Special reports

Nuclearwaste disposal:- Understanding

what happens undergroundAssessing "near-field" effects for safe long-term isolation

Many different kinds of repository systems are underconsideration, development and - for certain typesof waste - even in operation. These systems are basedon different host rocks and repository designs.

In simple terms, the concept of multiple barriersprevails in radioactive waste disposal. Repositorieslocated in host rocks characterized by a certain potential

by J. Heinonen and F. Gera

for radionuclide transport will require that other compo-nents of the disposal system compensate for this bypreventing or restricting mobilization of radionuclides.

Effects in the "near-field" - a term that generallyrefers to the excavated repository itself, the waste package,and the host rock (see accompanying box) - are fairlywell understood for certain types of rock and repository

Mr Heinonen is a former staff member in the Agency's Division of Nuclear Fuel Cycle, and Mr Gera is on the staff of the InstituteSperimentale Modelli e Strutture, S.p.A. (ISMES), Via dei Crociferi 44, 1-00187 Rome, Italy.

Photo shows tunnel in basalt rock, which is one geologic medium for nuclear waste disposal. Basalt is a dense volcanic rock deposited bylava flows that slowly covered the earth's surface up to 16 million years ago. (Credit: Atomic Industrial Forum)

IAEA BULLETIN, SUMMER 1985 35

Special reports

designs, and when considered separately.* Near-fieldeffects discussed here are waste package corrosion,leaching, chemical and mineralogical changes, modifica-tions of groundwater flow, and transport of radionuclidesto the far-field.

These various individual effects have to be consideredtogether to assess the performance of disposal systemsin containing radioactive wastes. This coupling is themost difficult part of the assessment and the one whereadditional work is still required to reduce uncertaintiesabout the performance of the various near-fieldcomponents.

For example, the thermo-mechanical effects in a hardrock repository may affect the permeability of the rockand the groundwater flow in a way that may be difficultto model satisfactorily. In addition, the chemicalenvironment may be changed by the presence of thewaste, and oxidizing conditions may result from radiolysis.On the other hand, the engineered barriers surroundingthe waste can be designed in such a way that anyuncertainty about coupling of near-field effects andabout long-term behaviour of the various componentswill be counterbalanced by their great capacity to isolatethe critical radionuclides.

Deep in situ experiments still are needed, for example,to help clarify certain near-field effects, as well as toquantify the impact of certain parameters. All this willaid in the integration of near-field effects in the perfor-mance assessment of the entire disposal system. Largeprojects in several countries should improve the under-standing of near-field phenomena and promote dissemina-tion of this specific knowledge. The development ofspecific disposal systems, in particular for high-level,long-lived waste, necessitates complex and extendedsite-specific investigations.

In general terms, the safety of waste disposal will beimproved by the increased knowledge and maturity oftechnology, and experimental data will allow the verifi-cation of models often based on laboratory experimentsand theoretical considerations.

Waste package corrosion

No mobilization of radionuclides — which is assumedto depend on leaching by groundwater — is possible aslong as the waste package maintains its integrity. Thewaste package consists of the waste form, the canisterand, when applicable, other barriers such as overpack,absorber material, and chemical conditioners, providedthey are integral parts of the package. It is possible todesign waste packages that are expected to last for almostany length of time. Designs for lifetimes in excess of

100 000 years have been considered, but most wastepackages are expected to last much shorter times.

Different kinds of materials can be used in wastepackages. Some concepts rely on materials that arethermodynamically stable in the environment of the

* For fuller technical information, see "Deep UndergroundDisposal of Radioactive Waste - Near Field Effects", the draftof an IAEA Technical Report, June 1984; and "Effects of Heatfrom High-level Waste on Performance of Deep GeologicalRepository Components," the draft of an IAEA TechnicalDocument, October 1983.

"Near-field" effects - defining terms

In assessing a geological disposal system for long-livedradioactive waste, it is necessary to understand the long-term behaviour of the various components contributingto waste isolation. It is obvious that the importantcharacteristics of an isolation system are those affectingoverall performance, in particular from the safety viewpoint.

However, to facilitate the performance assessment ofunderground repositories the relevant phenomena aredivided into near-field and far-field ones. This is anarbitrary distinction since no well-defined physicalboundary exists between the two. This article discussesprocesses taking place in the near-field in different hostrocks and with different disposal concepts. The near-fieldis defined as "the excavated repository, including the wastepackage, filling or sealing materials, and those parts of thehost medium whose characteristics have been or could bealtered by the repository or its contents."

