1970s.

Each of these groups has its own set of particular strengths andadvantages with respect to containment and isolation (see Fig. 4)and there is also a wide range of variability of these strengthswithin any one group and between specific sites that have beeninvestigated for disposal. In this sense, it is important to recognisethat adequate safety can be achieved by different balances ofcharacteristics and strengths of the safety functions of thegeological barrier. There is thus no unique solution that is the‘best rock’, the ‘best environment’ or the ‘safest site’.

It is common practice to represent the different characteristicsof host formations by the shorthand ‘THMC properties’: thermal,hydrogeological, mechanical and chemical characteristics. It can beseen that each of the containment requirements in Fig. 4 is basedupon different combinations of the T, H, M and C characteristics ofthe rock.

Because we are concerned with such long times into the future,the issue of continued geological stability is of central concern.Geological environments, and specific sites within them, areselected with a view towards both past and future stability – withevidence for the former being used as an indicator of likely futurestability and of the periods over which reliable forecasts of stabilitycan be made. In order for the EBS to function as intended it isimportant to locate the disposal regions of the GDF in a hostformation where dynamic processes (such as groundwatermovement or rock deformation) are extremely slow and wherethe hydrogeological, geochemical and mechanical conditionsrequired for containment (as discussed above) will remain asinitially characterised when the site is investigated. Rapidprocesses and changing conditions at depth make it hard todevelop a safety case for a GDF.

A GDF site and host formation needs to be stable with respect totectonic processes and events (see Connor et al., 2009 for acomprehensive overview) and to external environmental process-es – dominantly those associated with changing climate – indeed,the depths of several hundreds of metres chosen for GDFs arepartly to minimise the effects of changes and processes in thesurface environment. Tectonic and environmental processes canaffect the THMC properties of the deep environment and mightshift them away from the values around which the GDF disposalsystem is designed. When judging future mechanical stability ofthe rocks and the hydrogeological and chemical stability ofgroundwaters in the vicinity of a GDF, the likelihood of occurrenceand the nature of the following processes and events arecommonly evaluated:

� Rock deformation: all rocks deform slowly and it is important tounderstand which tectonic processes might lead to volumetricstrain or displacements in the host rock and the rates at whichthese can occur. In most environments, the tectonic drivingforces for slow deformation are well understood and the rateswill be so slow as to be of little relevance over a period of a fewhundred thousand years.� Seismic events: in the deep underground, in a backfilled and

sealed GDF, even extremely large magnitude events do not giverise to shaking that is a cause for concern with respect to EBSintegrity (see Stuckless, 2006, for a recent review of theliterature). However, it is critical to avoid sites where seismicallycapable fractures (i.e. large fracture zones – in this context, of theorder of kilometres or more in length – that may move and giverise to an earthquake) would run through or close to a GDF. Here,‘close to’ may mean hundreds of metres, depending on the


location. Although much of the strain of a major seismic event istaken up within such fracture zones, there is a potential for smalldisplacements on fractures some hundreds of metres away fromthe main displacement and these could damage the EBS. Caremust thus be taken in siting a GDF in a seismically active regionwith known active faults that are prone to large earthquakes.Even in areas not noted for seismicity, this can also be an issue, tothe extent that GDF design and layout decisions need to take lowprobability seismic events into consideration. For example, inSweden and Finland, large magnitude post-glacial earthquakeshave occurred on some major fracture zones in the otherwisehighly stable Fennoscandian Shield rocks (e.g. Lagerback, 1979;Olesen, 1988; Lund and Naslund, 2009). These events took placeduring the retreat of the last ice sheets, around 10,000 years ago,as a result of unloading of the stresses imposed by up to 3000 mof ice cover. Similar events could occur in future glacial cycles,which, with approximately 100,000 year average cycling, may liewithin the time period of concern for GDF stability.� Volcanicity: either direct intersection of a GDF by a new volcanic

vent, conduit or associated intrusion (dyke, sill) or peripheralhydrothermal impacts could have a major impact on GDFperformance, although this is only an issue in some countries onthe timescales of concern. The USA and Japan have been at theforefront in considering how to incorporate volcanic hazards intoGDF safety analyses or GDF siting. At the Yucca Mountain site inNevada that was, until recently discontinued, being developed asthe US national HLW and spent fuel repository, the potentialvolcanic hazard was not fully recognised until after the site hadbeen selected. Safety assessments then had to evaluate theimpacts of volcanic intrusion (by dyke or vent formation) on theGDF and, most importantly, the probability that these eventscould occur (Connor et al., 2000). Probabilistic volcanic impactassessment, similar to the approaches used for probabilisticseismic hazard assessment of nuclear power plants, had to bedeveloped (IAEA, 2010). Of course, the time periods underconsideration for the former were very much longer. In Japan,where a volunteer siting programme has been adopted, theapproach has been to avoid volcanic impacts by eliminating siteswhere the hazard is high. A simple yardstick has been used torule out sites that may be proposed if they are within a radius of15 km of an existing volcano that has been active in theQuaternary (NUMO, 2004). A major programme of work is underway to establish probabilistic risk assessment techniques forareas more than 15 km away, but which are in areas known(from tectonic first principles) to be susceptible to volcanism,although there has been no Quaternary activity (Chapman et al.,2009b).� Uplift/denudation: regions where slow tectonic uplift is occurring

can give problems for GDF siting, if uplift is accompanied byequivalent rates of erosion (McKinley and Chapman, 2009). Acombined uplift-erosion (effectively, denudation) rate of only1 mm/a can result in removal of a kilometre of overburden rockabove a GDF if sustained over one million years – a repositorylocated at 400 m depth would enter regions of de-stressed rockwith higher groundwater flow rates after only a few hundredthousand years. Significant uplift-erosion rates are thus to beavoided. Conversely, slow subsidence may add to the isolationprovided by the overlying formations.� Glacial erosion: in northern latitudes, the potential for localised

glacial erosion needs to be taken into account, especially in areasof moderate to high relief. Although it is unlikely to result in deeperosion (except on topographic features such as valleys, orcrustal weakness that are susceptible to major channelling of iceflow or sub-glacial water and consequent erosion), the localisedremoval or deposition of eroded material in a future glaciationcould affect groundwater movement at depth. Uniform erosion

rates, especially in low relief hard rock terrane, are normallyestimated to be in the order of metres per glacial cycle, which isinsignificant for GDF performance (Hildes et al., 2004).� Ice loading: the development of deep permafrost or the build-up

