Table Of ContentStudies in Environmental Science 57
NATURAL ANALOGUE STUDIES
IN THE GEOLOGICAL DISPOSAL
OF RADIOACTIVE WASTES
William Miller
lntera Information Technologies, U.K.
Russell Alexander
RWIG, Berne, Switzerland
Neil Chapman
lntera Information Technologies, U.K.
Ian McKinley
NAGRA, Switzerland
John Smellie
Conterra A.B., Sweden
ELSEVIER
Amsterdam - London - NewYork - Tokyo 1994
ELSEVIER SCIENCE B.V.
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Time present and time past,
Are both perhaps present in time future,
And time future contained in time past.
T.S. Eliot, Four Quartets (1936)
ix
Acknowledgements
In writing this book the authors have drawn on their experience of niariy iiational
and interr1a.tiona.l natural analogue programmes ovcr t,he last ten years. 'I'liese
programmes have involved a growing arid voluble family of researchers arrcl data
users and the authors wish to express their thanks to all their colleagucs in many
countries for fruitful, demanding and protracted discussion sessions in iiicctirig
rooms and watering holes around the world. Without, t.hese int.eracl.ions the
opinions expressed here would never have heen tlevclopc.tl or tcst,ctl. \Vc oiily
hope that. we havc done a.ll your work ,jiist,icr. in t,liis revicw.
The book went through several dra.fts and Imth Frctl I<;lrlssori at SKI3 irlltl 1111-
dreas Cautschi at Nagra provided useful comments on the various versions, as
well as contributing items to improve some of the scct,ioris. Tlie task of iirtlcxirig
was leavened by the assistance of Joy Ball at Intera. The iineirvial)lc I)III~~CIoI f
f i d t echnical editing was horne by Lintla h,lcIiiiiley at, Nagra.
Fiiia.lly, we would like to thank Nagra, SKB arid I~MIP/~D~OIEi f or tlrcir support,
in funding this project, and t.heir tolerance of t.lie incvit,al)led elays in hriiigiiig it
to press.
1
Chapter 1
Introduction
1.1 The concept of geological disposal
For certt.uries, the coininonly a.ccepted practice for disposiiig of most, t,vpes of
solid waste that, cannot, be burned has beeri to bury them. This has all too often
and, indeed, all too recently, inearit, a 1ia.sty aIid convenient, shallow gra.vc for all
kirtds of environnientally unfriendly material. However, during the last 1.5 years
thr concept, of underground disposal has been ctiibraced by radioactive wa.stc
r~ia~~ageirieotrigta~n isations and developed to a degree of sophistication t,hat. is I'ar
removed from practices in any other industry, reflecting t,he high stanchrcls of
safrty that the nuclear industdry is expect.ed arid legally required to achieve.
Most, corinti,ics wliicli ha.ve opt,cd to dispose of their longer -lived radioactive waste
products, rat,lier than st,ore t,hem indcfitiitely, are actively exploring the powi bili-
t,ies for burial in suitable geological format,ions. Several uIiclerground repositories
for the disposal of lower activity wastes arc eit.lier operating or under cortstruc-
t,ioii, a.ltlioiigh disposal of the highest level, heat -eniitting wastes is st,ill largely at
t.lit, conctrpt. tlevcloptiient, or site investigation si,age. 'rhe location of a repository,
its tlesigti a.ntl tlte tlttptli of burial deperid wry riiuch on the types of wastt it
is ittteridctl t,o cont,airi, iri ternis of tlieir ra.clionnrlide coiitcrit and physical and
cheiriical propertirs. The wast.e materials and the eiigineerecl harriers that itii-
tially coiitaiii theni within the reposit,ory are expect.ed eveiit,rtally to degrade md
it is ariticipat,ed t,liat, sonic small portion of t,hc radionuclitl(~sm ight, returu tto t,lw
I)iosplicrc at sotiic time in the distant future.
