Investigating deep disposal of radwaste - Part 1

In Belgium, the long-term management of radioactive waste has been on the agenda for many years. For 25 years, intensive research has been done on the possibility of an underground repository for high-level and/or long-lived waste (category B & C waste) in clay layers. Belgium is a pioneer in this field, led by ONDRAF/NIRAS, the Belgian radioactive waste management agency. Since the 1980s, the country has had an underground research laboratory (HADES-URL) in Boom clay, sited 225m under the Centre d'Etude de l'Energie Nucléaire (nuclear research centre) (CEN·SCK) in Mol. The research there should help find out whether the final disposal of high-level and/or long-lived waste in such clay layers, can guarantee the long-term protection of man and the environment.

The second Safety Assessment and Feasibility Interim Report (SAFIR 2) was published in 2002 to allow ONDRAF/NIRAS, its supervising minister and the safety authorities to evaluate progress made during 1990-2000. It describes the progress made between the publication of SAFIR in 1989 and the end of 2000.

The SAFIR 2 report has three main objectives:

• To provide a structured synthesis of the technical and scientific information relevant to disposal of category B and C waste into a poorly-indurated argillaceous formation, to enable them to assess progress made in terms of technical feasibility and assessment of long-term radiological safety.

• To promote interaction with the nuclear safety authorities to reach closer agreement on the research efforts still required and on the principles of safety assessments, and to specify the modes of enforcement of the regulations that are applicable to a deep repository.

• To be one of the technical and scientific bases for a broad dialogue with all of the parties affected by the long-term management of radioactive waste.

SAFIR 2 is not a licence application, but a state-of-the-art report. However the information is presented in the framework of a safety report to make it easier to identify all the technical aspects of the disposal solution and examine the areas where knowledge is still lacking or insufficient.

CATEGORISING THE WASTE

ONDRAF/NIRAS uses a systematic and hierarchical classification structure for waste arisings.

Two groups are defined at the top of the classification hierarchy. The 'open' group is conditioned waste with radiological characteristics that do not require geological isolation, while the 'geological' group must be permanently isolated from the biosphere. Limits of chemical toxicity may be included in the future. Depending on the definition of these groups, conditioned waste is assigned to four categories corresponding to disposal methods presently considered feasible.

• Category A waste has radionuclides with short half-lives or low level activity. It can be assigned to the open group and disposed at surface.

• Category B waste has higher activity than that of A, but lower heat generation than category C waste. It belongs to the geological group.

• Category C waste has high alpha and beta emitters and generates over 20 W/m3. It belongs to the geological group.

• Category R waste, unique to Belgium, refers to waste contaminated with radium and stored at Olen refinery, a former radium production facility. A specific solution is under study.

Conditioned waste which has similar packaging, storage and potential disposal methods are then assigned to twenty classes for technical and financial management purposes.

Finally, the packages of conditioned waste are assigned to waste streams, each consisting of packages with identical physical, chemical and radiological characteristics. The technical and scientific breakdown of this classification is comprehensive there are over one hundred waste streams.

SAFIR 2 deals only with B & C wastes (HLW or ILW). HLW includes vitrified waste arising from the reprocessing of spent fuels as well as spent fuel itself.

THE BOOM CLAY

The study of a possibile repository for radioactive waste in clay layers is an innovative project. Consequently, it has to go through all the stages that characterise a project of this type: fundamental research, methodological research, applied research, defining the components of the technical concept and ensuring their consistency, characterising the host formation and the interactions between the materials of the concept and the disposal environment, developing methodologies to evaluate performance and safety, pilot projects, implementing a preliminary project, applying for authorisations, and so on.

Following a general assessment of the geological opportunities for disposal in Belgium, and based on an internationally recognised suitability of argillaceous media as potential host formation, SCK•CEN initiated an R&D programme in 1974 to study the disposal of category B and C waste in the Boom Clay formation beneath its own site.

ONDRAF/NIRAS considers the Boom Clay as a reference host formation and the Mol-Dessel nuclear zone as a reference site for methodological studies related to the Boom Clay. It is also considering Ypresian Clays as an alternative host formation and the Doel nuclear zone as a reference site for studies related to the Ypresian Clays alternative.

So far, work has focused on the Boom clay in the Mol-Dessel region and, to a lesser extent, on the Ypresian clays in the Doel region. The work concerns the potential host rock and does not imply the choice of a site, which involves many other factors, both technical and social. Today, research and development is therefore still essentially methodological.

Initially, the reference design referred only to vitrified waste, because up to the first SAFIR report it was assumed that all spent fuel would be reprocessed, so vitrified waste appeared to be the most demanding in thermal and radiological terms.