of hundreds of metres of ice in a future glaciation will havemarked impacts on key properties of the deep environment. TheEBS will be subject to increased loading stresses and groundwa-ter movement and chemistry will be modified in different waysin various regions of the host rock and surrounding formations(e.g. Jaquet and Siegel, 2006). The possible impacts of effects suchas flushing of the upper regions of transmissive fracture zones byfresh, oxygenated sub-glacial waters need to be considered –such effects may be encountered deep into certain rockformations. Permafrost development can extend to hundredsof metres, causing localised flow cells and surface dischargepoints to develop. Salt exclusion by freezing of deep groundwa-ter, apart from modifying hydrochemical conditions, can giverise to density-driven flow at depth. A detailed site-specificmodelling study of permafrost impacts for the chosen GDF site inSweden (Forsmark) is presented by Hartikainen et al. (2010). Insites that may be subject to ice cover or deep periglacial effects,the design of EBS components needs to account for such transientstresses and the possible impacts on EBS behaviour (such as theswelling behaviour of bentonite) have to be evaluated. Thepossibility of post-glacial fault movements was also mentionedpreviously. Current global climate forecasts, based on acombination of Earth’s orbital climate forcing factors andconsiderations of atmospheric CO2 scenarios suggest that thepresent interglacial (into which we progressively emergedduring the last 18,000 years since the last glacial maximum)may continue for at least 50,000 years and possibly up to 200,000years or more (e.g. BIOCLIM, 2001). If this occurs, then glacialimpacts would not be relevant in a safety evaluation until timeswhen the hazard potential of the waste would already beenormously reduced. Clearly, however, there is considerableuncertainty in global climate forecasts, so glacial cycle scenarioscontinue to be included in GDF safety assessments.� Sea level changes: one of the impacts of changing climate is to

modify global sea level. Increased global warming could raise sealevels by several metres but, at the peak of a major glaciation, theimpact is much more pronounced, with global sea levels fallingby around 150 m, as water is taken up into ice sheets. Changes insea level affect coastal erosion rates, surface drainage patterns,the down-cutting of river valleys and the discharge regions ofdeep groundwater systems. If a GDF is located close to thepresent day coast, changes in water flow and chemistry indifferent regions of the geological barrier could be pronounced.An additional factor during glaciations is down-warping ofEarth’s crust where it is subject to ice load, which would affectthe relative location of a GDF host rock volume with respect tosea level. Much of central and northern Sweden and Finland iscurrently rising as the land surface and present-day Baltic seabedrecover from the removal of the last ice sheets, with the effectthat the coastline is receding as land continues to emerge fromthe sea (e.g. Pimenoff et al., 2011). Although it may be importantto consider such scenarios, again the timescales of such changesneed to be considered with respect to the declining hazardpotential of the waste.

In order to establish whether a site will be suitable for a GDF, itis clearly necessary to evaluate whether there is any evidence forsuch dynamic processes and events having affected the region inthe past. This is one aspect of the extensive programmes of siteinvestigation that are necessary to characterise the geologicalenvironment in great detail to provide information for design ofthe GDF and for the forward modelling that is central to long-term


safety assessment. Not only must the basic geological character-istics of the host rock and surrounding formations be adequatelyunderstood, but also the THMC properties of the rocks, ground-waters and porewaters must also be investigated in detail and anintegrated picture built up of the dynamic evolution of the deepenvironment, in particular during the last tens of thousands to afew million years. Evidence to allow forecasting and likelihoodestimates to be made for dynamic events may require compilationand interpretation of observations made by many field, laboratoryand remote sensing techniques, at a huge range of spatial scales.Similar breadth of effort is required to assess slow, progressiveprocesses of deformation, flow and chemical evolution in the deeprock-water system. Synthesising all this information into acomprehensive understanding of the past and future evolutionof a site defines the level of confidence that can be assigned tosafety assessments that evaluate future GDF behaviour over longtimes into the future. Uncertainties abound in such evaluationsand this is a unique area of geosciences – no other area ofengineering development asks for such a multidisciplinary, multi-technique approach to make forecasts over such long times intothe future. Identifying, scoping and managing uncertainties is thusa key activity.

In general both practitioners of repository safety assessmentand the regulators who set standards of performance for GDFsacknowledge that detailed and quantitative predictions of behav-iour can only be made for periods of, at most, a few thousands ofyears. As we move further into the future, many uncertaintiesbroaden and, somewhere in the period 100,000 to 1 million years,it becomes difficult to make anything other than qualitativeforecasts of system behaviour. Nevertheless, studies of analogousnatural systems can give much confidence that isolation andcontainment can be maintained for enormous time periods – wellbeyond the times when any ability to make justified quantitativecalculations of system behaviour has lapsed (see Miller et al., 2001,for a comprehensive review). For example, uranium ore bodiessuch as Cigar Lake in Canada, located a few hundred metres belowthe surface, have chemical, mineralogical and geometricalsimilarities with a spent fuel GDF and have been stable for over1000 million years, exhibiting no radiometric signature at thesurface (Cramer and Smellie, 1994).

The ability to model system behaviour in massive, solid, slowlyevolving, engineered materials emplaced in a deep naturalenvironment selected for its stability and lack of dynamicprocesses is, of course, variable from barrier to barrier, fromdesign to design and from process to process. In a well-chosen andwell-characterised site, we might be highly confident, withminimal uncertainty, that the GDF will not be exposed by erosionor affected by rock deformation, volcanicity or other tectonicprocesses over the next 100,000 years. We can also say, with a highdegree of certainty, that over tens of millions of years almost anyGDF will eventually be eroded away and the degraded and decayedvestiges of the wastes dispersed into the oceans by naturalprocesses. Conversely, we have much less certainty about someaspects of the behaviour of the man-made components of the EBS.Consequently, both the design and regulations governing accept-ability need to take all of these factors into account in a sensiblemanner that matches hazard against time against uncertainty. Forseveral aspects of design and of the weight to be placed on specificsafety functions in a safety case, this argues for a carefully appliedconservative approach to be taken, allowing certain types ofuncertainties to be absorbed and accommodated.

5. How geological disposal is being implemented

As noted in Section 3, the design and location of a geologicaldisposal facility within a geological environment must take

account of a number of factors. In particular the facility mustdeliver the required degree of isolation and containment in respectof long-term safety, it must be safe and practicable to implement,and its implementation must take account of environmental andsocio-economic impacts. The focus of this paper is very much onthe long-term safety aspects but this does not mean that the othersare unimportant in decision-making. A good example is theexpressed desire in public consultations for ‘‘reversibility’’ or‘‘retrievability’’ where the design might be required to keep openthe option for as long as possible for future generations to retrievethe wastes from the facility in a relatively straightforward mannerif they so wished (e.g., NEA, 2010a,b).

When designing for long-term safety a key consideration is thepotential for transport of radionuclides from the wastes to thesurface environment and how best to combine man-madeengineered barriers with the natural barrier afforded by thegeological environment to prevent or minimise such transport.Typically the main potential for transport comes from radio-nuclides entering the groundwater system so the nature of thatsystem in a given environment is a very important consideration indetermining the engineering design. For long-lived, high activitywastes there is a requirement for a high degree of containmentover long periods of time: the design must take account of theextent to which the natural geological barrier can be relied upon toprovide absolute containment and complement this with thecontainment performance of one or more of the engineeredbarriers. Even in cases where the geological barrier supports noadvective flow and potentially limits the movement of solutes ingroundwater to control by physical diffusion, the GDF designsproposed to date envisage the use of a metallic waste containerthat will remain intact for thousands of years until most of theradioactive inventory in the wastes will have decayed.