( 'otitrollrd rinplaceiiicnt in an rngineered repository, whicli may be purposcc-huilt
2
or a modified mine or other excavation, can be distinguished from other types of
underground disposal which have been practised in the past, such as the injec-
tion of liquid wastes or waste-containing grouts in deep boreholes. All types of
engineered repository utilise the ‘multibarrier’ concept (Figure 1. l), whereby the
wastes are emplaced inside a series of engineered structures and natural barriers
which act in concert to control the rate of release of radionuclides over long pe-
riods. The solidified wastes are packed into containers which may themselves be
placed inside an ‘overpack’; both containers and overpacks are usually made of
meta,l or cement. The waste packages are then emplaced at some depth in an ex-
cavated gallery or borehole in a repository. The surrounding space ma,y be filled
with some suitable ‘buffer’ material to provide long-term structural, hydraulic
and chemical stability for the package. After repository operations have been
completed, access excavations would be backfilled and sealed to the surface. A
variety of materials and mixtures of materials (often making use of processed rock
from the excavations) is available for backfilling and sealing operations. The host
rock in which the repository is excavated is generally selected to be adequately
stable for the construction and operation of the facility and, most importa,ntly,
to provide a stable environment where groundwater fluxes through the repository
zone are small, and other natural geological and geochemical processes are slow
and predictable.
Some low activity wastes are currently disposed of by burial in ‘shallow’ or ‘near-
surface’ repositories, often in concrete chambers situated in trenches or on the
surface, which are then covered with earth. These types of repository make only
limited use of natural barriers, as it is generally acknowledged that only rela-
tively short containment times are required for the wastes (perhaps only around
300 years), and that these times can be achieved by confinement within the en-
gineered components of the repository. Until the late 1970s, solid wastes were
also dumped at sea, with no attempt at careful emplacement. Much research
wa.s also carried out into remote emplacement of highly radioactive wastes in
sea.bed repositories. The options considered promised very considerable levels of
sa.fety and long-term predictability, possibly more so than deep repositories on
land, but ocean disposal has effectively been abandoned for the time being owing
to the difficulties in achieving consensus on how to proceed outside the confines
of national boundaries. However, none of these types of repository is generally
cla.sset1 as geological disposal. This book is concerned with what is commonly
termed ‘deep geological disposal’ the emplacement of wastes in repositories con-
~
structed at depths of several tens to many hundreds of metres below the surface,
making use of the surrounding geological environment as one of the principal
containment barriers. For a comprehensive description of the background to the
Figure 1 .I: The near-field of a generic high-level radioactive waste repository demon-
strating tile multibarrier concept. The wasteform is either spent fuel or vitrified repro-
cessing waste packaged in a metal canister wliich is further encased within an envelope
of compacted bentonite. Physical containment is assured by the wasteform, the canis-
ter, the bentonite and the rock. Chemical containment is assured by low groundwater
flows in a chemically reducing, mildly alkaline environment bufft.red by the metal, bell-
tonite and the rock.
4
geological disposal of radioactive wastes, see Chapman & McKinley (1987).
1.1.1 Geological disposal environments
The fundamental requirement of a suitable geological environment for a repository
is that it should be relatively stable and its behaviour adequately predictable.
The need for predictability arises from the requirement to make scientifically
sound evaluations of the long-term radiological safety of a disposal facility. Such
evaluations are necessary in order to demonstrate that a repository will comply
with safety regulations and are generally termed ‘performance assessment’ or
‘safety assessment’ exercises; in this book the kwo terms are used synonymously.
Unlike environmental impact assessments in many other areas, ‘long-term’ in
the context of radioactive waste disposal really does mean what it says, and it is
not uncommon to find performance assessment predictions of the behaviour and
health impacts of radionuclides released from the waste which look at timescales
of tens of thousands to millions of years into the future. Knowledge of the nature
and evolution of the geochemical and hydrochemical regime is fundamental to
making such predictions. This is particularly important for deep radioactive
waste repositories, situated several hundreds of metres below the surface and
designed to contain the longest-lived radioactive wastes.