The number and nature of components actually presentin the near-field is specific to particular disposal systemsand depends on the nature of the host rock and the wasteand on the engineering aspects of the repository. Mostnear-field effects discussed here are relative to disposal ofhigh-level vitrified waste and spent fuel, but, wheneverappropriate, effects specific to other types of waste havebeen considered.

Specific components

The components of the near-field can be grouped asthe waste package, the repository, and the host rock.

The waste package includes the waste form, its con-tainer, and any additional barriers, provided they areintegral parts of the package.

The repository is the underground facility into whichthe waste packages are emplaced. It includes any engineeredbarriers that separate waste packages from host rock,structural materials such as tunnel lining and hole liners,and materials used for backfilling and sealing.

The host rock included in the near-field is that part ofthe disposal medium where physical properties and chemi-cal conditions are significantly modified by the repository.The actual extension of this altered zone will varydepending on waste type, repository design, and host rock,but it will range up to a few tens of metres at the most.

The near-field is surrounded by that section of the hostrock where repository-induced changes are minor and byany additional geological materials that may exist betweenthe host rock and the ground surface. The geologicalenvironment around the near-field, or the far-field, is notthe subject of this article. But it has strong interactionswith the near-field since near-field processes determine thesource term for transport of radionuclides in the geosphere.Conversely, the migration potential in the far-field deter-mines the number and nature of barriers installed in thenear-field.

The types of rocks presently considered as potentialhost rocks for long-lived radioactive waste include rocksalt; granite and similar crystalline rocks; argillaceousrocks; tuff; and basalt.

36 IAEA BULLETIN, SUMMER 1985

During a series of experiments from 1977—80 atthe Stripa mine in central Sweden, scientistsassessed the deep underground disposal of nuclearwaste in hard crystalline rock. Above, ultrasonicmeasurements were performed during a full-scaleheater experiment. Below, the macro-permeabilityexperiment measured water pressures in about100 sections and 15 boreholes.(Credit: SKBF KBS, Stockholm).

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repository. Other materials, although unstable, wouldform a protective coating that would drastically reducefurther attack. Finally, it would be possible to userelatively thick layers of materials that corrode with aknown rate. Decisions about waste packages in specificrepository conditions will depend on a number of factors.These include regulatory requirements, the isolationcapability of the other barriers, and economics.

Leaching by groundwater

After failure of the packages, waste will eventuallycome into contact with water or brine. Reactionsbetween the waste matrix and the water will cause someradionuclide dissolution. The rate of reaction willdepend on the waste form, the waste composition, theratio of water volume to exposed surface area, theeffective flow-rate of the water past the waste, thetemperature, and some other factors, such as radiolysis.The composition of the water contacting the wastedepends on the composition of the interstitial water inthe host rock. It can be modified by substances derivedfrom buffer and backfill materials and from the wastepackage. The flux of water past the waste is determinedto a large extent by the groundwater flow in the far-field,

as this will control the amount of water which reachesthe repository. The flow-rate past the waste packagesmay be further limited by low-permeability materialsplaced around the waste. These additional barriers canreduce the water flow to very low values and reducesignificantly leaching of many radionuclides.

Radiation effects

The near-field environment can be modified byradiation by way of three different primary effects:alteration of the waste form and any buffer material, ofany fluids present around the waste, and of the host rock.Due to the shielding capacity of solid matter, gammaradiation will be adsorbed completely within about onemetre from the waste containers. Beta and alpha radiation,which are much less penetrating, will be adsorbed withina very small volume of material. However, radiolysis mayproduce corrosive substances resulting in acceleratedcorrosion of containers and enhanced leaching of waste.Other radiation effects such as energy storage and changesin physical properties of both rocks and man-madematerials have been found to be irrelevant in the contextof waste disposal.

IAEA BULLETIN, SUMMER 1985 37

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Thermal effects

Radiation energy absorbed in matter is transformedinto heat. Heat generation is highest in spent fuel andhigh-level waste but some heat is generated in all typesof radioactive waste. In a repository, heat will flow fromthe waste through container and buffer materials to thehost rock, thus raising the temperature of all componentsof the near-field and affecting their physico-chemicalproperties. The actual temperature distribution in therepository depends on many factors, including:• Type and age of waste• Dimensions and distributions of waste containers• Volume and nature of buffer and backfill materials• Repository design• Type of host rock

Peak temperatures in the repository will be reacheda few decades after waste emplacement. Highly reliablecomputer programs, capable of predicting the tempera-ture distribution in the repository at any particular time,have been developed and verified.