As noted in Table 2, the geological barrier may not simplycontribute to containment in its own right. It can play an importantrole in the stage of evolution of the GDF after closure, when thewaste container is intended to be intact, by providing a benign andbuffered chemical and mechanical environment to minimisecorrosion and stress damage to the container. Thus the materialof waste containers will be chosen to have good corrosionresistance under the hydrochemical conditions of the geologicalenvironment. Conversely it is important that the materials of theEBS do not significantly impair the containment properties of thegeological barrier: substantial research has been carried out toensure that, for example, the reactions of alkaline leachate fromcement-based materials used in the EBS and in the construction ofthe GDF with rock minerals are well-understood and that the use ofthese materials will not compromise containment (e.g. Nirex,2002).

In geological environments where groundwater flow occursmainly in fractures, the engineering design may well includeavoidance criteria for the positioning of waste containers. Thesewould possibly be based on the flow properties of individualsmaller fractures intersecting a potential container position buttypically extend to exclusion of locating any disposal tunnels orvaults within a certain distance of larger fracture zones. Similarcriteria can be applied to protection of waste containers againstshear failure in the event of seismic activity where rockdisplacements are likely to occur on existing fractures and thesize of a potential displacement will be related to the fracturegeometry. For heat generating wastes, another important con-straint on design will be the ability of the geological environmentto conduct the heat away from the waste container and prevent arise in temperature that may be detrimental to components of themultiple barrier system. Thus, the spacings between containers ofheat-generating waste will be determined by the thermalconductivity of the host rock, with larger spacings and therefore


a larger disposal area being required for rocks with lowerconductivities (e.g. Hokmark et al., 2009).

The mechanical properties of the host rock are of courseimportant constraints on the dimensions and orientations of thedisposal tunnels and vaults. Typically, strong ‘‘crystalline’’ rockswill allow the excavation of relatively large spans of tunnels orvaults that will remain stable for extended periods of time withstandard rock support. In this situation, waste containers can bemanoeuvred relatively easily, allowing the designer a range ofoptions for their final disposition. In weaker rocks, the span is likelyto be limited and substantial rock support will possibly berequired: this, in turn, limits the designer’s options and manyconcepts in such rocks envisage emplacement of wastes inhorizontal tunnels having relatively narrow diameters. As canbe seen in Fig. 3, concepts for disposal of SF and HLW in allgeological formations predominantly use narrow diameter tunnels(typically around 3–5 m – although some large-span cavernconcepts are being considered and are also shown). On the otherhand, concepts for the generally larger packages (and in somenational programmes, such as the UK, much larger volumes) of ILWfocus on larger span tunnels or caverns. The mechanical propertiesof the rock must also be taken into account when considering thedisturbance to the hydraulic and mechanical properties that willbe caused by excavating the access to the disposal horizon and thedisposal areas themselves. In low permeability geological envir-onments the ‘‘excavation disturbed zone’’ potentially presents apreferential pathway for the transport of radionuclides ingroundwater that needs to be accounted for in the safetyassessment. In hard, crystalline rocks, it seems likely fromexperimental evidence in underground laboratories that the EDZcould be discontinuous and, in some clay formations it may be self-healing. Nevertheless, depending on the specific properties of thehost rock, the design of the facility may need to limit thisdisturbance, perhaps by including an engineered sealing system toblock off this pathway.

A few examples of different geological disposal concepts havebeen selected to reflect the range of designs developed in variouscountries appropriate to different types of host rock formation.

The KBS-3 concept developed in Sweden and Finland for thegeological disposal of spent fuel in granitic bedrock is based on theuse of a waste container comprising a cast-iron insert surroundedby a copper shell (see Fig. 3, top row, first image on the left). Thiscontainer will be lowered into a vertical deposition hole drilled inthe floor of a deposition tunnel. The deposition hole is to be linedwith highly compacted bentonite, which swells when contacted bywater, thereby bringing it into intimate contact with the wastecontainer and the rock wall. Under the hydrochemical conditionsin the bed rock, the copper shell is expected to remain un-breachedby corrosion for a period of order 100,000 years. Thus, the conceptrelies upon the containment properties of the copper shell and theprotection of the copper by the geological barrier. The bentonite‘‘buffer’’ plays an important role in limiting access of solutes ingroundwater to the copper shell and preventing microbial activity,which might perturb the benign conditions. In the event of failureof a waste container, the geological barrier will retard themovement of radionuclides in groundwater to the surfaceenvironment through a combination of chemical and physicalprocesses so that most will have decayed completely and thelongest lived will reach the surface only in small quantities.

France and Switzerland are developing concepts for thegeological disposal of spent fuel and high-level vitrified waste inJurassic sedimentary formations (the Callovo-Oxfordian Clay inFrance and the Opalinus Clay in Switzerland). Both concepts arebased on the use of an outer thick-walled, carbon steel wastecontainer. This container will be placed in a horizontal disposaltunnel or borehole and, in the Swiss concept, the space between

the container and tunnel wall filled by a thick bentonite clay buffer(see Fig. 3, top row, third image from the left). Investigations of therespective geological formations have provided conclusive evi-dence that the movement of solutes in groundwater has beencontrolled by physical diffusion over millions of years. Therefore,these concepts place a strong reliance on the containmentpotential of the geological barrier. Nonetheless the thick-walledcarbon steel waste container has been shown to corrode veryslowly under the hydrochemical conditions found in theseformations and it is likely to remain intact for many thousandsof years. Therefore, only the very long-lived radionuclides in thewaste will be released into the geosphere at depth.

Germany is planning to operate a GDF for non-heat generatingwastes within a couple of years at the former Konrad iron ore minewhere the overlying Mesozoic clay formations provide contain-ment potential in much the same way as described for the Frenchand Swiss concepts. However, for the disposal of heat-generatingwastes Germany has for a long time studied the potential of saltdomes. Various conceptual designs have been developed but withthe common basis of using the high containment potential of haliteformations, as described in Section 4. Thick walled carbon steelwaste containers would be placed in deposition holes or shafts andsurrounded by crushed halite (see Fig. 3, top row, third image fromthe left). Halite exhibits plastic creep behaviour, which isaccelerated at elevated temperatures, so the concept envisagesthat the immediate environment of the waste container willrapidly be restored to the undisturbed condition. As in the case ofthe French and Swiss concepts in low permeability rocks, the thickwalled container is expected to remain intact until shorter-livedradionuclides have decayed away.