The search for suitable disposal environments has concentrated on four principal
types of rock formation: (1) hard, fractured basement rocks, (2)l ow-permeability
sediments, (3) thick sequences of extrusive volcanic rocks, and (4) thick or diapiric
evaporite deposits. The variety of different environments that has been investi-
gated or selected for repositories is wide, and the useful features of more than one
type of environment have been combined (as with basement rocks buried beneath
thick sediments). However, the essence of any suitable host environment is that
it should provide a stable cocoon for the repository in which the rates of all pro-
ceses which might disturb the engineered barriers surrounding the waste are very
slow and are not likely to be subject to significant disturbances or modifications
over the time period of containment.
1.1.2 Evolution of a waste repository
The concept of a planned ‘time period of containment’ stems from two mecha-
nisms which inevitably will occur during repository evolution. The first is that
of radioactive decay. Many isotopes in radioactive waste decay relatively rapidly
5
and, a.lt,hough highly act,ive nieinbers of decay chairis may he formed, there is
a steady decrease in the overall activity of the wastes with time. A number of
the more radioactive and radiologically problematic radionuclides are suffciently
short-lived that they are expected to dccay to insignificant levels before there is
any red possibility of their release from the repository near-field. Consequently,
it is sensible to think in ternis of a containment period in which no release of
such ra.dionuclides should bc expected to occur. However, other radionuclides
decay very slowly and, consequently, it. is much ha,rder to conceive of contain-
ing them for any atlequa.te length of tirile. This is due to t,he second inevitable
meclianisni, namely t,he progressive degradation of the engineered Imrriers witliiii
thc repository (Figure 1.2). ‘The allility to predict how this may happen is t,he
ceiit,ral issue in demonstrating the long-term safet,y of any waste reposit,ory. The
length of time for which it is considered reasoiiabl(. to expect largcly complet,e
conta,ininent within the engineered barrim va.ries from concept to concept; in the
examples dealt wit.h in this book, from a few tens of years to 100 000 ycars or
more.
The essence of predictive safety a.ssessment8si s to use models which describe,
in a simplified but adequate fashion, the many processes leading to the degra-
dataion of the wask coritainment system (metal containers and other harriers
engineered into a rcpository), the mobilisa.tioii of radionuclides from the waste
int,o porcwaters and groundwaters and t,heir thnsport through the surrounding
rock formations to the biosphere (for example, to wells or surfacc water bodies:
Figure 1.3). In the ‘near-field’ of a wa.ste repository (the engineered harriers
arid imriicdia.tely surrounding rock), the geocheniistry of these processes will be
dominated by intemctions between local pore fluids, inflowing groundwaters a.nd
the wa,stes and other repository compoiien1.s. In the ‘far-field’ (the rest of t,he
rock around the repository), released radionuclides may eventually he dilutd
to Ijeconie very minor coinponerits of a largely uiiperturbcd natural geochcinical
syst,em. Their subsequent Ilehaviour will bc doinina,ted by the slow geochemical
fluxes of a.ii evolving natural hydrochemical system. During the last decade or
so, a coiisirlerable amount of field and laboratory geocheniical investigation a.nd
research ha,s helped to establish which processes are important at each shgc of
this modclliiig and what, the scalcs and rates of these processes might be. A
iption of tlie geochemistry of the natural environment of a proposed
rcpository is clearly the starting point for sucli aiialyscs.
6
Figure 1.2: Over a long time period the components of the near-field will degrade and,
eventually, the wasteform will be exposed to the groundwater and begin to corrode.
Dissolution of the wasteform may take place in a locally oxidising environment due to
a-radiolysis of the groundwater. Migration of radionuclides may be retarded either by
sorption onto the rock, bentonite or the degradation products of the wasteform and
canister or by precipitation at the redox front.