The effects of elevated temperatures on different wasteforms, on buffer and backfill materials, and on host rockshave been and continue to be investigated. A thermaleffect of particular significance for repositories located ingranite or similar crystalline rocks is the possible modifi-cation of groundwater flow. If the rock is fractured andwater is contained in the fractures, which is practicallyunavoidable in this type of rock over the dimensions of arepository, then heating may decrease viscosity and densityof water and increase the permeability of some fractures,thereby initiating thermally induced flow of water or evenconvection cells.

The objective of the studies on thermal effects is todevelop thermal criteria for the various repository com-ponents. The respect of the thermal criteria should ensurethat no effects capable of weakening the isolation capabi-lity of the disposal system take place. At this time itappears that the most restrictive temperature limits applyto clay minerals present either in the host rock or in thebuffer and backfill materials. It is considered prudentto keep the temperature of clay materials below 100°C.Somewhat higher temperatures would be permissible inrock salt; this, combined with the higher thermal con-ductivity of salt, means that a significantly greater thermalload could be placed in a salt repository.

Thermo-mechanical effects

A particular effect of heating the host rock is itsthermal expansion and the resulting stresses in both near-and far-fields. In an excavated repository the rock isalready exposed to overburden stresses and possibly totectonic stresses, which are locally modified by thepresence of the excavations. To this relatively complexstress field, thermally induced stresses are superimposed.A large amount of data exists on thermo-mechanicaleffects in salt. The fit between calculations and observa-tions is very good. In hard rocks like granite, the strains

are absorbed by joints and fractures, which are practicallyimpossible to model successfully. As a result somedifferences exist between theoretical and experimentaldata.

As far as clays are concerned, few data are availableabout their mechanical behaviour at repository depths.Even less is known about their thermo-mechanicalbehaviour since no deep in situ tests have been carriedout yet. The same is true for other potential host rocks.No particular difficulty is anticipated as a result ofthermo-mechanical effects; however, they must be knownin order to optimize the design of the repository.

Chemical and mineralogical changes

The excavation of a repository and the emplacementof waste packages and other materials in the host rockis going to disturb the natural equilibrium between hostrock and interstitial water. The excavations will intro-duce oxygen and cause changes in in situ stresses andhydraulic gradients. All these disturbances provide thepotential for changes in host rock mineralogy andgroundwater chemistry. The potential for chemical andmineralogical changes is enhanced by the temperaturerise due to the waste. However, these changes areexpected to be minor and restricted to a small volume ofmaterial if sound thermal criteria are used in the reposi-tory design.

Groundwater flow

Since water is the main transport medium for anyradionuclides released from waste packages, any modifi-cation of groundwater flow can be of particular interest.As far as near-field effects are concerned, groundwaterflow can be modified by four basic mechanisms:• Introduction of low-permeability barriers• Draining effect of underground excavations• Changes in the physical properties of the host rock• Heating caused by the waste

The potential for water migration in the near-fieldis different for each of the proposed host rocks. Salt isusually extremely impermeable and free from circulatingwater. The only water in the disposal zone will bepresent as brine in small inclusions. The overall watercontent in salt is normally in the order of l-to-2% inbedded salt and well below 1% in diapiric salt. The mainnear-field effect in salt would be the migration of thebrine inclusions towards the heat sources. While thisphenomenon can have significant effects on the chemicalconditions around the waste and on container corrosion,the volumes of brine are too small to cause any significantmigration of radionuclides.

Groundwater circulation in granite and similarcrystalline rocks is controlled by extent, characteristics,and interconnection of fractures, and by the regionalhydraulic gradient. A body of hard rock consideredsuitable for waste disposal must be characterized by verylow hydraulic conductivity; that is, few fractures mustbe present and they must transmit very little water since

38 IAEA BULLETIN, SUMMER 1985

Isolation: Tuff formations at the Nevada Test Site are one of several areas in the US being investigated as potential sites for a geologicrepository for disposal of high-level radioactive waste. Tuff is a rock made up of solidified and welded volcanic ash, with separate layersdeposited by successive eruptions millions of years ago. (Credit: Atomic Industrial Forum).

they are closed by the overburden pressure, by theaccumulation of secondary minerals, or by a combinationof the two mechanisms. The excavation of a repositoryin such a geologic environment will modify groundwaterflow even before any waste is placed underground. Thiswill be due to the draining effect of the excavations,which being at atmospheric pressure will act as a pointsink, and to the changes in permeability mat the modifi-cation of the stress field may induce in fractures adjacentto the openings.