The USA has operated the Waste Isolation Pilot Plant (WIPP) forthe geological disposal of transuranic wastes from its defenceprogramme for over ten years. This GDF is based on thecontainment potential of a Permian bedded halite formation.Wastes in steel drums are stacked in disposal vaults excavated inthe halite. In a relatively short period of time the salt formationencloses the wastes (and buffer) through natural creep processes.In view of the long-lived nature of the radionuclides in the wastesand the containment properties of the salt formation, the wastecontainers do not have a long-term safety function. However, incommon with other low permeability host formations there is apotential for disruption through over-pressurisation caused bygases released from waste materials by chemical or microbiolog-ical reactions. To prevent a possible over-pressurisation bymicrobially generated carbon dioxide the waste containers aresurrounded by a magnesium oxide ‘‘buffer’’ designed to consumeany carbon dioxide released.

The national programmes responsible for these exampleconcepts were all initiated between the mid-1970s and early1980s. They have encountered scientific and technical problems intheir development of concepts, in some cases causing a majorchange of direction, but the most significant issue held in commonconcerns the socio-political nature of the siting process. Thewillingness of the local community to host a disposal facility onbehalf of the nation and hence the legitimacy of the siting processnow receives great attention as a result of this experience.

A detailed account of each programme is beyond the scope ofthis paper, but it is worth commenting on important develop-ments.

For the KBS-3 concept pursued in Sweden and Finland, a keytechnological issue has been the capability to produce severalthousand copper shells for the waste containers and, in particular,to effect the closure weld to the quality consistent with therequired containment performance. An Encapsulation Laboratorywas established to develop and demonstrate reproduciblecontainer fabrication to the required standards. The flow of


groundwater in fractures in the granitic bedrock is important tothis concept. An innovative borehole logging device was developedby the Finnish waste management agency, Posiva, to detect andcharacterise flowing fractures (Rouhiainen, 2008). Following trialsat the Aspo Hard Rock Laboratory, constructed to support theSwedish programme, measurements taken with this device atprospective GDF sites were coupled with specially developedgroundwater flow modelling techniques to simulate tracks ofparticles travelling through the groundwater system from disposallocations (Svensson, 2001). In Finland, a preferred site has beenselected near the Olkiluoto nuclear power plant and is beinginvestigated in detail through an underground rock characterisa-tion facility ‘‘ONKALO’’. Posiva will be applying for a constructionlicence in 2012 and, provided that conditions at depth are shown tobe suitable, disposals of spent fuel are planned to start around2020. Sweden is following a similar path and has recentlyannounced a preferred site near the Forsmark nuclear powerplant. Subject to the outcome of reviews of the recently submittedsafety case ‘‘SR-Site’’ (SKB, 2011) by the statutory regulators, theEnvironmental Court and other bodies, it is planned to conductdetailed underground investigations leading to a start to disposalsof spent fuel around 2022.

In France, a revised approach to siting was embodied inlegislation in the early 1990s, following earlier public protests. Thisemphasised the role of an underground research laboratory (URL)in potentially suitable geological environments, to be located inadministrative areas expressing willingness to host such a facilityon condition that the URL would not be developed subsequently asa disposal facility. This led to the investigation of the JurassicCallovo-Oxfordian Clay in NE France and the subsequent develop-ment and operation of a URL in that formation at Bure. Theinvestigations in and around the URL have led to greatly improvedunderstanding of important THMC processes that occur in lowpermeability sedimentary formations. The extensive nature of thebasin in which the clay formation is located will allow the scientificinformation obtained from these investigations to be transposed toa potential GDF site in the same region, subject to the willingness ofthe local community to host such a facility. A programmeembodied in recent legislation envisages the start of disposal ofhigh-level, long-lived wastes in 2025.

The Swiss programme has been characterised by a systematicprocess of periodic feasibility studies, initially in the mid-1980s, todemonstrate that there was a safe technological method fordealing with the wastes from the country’s nuclear powerprogramme and thereby justifying its continued operation. Atthat time, much of the focus of the programme was on crystallinehost rocks but, with the advent of improved geophysical andgeochemical techniques for characterising sedimentary rocks, andthe consequent building of confidence in their long-term contain-ment potential, the focus is now on sedimentary rocks and, inparticular, the Opalinus Clay formations that are found inSwitzerland. Recently, a siting process has been agreed. Much ofthe extensive international R&D programme conducted in theMont Terri URL is expected to be transferable to prospective siteswith clay host rocks.

In Germany, the knowledge gained from research intoradioactive waste disposal at the disused Asse Salt Mine and fromthe disposal of chemo-toxic industrial wastes in salt domes led to astrong focus on location of a GDF for heat-generating wastes inhalite. A salt dome at Gorleben was investigated in great detail,with two access shafts being sunk to the depth of a prospectivedisposal facility. All aspects of the German waste managementprogramme have been subject to political headwinds andinterventions, reflecting a long-standing national aversion tonuclear energy in some influential sectors, and a ten-year politicalmoratorium was declared on further work at the site. A substantial

review was initiated on the respective merits of crystalline,sedimentary and evaporite host rocks for the disposal of heat-generating wastes. This moratorium is now over and the nuclearindustry wishes to move ahead with developing Gorleben site asthe national waste repository.

As already noted, the WIPP facility, located in bedded halite inNew Mexico, USA, has been receiving long-lived radioactive wastesfor disposal for over a decade. The regulatory certification requiredto operate the facility has to be re-applied for every five years andthe applications take account of any improvements to disposalpractices that can be made in the light of operating experience,including the creep behaviour of the salt surrounding the wastes inthe excavated disposal tunnels, and of the results from ongoingsupporting R&D. The Obama administration halted work on theprospective disposal of spent fuel in unsaturated tuffs at YuccaMountain in Nevada, which had been the subject of a substantialprogramme of site investigations, R&D and underground investi-gations for over 20 years, and announced a ‘‘Blue Ribbon’’ enquiryinto the long-term management of the nation’s spent fuel. As thispaper goes to press the findings of this enquiry have beenpublished. They appear to re-affirm support for geological disposalas the preferred method for safe management of spent fuel wastesin the USA.

5.1. Addressing uncertainties and reaching a safe solution

As has been discussed in Section 4, the predictability andforecasting of the evolution of both the natural, geological barrierand of the engineered barriers of a GDF are subject to uncertainties.Of course, these need to be accounted for in the first instance whenaddressing safety during the open, operational period of a GDF,when the hazard potential of the wastes is highest, but when thesystem is under active control. In this paper, however, we areprincipally concerned with the post-closure period, when the GDFis functioning passively. The uncertainties increase with the lengthof time after control over conditions in the facility is relinquishedbut, as shown by Fig. 2, the radiological hazard of the wasteinventory declines with time, through the process of radioactivedecay. It is of great importance in making a safety case for a GDFthat uncertainties are properly characterised and accounted for.

There is a great deal of academic work on the classification ofuncertainties in a general sense but, for the purposes of geologicaldisposal, the following sources of uncertainty are often identified:

� Uncertainty about the value of a parameter of interest.� Uncertainty about a safety-relevant process and how this should

be described in a conceptual model.� Uncertainty about future human behaviour.� Uncertainty about the future evolution of the geological system.� Uncertainty about the mathematical models used to calculate

the safety and environmental consequences of any releases ofradionuclides from the GDF.