The region in which the Boom Clay is present corresponds roughly to the northeast of Belgium

ONDRAF/NIRAS followed the recommendations of the 1990 SAFIR Evaluation Commission and started a study of the Ypresian Clays, focusing on Doel. In 1994, ONDRAF/ NIRAS commissioned the University of Gent to establish the state of the art relating to these clayey formations and to identify areas where the formations offered characteristics suitable for the disposal of high-level and long-lived waste

The Kortrijk Formation seemed to be the most suitable of the formations in terms of lithology and geometry. The University identified a zone where the Kortrijk Formation could be considered for additional research into deep disposal. This zone encompasses a narrow strip of Belgium close to the border with the Netherlands, mostly within East Flanders.

Among the issues raised in the discussions of the SAFIR Evaluation Commission (1990), those that have attracted particular attention over the past decade relate to the geometry of the Boom Clay and its surrounding formations, to the hydrogeological roles played by the aquifers beneath the Boom Clay and by the clay itself, and to its heterogeneity.

The regional geometry and heterogeneity of the underground formations are being studied using boreholes and seismic surveys. The characterisation programme is using high-resolution logging tools to investigate in detail the geology and structure of the Boom Clay. The vertical resolution of these tools ranges from a few centimetres to half a metre.

A three-dimensional seismic survey would be very difficult to carry out in Mol-Dessel. No measurements would be possible on the built-up areas covered by the nuclear research centre, the other nuclear facilities, or the fuel producers, causing large gaps in the survey network. It was therefore decided to carry out a dense 2D seismic network instead.

Connection between seismic characteristics and borehole stratigraphy were studied with vertical seismic profile. The integrated interpretation of all this data is ongoing.

Hydrogeological research

The period 1990-2000 was marked by three successive phases in hydrogeological research:

• Compilation of new hydrodynamic data in 1990-1993.

• Construction of a new regional hydrogeological model in 1993-1994.

• A programme of hydrogeological characterisation, designed in 1995 and carried out in 1996. The subsequent interpretation is presented in a series of reports and used in the construction of a new regional model.

Regional modelling, based on a quasi 3D multi-layer approach, accurately reproduces the behaviour of the Neogene Aquifers that overlie the Boom clay, and can provide information about hydraulic potentials, rates and directions of flow. The regional model also shows that for the aquifers beneath the Boom Clay the supply coming from the south is essentially diverted into flows that move from east to west. However, the calculated hydraulic heads are significantly less than those measured from the piezometric network. This discrepancy is thought to be due to the lack of detailed information on the geometry and heterogeneity of these aquifers, their hydraulic properties and the potential transient behaviour. This is why the 1994 hydrogeological investigation programme focussed on these deeper aquifers.

Among the constraints which have hampered modelling are limitations imposed by the location of the Belgian/Dutch border. Efforts to obtain information in the Netherlands should be continued, to obtain as much data as possible to calibrate the model and compare the results of the model with experimental results. The geometry of certain hydrogeological units is difficult to assess due to the series of faults associated with the Roer Valley Graben, especially if the faults extend into Dutch territory.

Although data from the hydrochemical investigations have not been used directly for verification and validation of the regional hydrogeological model, they have already been studied, and they give a better understanding of the groundwater flow system by providing answers to the origin and evolution of the different groundwaters. The groundwater sampling programmes and other studies carried out are in general agreement and suggest the same overall tendencies for groundwater in deep aquifers the presence of a marine component in the northern part of the regional model, ground-water residence times based on carbon-14 that increase from south to north, to reach 40,000 years. However, the number of reliable analyses is spatially limited, and need to be extended.

More geochemical analyses are required to study the hydrogeological system. Emphasis must be placed on measurement of trace elements, isotopes and noble gases to determe the age, origin and mixtures of the various groundwaters. Obtaining high quality geochemical data is difficult, and calls for rigorous sampling and analysis programmes.

Another recommendation for future programmes is enhancing the geoprospective modelling of the hydrogeological system, because this has not been considered in much detail over the last ten years. Firstly, however, a new version of the regional hydrogeological model will be developed.

Efforts were made to confirm the value of hydraulic conductivity of the Boom Clay measured on core samples by in situ tests (injection or slug tests) in boreholes, or using the small lateral shaft to the HADES underground research facility as a large-scale permeameter and assess the anisotropy of hydraulic conductivity in this formation. As part of the assessment of the vertical variability of parameters controlling radionuclide migration, a profile of hydraulic conductivities over the full thickness of the Boom Clay was established from cores. This profile shows the different behaviour of the Belsele-Waas Member compared with other members of the Boom Formation, indirectly confirming the continuous qualitative profile of hydraulic conductivity obtained by NMR logging in the Mol-1 borehole.

All of these investigations yield coherent values of hydraulic conductivity of the order of 1012m/s for the most argillaceous part of the formation. Furthermore, the natural lithological variability does not impart the overall radionuclide migration parameters over the Boom clay.

In parallel with all other actions, measurements of SCK•CEN's piezometric network have been carried out over the last ten years. The piezometric network was maintained and expanded following recommendations from an update of the regional hydrogeological model. This network will need to be maintained in the future so that it can monitor long-term changes in the different aquifer units.