After waste emplacement has taken place and therepository has been backfilled, the near-field hydrologycan be further modified by the thermo-mechanical effectsalready discussed, and by the presence of buffer andbackfill. In conclusion, hydrogeological modelling of arepository in hard rock can be complex. The resultinguncertainties can be compensated either by the greatcapacity of the far-field to restrict radionuclide migrationor by the engineered barriers placed in the repository.

Argillaceous rocks are usually characterized by verylow permeabilities. In plastic clays all water flow isthrough the pores, while in stiff clays and shales somewater-bearing fractures seem to be possible. It is antici-

pated that any argillaceous formation considered suitablefor waste disposal would not be intersected by permeablefractures. After waste emplacement and after backfillingand sealing of the repository, near-field conditions willdepend on the repository design.

For example, in a repository based on a matrix ofboreholes drilled from the surface, each borehole wouldbe isolated from the others and containers would bemore or less directly in contact with the clay. In case ofa mined repository, the waste containers could be placedin disposal holes drilled from the galleries or within thegalleries themselves. In either of the latter two cases,the galleries would be filled with a material withhydraulic properties probably different from those ofthe undisturbed clays.

Consequently, the near-field hydrogeology would besignificantly different from the initial conditions in theformation. However, provided that connections betweenthe repository and the surface are plugged and that nonew flow paths are generated by subsequent events, thennear-field changes of groundwater flow are local andshort-lived and are expected to have little impact on theoverall performance of the disposal system.

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For a repository located in tuff in the unsaturatedzone little effect would be expected on the flow ofgroundwater as a result of waste emplacement. Thethermal field in the disposal area would be expected todrive away water and reduce the flow past the wastepackages.

For a repository in basalt the effects on groundwaterflow would be similar to those already described for arepository in granite.

Radionuclide transport to the far-field

All near-field effects described so far should not beconsidered in isolation; on the contrary, the analysisshould concentrate on their coupling and lead to arealistic assessment of the potential for radionuclidetransport to the far-field.

For a repository in granite and similar crystallinerocks, the release and transport of the radionuclides willmainly be determined by the rate of water flow in thefissures in the rock. The presence of ferrous iron mineralscontrols the redox potential of the water and may limitthe solubility of many of the actinides to very low values.The radionuclides may also diffuse into the matrix of therock and sorb into pore surfaces. This will withdrawthem from the water flowing through the fissures.

For a repository in salt, no transport of radionuclidesto the far-field is possible if no additional fluids enterthe near-field from outside the repository. In case offlooding of the repository — an event that suitable designcan make extremely unlikely - transport of radionuclidesto the far-field would be strongly dependent on therepository layout.

For a repository in argillaceous rocks, assuming thatno abnormal events take place and no preferentialpathways are generated through the formation, themigration of radionuclides would be controlled bydiffusion. For a clay formation of suitable dimensionsand with representative hydraulic properties, it can becalculated that sorbing radionuclides would decay toinsignificant levels within a few metres of the wastepackages. On the other hand, non-sorbing radionuclides,such as Iodine-129, could migrate to the far-field andpartially escape the disposal formation.

For a repository in tuff the migration of radionuclidesthrough the near-field would depend, as in other rocktypes, on the rate of mobilization of radionuclides andon the magnitude of fluid flow. Sorbed species will beretarded mostly by reactions with zeolites, which aresecondary minerals formed by the alteration of ash.The effectiveness of tuff as a geochemical barrier willdepend strongly on the zeolite content. Consequently,any assessment of the potential transport of radionuclidesin a particular tuff formation will require specific dataabout fluid flow and mineralogical composition ofthe rock.

For a repository in basalt the situation would besomewhat similar to the one described for granite,since radionuclide transport would occur as a result ofwater flowing through fractures. The same uncertaintiesabout coupling of thermo-mechanical effects withfracture permeability and groundwater flow wouldexist. The uncertainties can be reduced by site specificdata and counterbalanced by effective engineered barriers.

Ongoing and future studies

The following list consists of examples of activitieseither ongoing or planned, which can contribute to abetter understanding of near-field effects:

• Investigations of groundwater flow in differentgeological environments

• Studies on migration of radionuclides in variousrocks and in various hydrogeological conditions

• Verification of models; in particular identification,quantification, and verification of certain parameters

• Investigation of different buffer materials, includingbuffer mass tests and comprehensive analyses of the results

• Heat transfer experiments in various potential hostrocks

• Study of thermally induced rock movements• Improvement of methods for evaluation of rock

behaviour, including effects of rock stress and fracturegeneration

• Chemical and physico-chemical behaviour of certainimportant radionuclides, for example mechanisms ofdissolution, leaching, and migration of actinides.

40 IAEA BULLETIN, SUMMER 1985