Uncertainty about the value of a parameter of interest, such asthe solubility of a radionuclide in groundwater, can derive from anumber of sources, including experimental measurements or thespatial and/or temporal variability of the chemical conditions thatcontrol the solubility. There are relatively standard methods fordealing with parameter uncertainty, involving either probabilisticsampling of the range of possible values, usually expressed as aprobability distribution function for this purpose, or exploring theconsequences of using different values for the parameter inindividual deterministic calculations. Irrespective of the methodchosen to deal with the uncertainty in terms of its consequences, itis most important that the full range of uncertainty is properlyidentified at the outset and this typically involves a formal process


involving recognised scientists having an awareness of all relevantscientific information.

Uncertainty about a safety-relevant process, such as the flow ofgroundwater through a fracture network in crystalline rock and theretardation of dissolved radionuclides by interactions with therock, is typically addressed by developing a range of so-calledconceptual models that span the range of possible descriptions ofthe way in which the process operates, consistent with empiricalobservations of the process in the field or laboratory. Theconsequences of these alternative conceptual models can thenbe analysed in a safety assessment.

For some aspects of safety, the interaction of a number ofprocesses may require consideration. A good example concerns thegeneration of gas in the repository and its subsequent migrationout of the engineered system and in the geosphere. Particularly forthe case of typical intermediate-level wastes, there is the potentialfor the generation of hydrogen gas by corrosion reactions whengroundwater contacts certain metallic waste materials or wastecontainers, and for the generation of carbon dioxide and methanefrom microbially mediated reactions involving waste materials. Inorder to calculate the volumes of gases generated, and their rate ofgeneration, information is required on a number of chemical andmicrobiological reactions under repository conditions and on theavailability of groundwater to the reactive materials (see, forexample, NDA, 2011a). The generation of gas has the potential tocause over-pressurisation of the engineered system, subject to theextent to which it would, variously, dissolve in groundwater, reactwith a component of the engineered system (as in the case ofcarbon dioxide and cement) or disperse into the pore spaces of theengineered barrier materials and any excavation disturbed zone inthe surrounding rock. The fate of the gas remaining as a discretephase would be determined by the two-phase gas/water char-acteristics of the repository host rock and any overlying geologicalformations. In the case of low-permeability, argillaceous sedimen-tary rocks, or indeed clay-based engineered barriers, it is suggestedthat pressure is relieved by the formation of micro-fissures that‘‘heal’’ after the pressure is relieved, because of the intrinsicproperties of the rock or clay and the compressive forces acting atdepth. In this way, the micro-fissures would be a transient featureand not represent a ‘‘fast pathway’’ for groundwater flow (see, forexample, Nagra, 2004). There are potentially significant radiologi-cal consequences from the localised release of gas containingcarbon-14, which will be mediated by the uptake of carbon-14 ingaseous form into soils, water, flora and fauna in the biosphere.Thus, in order to develop a suitable repository design to deal withgas generation, and to develop a robust safety case, an adequateknowledge is required of how these often complex physical,chemical and biological processes may interact. Many of thescientific challenges in this area are held in common with othermajor environmental protection initiatives, including, notably,carbon dioxide sequestration.

Uncertainty about future human behaviour is important in tworespects. As will be discussed later, the long-term safety standardfor a GDF is typically expressed as a radiological risk or dose to anindividual living near the GDF at some distant time in the future,when radionuclides might be transported from the disposal systemat depth to the surface environment. Although this is clearly ahypothetical construct, since there is no way of knowing if and howanyone would be living in such a location many thousands of yearsin the future, it is important to assign habits to this hypotheticalindividual, particularly concerning sourcing of food and water, totest whether an appropriate level of protection will be afforded bythe design and location of the disposal facility. This is addressed bydeveloping a stylised approach, usually with the strong involve-ment of national environmental and safety regulators, defining arepresentative individual member of a group of people living in the

location of highest potential exposure to radiological risk andsourcing all their food and water requirements from the localenvironment. The second aspect of future human behaviour thathas to be considered concerns the potential for people, having noknowledge of the location and hazard of the disposal facility, todisrupt its containment barriers inadvertently. This might happen,for example, by drilling into the facility in search of naturalresources. The chances of this happening are greatly reduced byensuring that a disposal facility is not sited where there is knownresource potential. However, it is typically considered necessary todevelop scenarios to cover the range of credible and realistic futurehuman actions that might lead to disruption of the facility in thisway. The radiological harm that would result to a person carryingout some aspect of the disruptive action can then be assessed, aswell as harm to people living in the vicinity of the disposal facility.

Uncertainty about the future evolution of the geological systemis usually dealt with by developing scenarios to cover the range ofpossible evolutions and then exploring the containment perfor-mance of the natural, geological barrier and of the engineeredbarriers, under the conditions imposed by the different scenarios.As explained in Section 4, at time periods of more than a millionyears or so into the future, significant changes will occur and theuncertainties are sufficient that a conventional forward modellingapproach cannot be relied upon to forecast performance mean-ingfully. However, by this time, all but the longest-lived radio-nuclides in the wastes will have decayed and the remainder of theinventory will have radiotoxicity characteristics similar to anaturally occurring uranium ore deposit.

Uncertainty about the mathematical models used to calculatethe consequences of any releases of radionuclides is dealt with byrelatively standard methods. The models are subject to verificationand quality assurance procedures that are commonplace inmodern computing practice, including inter-comparison betweenthe outputs from different models that are designed to carry outthe same assessment exercise. Increasingly, simpler analyticalmodels are being developed to model the totality of the disposalsystem, or compartments of it, so that the outputs of the moredetailed mathematical models can be compared with the relevantoutputs from these (e.g. the ‘‘INSIGHT’’ model developed by Nirex;see Nirex, 1997).

The continual analysis of safety-relevant uncertainties isimportant in informing decisions on disposal facility design andsiting. The choice of materials and designs for engineered barriercomponents seeks to ensure that the performance of the disposalsystem is robust to any uncertainties about the evolution of thecomponents. In the case of the natural, geological barrier, theuncertainties about the movement of radionuclides in groundwa-ter can be significant in determining the suitability or otherwise ofa prospective site, so the ability to characterise the rock-watersystem adequately, so as to constrain such uncertainty, is animportant consideration.

5.2. Regulatory targets for post-closure GDF safety

The level of protection to be afforded to the environment andfuture human populations is set by national regulations, which inturn are developed taking account of international standards andguidance. A key question is the period of time for which it isreasonable to demand assurance of a given level of protection, asthe uncertainties about the evolution of the disposal system, andhence its containment performance, increase with time.

Internationally, the quantitative regulatory standards for long-term safety are expressed in terms of an annual radiological risk ordose to a representative individual member of a group of peoplewho would potentially be exposed to the greatest releases to thesurface environment of radionuclides derived from the wastes. In


most cases, these standards are set at a level equivalent to between0.1 and 1% of the annual radiological risk to which every individualis currently exposed through the naturally occurring radiationbackground. This approach is based on studies of intergenerationalequity and the principle that this is a level of exposure from anartificial source that society finds broadly acceptable (NEA, 2007).In order to deal with the uncertainties about human behaviour andthe precise nature of the biosphere at long times in the future,some national regulations set different standards that apply, attimes 10,000 years or more after closure of a disposal facility. Theseare based on allowable levels of release to the surface environmentof given radionuclides. In addition to such standards that apply tothe normal evolution of the disposal system, further standards areoften developed for the protection of people who would potentiallybe exposed as a result of human disruption of the disposal facility.Recently, there has been an increased recognition of the need toprotect future ecosystems themselves from radiological harm andstandards have been derived for the protection of a number ofreference living organisms for each main type of habitat (Brownet al., 2008).

Although it is clearly important to show that the disposalsystem can comply with these numerical standards, this can leadto an undue focus on the eventual release of radionuclides from thedisposal system rather than on the quality of the system inpreventing their release. Geological disposal facilities are designedto ensure complete containment of the highly radiotoxic relativelyshort-lived radionuclides within the engineered barrier systemand, taking account of the relevant uncertainties about the normalevolution of the system, can reasonably be expected to preventrelease of radionuclides to the surface environment for 10,000years or more. As noted in the descriptions of some typical disposalconcepts for the most highly radioactive wastes, either the wastecontainer or the natural, geological barrier can potentially affordcomplete containment for a period of 100,000 years, but this issubject to greater uncertainties. Increasingly, safety cases forgeological disposal facilities explore the consequences of hypo-thetical losses of safety functions of barrier components and,reassuringly, show that for a well-designed disposal facility in asuitable location, only unlikely combinations of losses of multiplesafety functions would lead to potential radiological exposures ofpeople, equivalent to those received from natural backgroundradiation (e.g. SKB, 2011, Section 13.7.3).

These considerations are intimately linked with the question ofthe stability of the geological setting that was discussed in Section4. Of course, no geological system is stable in the sense of being atequilibrium, such that steady-state conditions prevail over longperiods of time. However, the concept of stability in the context ofgeological disposal does imply that the changes that occur in thegeological system do so to an extent and at such a rate that theireffects are unlikely to compromise the long-term safety of thedisposal system.

The stability of the geological setting for a disposal facility canbe accounted for in respect of some more extreme potentialperturbations, for example, large earthquakes or, where relevant,volcanism, by using siting criteria, as described in Section 4.However, the safety functions of the disposal system have to bedefined with respect to the less extreme evolutionary processesand events that may reasonably be expected to occur at a givensite.

In the case of highly impermeable sedimentary or evaporitichost rocks, the groundwater conditions might reasonably beexpected to be well-buffered against changes in conditions at thesurface induced by climate change, for example, and the mainsource of change in the host rock itself would be caused by slowprocesses operating over timescales of the order of one millionyears or more. These rocks are potentially more susceptible to the

impacts of introducing the wastes, in particular if gas over-pressures are generated. The engineering design has to bedeveloped to ensure that these impacts are suitably limited.

In rocks characterised by more dynamic groundwater flowconditions, including more permeable formations overlying highlyimpermeable host rocks, the hydrochemistry and mineralogy willrespond to changes in conditions at the surface. In this case, it isimportant to gain an understanding of the range of conditions thatwill prevail over the timescales of interest and ensure that thesafety functions, such as corrosion resistance of waste containers,or retardation of radionuclide transport in the geosphere, are notcompromised.

Climate change, inducing ice loading and unloading orpermafrost, represents the most significant potential source of achange of conditions that could affect the groundwater system ontimescales of interest before the radiotoxicity of the wastes willhave decayed to the equivalence with a uranium ore body. Wheresuch climate change is reasonably expected to occur, in higherlatitudes, possibly tens of thousands of years in the future, thedisposal system has to be resilient to a range of effects, in particularincreased compressive stresses, increased pore pressures orincreased hydraulic gradients.

5.3. Non-technical issues

The R&D effort directed, over more than 30 years, to addressingthe scientific and technical challenges outlined in this paper nowprovides a sufficient scientific and technical basis for manycountries to be well-advanced towards implementing geologicaldisposal of the most highly radioactive wastes. However, geologi-cal disposal is not a purely technological challenge and a failure torecognise the importance of socio-political aspects, particularlyconcerning the selection of a site for a GDF, historically causedsignificant reverses or delays in a number of national programmes.The willingness of a community to host a GDF is now recognisedalmost universally to be of paramount importance in the siteselection process. Although the details of site selection processesdiffer between national programmes there are a number ofcommon elements:

� The site selection process is arrived at through national debateand consultation and is made readily available along withinformation on the inventory of wastes requiring disposal andthe principles of safe geological disposal.� Communities are invited to express an interest or to volunteer to

participate in the site selection process.� Participation in the process is appropriately supported by supply

of information and resources so that the community is well-informed.� There is continual dialogue between the local community and

the prospective developer, and other organisations withdesignated roles in the process, so that the community canhave access to and influence decision-making.� All decision-making is characterised by openness and transpar-

ency.

The overall objectives are to conduct the site selection processthrough partnership with local communities and to ensure that thewell-being of the involved communities is enhanced rather thanharmed by participation in the process.

5.4. The international perspective

Work on all aspects of the geological disposal of high-activity,long-lived radioactive wastes has been subject to a remarkablelevel of international cooperation and information-sharing. The


results of a worldwide R&D effort, valued at well in excess of tenbillion pounds sterling, are readily available in published reportsand scientific papers. This situation recognises, of course, that inmost countries all the high-activity wastes produced to date can bemanaged through just one national GDF, so there are considerableadvantages in comparing ideas and information with one another.However, the nature and quantities of wastes, the types ofgeological settings available and the geological stability of thosesettings can be markedly different in the various countries, so thereis not a single best solution in the form of a universal geologicaldisposal concept. Therefore, each national programme generallyseeks to identify those aspects that are specific to it and for whichwork needs to be commissioned, but also to identify wherecooperation with other national programmes will produce a morecost-effective and efficient approach. The international pro-grammes of work in URLs provides the most striking example ofthis type of cooperation, but other forms of cooperation take place,such as the sharing of other specialised facilities or resources,developing and sharing scientific databases and secondingscientists and engineers to other programmes.

A number of countries have relatively small nuclear powerprogrammes, or no nuclear power at all, but still hold some higheractivity, long-lived industrial or research-generated radioactivewastes, so that just a small amount of wastes will eventuallyrequire geological disposal. This potentially implies that each ofthese countries will have to incur the considerable burden ofestablishing a GDF to deal with the wastes. Although the export ofradioactive wastes from one country to another is subject to strictinternational and national legislation and regulation, consider-ation is being given to the feasibility of developing shared, regionalGDFs, where the wastes from a number of neighbouring countriescould be disposed of in a single GDF. In Europe, this possibility isrecognised in the 2011 European Council Directive on radioactivewastes (EC, 2011) and is being explored by the EuropeanRepository Development Organisation Working Group (ERDO,2011). Clearly, this approach introduces further socio-politicalconsiderations, over and above those for the case of a countryhaving only a national programme for disposal of its own wastes.

5.5. Current developments in the UK

The UK began research into the geological disposal of high-levelradioactive waste (HLW) shortly after the publication of the RoyalCommission on Environmental Pollution report on nuclear powerand the environment in 1976 (the Flowers Report; RCEP, 1976),recommending this solution and proposing that a R&D programmegot under way. Similar programmes began in several Europeancountries in the late 1970s. At this time, the intention was toreprocess all UK spent fuel and no consideration was given todisposing of it directly. An extensive programme of field andlaboratory studies began, but the need to carry out field-basedresearch in a range of geological environments precipitatedconsiderable local opposition to the essential drilling programmesand the government of the day cancelled the R&D programme in1981. In this period, although there was work on defining andstudying potentially useful UK rock formations, there was noproject that aimed at locating a specific repository site.

Following cancellation of the HLW R&D programme, the UKposition for the next twenty years was that there was no urgentrequirement to dispose of HLW, which needed to cool for around 50years before it could be emplaced in a repository, and that a watchingbrief would be maintained on R&D developments in other countries.With the support of Government the nuclear industry establishedthe Nuclear Industry Radioactive Waste Executive (NIREX) in 1982,charged with the responsibility initially for managing sea disposalsof some low-level and intermediate-level wastes (although this

practice was almost immediately discontinued) and for developingland-based disposal facilities for these categories of waste in thelonger term. The organisation was subsequently incorporated as alimited company, United Kingdom Nirex Limited (‘‘Nirex’’) in 1985.Over this 20-year period, considerable advances were made in anumber of countries and several geological disposal concepts forboth HLW and SF moved from a simple conceptual level to well-researched engineering solutions, underpinned by extensive testingand safety analyses.

In 2001, following a number of failed projects to locaterepositories for lower activity wastes in the intervening 20 years,Government began a concerted project to address the legacy of UKwastes from operating and decommissioned nuclear facilities. Itinitiated the current Managing Radioactive Waste Safely (MRWS)process, formed an advisory committee, the Committee onRadioactive Waste Management (CoRWM), on how to moveforward with management of the wastes and, in 2005, establishedthe Nuclear Decommissioning Authority (NDA) with the responsi-bility for the decommissioning and clean up of publicly ownednuclear facilities and sites. In October 2006 the governmentaccepted CoRWM’s recommendations for geological disposal forhigher activity wastes (published in July 2006) and made the NDAresponsible for implementing this policy. Consequently, the NDAbecame responsible for all aspects of managing the UK’sradioactive wastes and, in April 2007, Nirex’s skills and knowledgebase were integrated into the NDA, which established a Radioac-tive Waste Management Directorate specifically to implementgeological disposal of higher activity wastes.

Following a public consultation on its proposals for implement-ing geological disposal the Government issued a White Paper inJune 2008. Building on the experience of other countries, this laidout in some detail how the approach to site selection would bebased on voluntarism and partnership. The site selection process isdesigned to progress in stages as shown in Fig. 5. The first stage wasimplemented with the publication of the White Paper in the formof an invitation from Government for communities to express aninterest in participation in the site selection process.

To date three local government bodies have expressed such aninterest, Copeland Borough Council, Allerdale District Council andCumbria County Council. Stage 2 of the MRWS site selectionprocess has been completed in respect of these communities in theform of the identification of any unsuitable areas of the deepgeology underlying the land covered by the Copeland and Allerdaleadministrations (Powell et al., 2010). This analysis was performedwith reference to sub-surface screening criteria published in theWhite Paper, which emphasised the need to avoid areas that haveknown resource potential including the need to avoid siting a GDFwithin an aquifer (DEFRA et al., 2008, Annex B). The communitieshave formed the West Cumbria MRWS Partnership to work withGovernment and others at these early stages of the site selectionprocess and this partnership is currently deliberating on the nextstage (Stage 3), a decision on whether to participate in the siteselection process which will lead to the community identifyingpotential candidate sites for evaluation in Stage 4.

In parallel with the development of policy on geologicaldisposal of higher activity radioactive wastes, the Government wasalso developing policy on the possible construction of new nuclearpower stations, as a low-carbon contribution to secure energysupplies within the next decade. Currently, it is planned that thewastes from such new nuclear power stations would be placed inthe same geological disposal facility as developed for legacy wastesfrom the currently existing nuclear facilities, provided that thisdoes not have unacceptable environmental and safety implications(BERR, 2008). The Government is also planning on the basis thatspent nuclear fuel from the new reactors will not be reprocessed inorder to recycle the uranium and plutonium it contains. The latest

Fig. 5. The stages in finding a location for and developing a geological repository in

the current UK ‘Managing Radioactive Wastes Safely’ programme, which is

overseen by the Department of Energy and Climate Change (DECC); see DEFRA et al.

(2008).


generation of power reactors that are under consideration produceconsiderably smaller volumes of operational and decommissioningradioactive wastes. Combined with the presumption of noreprocessing, the volumes of waste to be managed will beconsiderably less than from historical nuclear power generationfor a given power output. A significant feature of the newgeneration of power reactors is the greater amount of powerextracted from the fissile material in the fuel. This means thatsmaller amounts of fuel are required to generate a given amount ofpower but, when discharged from the reactor, the spent fuelcontains higher levels of fission products and correspondinglyemits greater amounts of heat. This could mean that, if the chosenGDF safety concept was based upon a specific temperature limit inthe engineered barrier system, each spent fuel element wouldrequire a larger area for disposal than envisaged for fuel elementsfrom currently operating reactors, or that the spent fuel wouldrequire a longer cooling period in interim storage before disposal(or that the GDF concept would need to be adapted). Fromassessments carried out to date this appears to be the mostsignificant difference between the wastes from current andpotential future power reactors with respect to their geologicaldisposal. Therefore, provided that a site can afford the appropriatevolume of suitable rock, the disposal of wastes from new buildreactors is unlikely to pose significant new challenges. Neverthe-less, it is to be expected that, whatever the final waste inventory,the technology of geological disposal will require regular updatingin response to the future evolution of energy and environmentpolicies and associated industrial practices and developments.

The UK Government recently launched a consultation on thelong-term management of the nation’s stockpile of separatedplutonium (see Table 1) in which the preferred option was re-useof the plutonium as so-called ‘‘mixed oxide’’ (i.e. plutonium anduranium oxide) or ‘‘MOX’’ nuclear fuel in new nuclear powerstations (DECC, 2011a). However, some of the plutonium stockpilemay not be suitable for re-use in this way. Therefore, theGovernment has proposed that the NDA will examine optionsfor the immobilisation of this material to create a waste productsuitable for safe, secure and cost-effective geological disposal. TheNDA has also been tasked with assessing the geological disposal ofthe spent MOX fuel that would result from the re-use of theplutonium.

When civil nuclear power was first introduced in the 1950sthere was little concern about the ability to manage the resultingradioactive wastes in the long term. The modern requirement forsustainable energy policy means that considerable attention isnow being given to the availability of technologies and facilities tomanage the wastes from the proposed new nuclear power stations.As part of a regulatory ‘‘Generic Design Assessment’’ of theproposed types of new nuclear power stations, prospectivevendors have commissioned disposability assessments for theradioactive wastes that would result from the operation anddecommissioning of the nuclear reactors (NDA, 2009a,b). Animportant element of Government policy is to ensure that thenuclear operators, and not the tax payer, will meet the full cost ofmanaging the wastes. A suitable cost model for a viable geologicaldisposal facility is required in order to determine realistic unitcosts that are transparent to all concerned in establishing thenecessary financial measures (DECC, 2010).

To date, intermediate-level wastes typically have beenimmobilised in cement-based grouts which can afford favourablephysical and chemical containment properties for the safedisposal of the wastes. However, there is increasing interest inalternative immobilisation technologies which may represent abetter environmental option for certain wastes. There is particularinterest in the use of organic polymers to immobilise reactivemetal wastes in a chemically inert matrix and in the incorporationof wastes in a glass waste form, which can potentially incorporatehigh concentrations of waste whilst being highly resistant toleaching by groundwater (NDA, 2011b, Sections 4.2.2/4.2.3).There is also interest in disposing of some intermediate-levelwastes, without immobilisation, in disposable cast-iron contain-ers which can also serve to provide stand-alone interim storage ofwastes prior to disposal, in line with practice in some othercountries (e.g. NDA, 2009a, Section 3), The time at which ageological disposal facility will become available to receivewastes is, of course, very important to the plans and costs both ofcleaning up and decommissioning existing nuclear sites, and ofoperating existing and future nuclear plants. Based on informa-tion supplied by the NDA (NDA, 2010a), the UK Governmentshows at www.decc.gov.uk an indicative implementation time-line for geological disposal leading to the first emplacementunderground of intermediate-level waste in 2040 (other wastesstarting to be emplaced underground at later dates). CharlesHendry, Minister of State at the Department of Energy and ClimateChange has recently stated that the first annual report on theMRWS programme shows that good progress is being made, butwent on to say (DECC, 2011b), ‘‘I still want us to be ambitious in our

timescales for delivery by setting a goal of putting the first waste into a

disposal facility by 2029. I have tasked the NDA to look at

opportunities to achieve this.’’These represent some of the more significant challenges to be

addressed by the scientists and engineers engaged in supportingthe implementation of geological disposal, whilst ensuring properregard for the socio-political context.

http://www.decc.gov.uk/


6. Conclusions

Geological disposal of radioactive wastes was first mooted bygeoscientists working with nuclear scientists more than 50 yearsago, but has taken decades of intensive scientific, political andsocietal effort to get to the current point, where several countriesare within a few years of realising their own national repositoriesfor wastes from the nuclear power generation industry. Geologistshave been involved in these developments from the outset, withalmost every field of the geosciences having something tocontribute. Building a safety case for geological disposal relieson a detailed understanding not only of the deep rock environmentwhere a GDF will be built, but also of the processes that have led toits past hydrogeological, chemical, thermal and mechanicalevolution and of how these processes will work together tocontrol its future evolution. This understanding encompassesinteraction with the surface environment, which is itself evolvingat a more dynamic pace, controlled by climate driven processesand impacted by our own behaviour.

Understanding of the important functions of the various hostrocks and surrounding formations that are being considered for aGDF has also evolved. Thirty years ago, much emphasis was placedon their role as a barrier to the migration to Earth’s surface ofradionuclides dissolved in groundwaters, and extremely long timeperiods were being assessed – out to hundreds of millions of yearsin some studies. Today, with the minute radiological impacts thatmight occur in the far future being seen in their relativeperspective as only potential contributors that are small relative

Fig. 6. The immense containment capacity of a clay formation or a copper container in the

common ancestor to populate the whole Earth, which began about 150,000 years ago (B

Oppenheimer, 2003; base map from: http://english.freemap.jp/). The times at which mo

Adjacent to each green dot is the typical distance that a non-reactive, weakly retarded rad

hosting a waste repository in the same period of time. It can be seen that for the whole p

typically move less than 10 m (data on typical diffusion timescales from Nagra, 2003, for t

of a copper container surrounded by a bentonite buffer in reducing conditions, on the s

whole period, this amounts to less than 3 mm.

to other, ever-present, natural environmental hazards, a morerealistic appreciation of the role of the geological barrier isemerging. This is coupled with, perhaps, a more rationalunderstanding of what is realistically achievable by geologicaldisposal. The rapidly declining hazard potential (over ‘historic’timescales of thousands of years rather than geological timescales)nicely balances an increasing uncertainty in forecasting systembehaviour with the passage of time and the total system remainsrobust even as containment deteriorates in a progressively lesspredictable way. Thirty years ago, safety modellers wanted tomake quite detailed predictions of impacts out to many millions ofyears – today practitioners and regulators are content to makebounding forecasts on much more reasonable timescales ofthousands of years that are consistent with the timescales ofdeclining radioactive hazard potential. Nevertheless, forecastingover these time periods has to be couched within knowledge ofmuch longer-term geosystem behaviour.

In summary, we can be highly confident of achieving anextremely high level of isolation, containment and consequentprotection over the next thousands of years, whilst wastes are attheir most hazardous, but we must accept that, in the very longterm, radionuclides will re-enter the natural environment at lowconcentrations that are well within the range of naturalbackground radiation. This long time period (even the first fewthousand years) needs to be placed in a human context. The wholeof recorded human history is only about 5000 years old; modernhumans only appeared about 150,000 years ago in Africa and thewhole diversity of modern people has evolved as human

context of a human timescale. The map shows the spread of modern humans from a

P) in central Africa and reached South America about 12,500 years ago (data from

dern humans arrived in different regions of the world are shown for each green dot.

ionuclide (such as iodine-129 or chlorine-36) would diffuse through a clay formation

eriod in which the diaspora of modern humans occurred, such radionuclides would

he Opalinus Clay formation). Also shown (in blue) is the uniform corrosion thickness

ame timescales, based on conservative data reviewed by Kim et al. (2007). For the

http://english.freemap.jp/


populations migrated and dispersed over the Earth during the last50,000 years. In a simplistic way, Fig. 6 places some radioactivewaste containment times within this context. Geological disposaldoes a very good, passive job of protecting humans and the naturalenvironment, with the degree of protection matching perfectly thediminishing hazard of the radioactive wastes.

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