DESIGNING THE REPOSITORY

The disposal design for the vitrified Cogema waste is based on a disposal volume of 3915 canisters (420 under current contracts and 3495 under possible future contracts). If the decision is taken to abandon the reprocessing of spent fuel and only to complete the existing contracts the amount of vitrified waste to be emplaced will be significantly less, and this vitrified waste will be integrated into the disposal solution for spent fuel elements.

The SAFIR 2 reference design allows separation of

vitrified waste and spent fuel from other waste, to prevent interaction, make thermal calculations easier and allow more convincing safety assessments. It uses corrosion-resistant watertight packages so radionuclides are contained, at least during the thermal phase, and avoid the need to consider complex physical and chemical interactions. The design uses watertight and corrosion-resistant disposal tubes to facilitate emplacement. In the reference design the vitrified waste would be disposed of in a network of horizontal galleries located in the centre of the Boom Clay formation, in the Mol-Dessel area, at a depth of approximately 240m.

These disposal galleries have an internal diameter of 2m and their walls will be lined with prefabricated concrete segments.

The stainless steel disposal tube (internal diameter 55cm) is positioned centrally over the total length of the disposal gallery. The gap between the disposal tube and the lining is occupied by backfill material (precompacted blocks of bentonite). The main gallery end of the disposal gallery is sealed.

The canisters, provided with a stainless steel overpack and fitted with rollers, are brought from the surface buildings and positioned in front of one of the disposal galleries by a transfer machine. The canisters are then pushed into the stainless steel disposal tube. The canisters will be inserted into the disposal tube by an automated system.

The eight disposal galleries are normal to the main galleries at 40m intervals. This limits the overall temperature rise in the Neogene Aquifer to 6oC. Each disposal gallery is 800m long, divided into three sections (200m, 400m, 200m). The total length of the disposal galleries is therefore 6.4km.

Overpacked canisters will be loaded from both ends of the disposal tube so each overpacked canister travels a maximum distance of 200m through the tube.

Access to these disposal galleries from ground level is provided by two access shafts, with minimum internal diameters of 6m, between which a horizontal connecting gallery is constructed. This connecting gallery also serves as an escape route for personnel from either of the main galleries. The two main galleries, with internal diameters of 3.5-4m, are excavated from the starting chambers at the base of the two shafts.

Following disposal of the overpacked canisters, a decision will be taken to seal off and backfill main galleries, connecting gallery and access shafts. A number of seals will be placed in each main gallery or shaft during this operation. The remaining voids in the shafts and galleries will be packed with a suitable material based on sand, possibly mixed with swelling clay.

As well as the two main galleries that afford access to the disposal galleries for the very high-level heat-emitting waste, two galleries will also be provided on the other side of the same access shafts for disposal of moderately heat-emitting category C waste (hulls and endpieces) and for category B waste.

During construction, excavated material must be removed and various components of the disposal system (lining segments, disposal tube, backfill material, hydration pipes) will need to be transported from ground level to the underground galleries. This will involve a large number of daily movements using small vehicles that can also be used in the disposal galleries and pass each other in the main galleries. The operational phase will see a small number of transport movements each day in which the waste will be taken from the surface facilities to the disposal galleries by a transfer machine that is larger and heavier than these vehicles.

The transport system will be adapted to suit each period of operation according to needs of frequency, size and weight. There will also be a transition from a non-controlled to a radiologically controlled zone. Non-nuclear transport is still to be investigated in further studies on logistics and operational feasibility. Internal and external reviews of the SAFIR 2 reference design have raised questions concerning its technical feasibility and have led to a complete revision of design requirements.

Principles of design

In designing the waste disposal facility, current thinking places emphasis on the assessment of the long-term safety of the disposal system as a whole rather than on individual performance of the different constituent elements of the system.

The underlying principles of this change in attitude include:

• The practical application of the multibarrier principle.

• The defence approach, based on multiple lines of reasoning, which calls for the use of different arguments as a basis for the safety of a disposal system.

• A better understanding of the features, events and processes that affect long-term safety for different types of host formations, wastes and engineered repository designs. The thickness of the host formation, for example, is more important for the migration of radionuclides in an environment where the migration of species in solution is controlled by diffusion, than in an advective environment which is likely to contain water-conducting features.

• The development of techniques and tools for assessing the performance and safety of the disposal system in its entirety (Integrated Performance Assessment) and the practical recognition of the major role of these assessments in the iterative development of a disposal system.

• Better integration within national programmes of issues relating to, and the teams responsible for, site characterisation of sites and the design and assessment of repositories. This evolution, as much 'cultural' as technical, has been regarded by the NEA as one of the major technical advances of the past decade.

The work of establishing that a technical solution is possible on Belgian territory for the final disposal of high-level and/or long-lived waste consists of two main activities:

• Developing and finalising methods required for assessment.

• Assessing the feasibility of the solution and the elements supporting its safety, and developing a repository architecture, based hypothetically on the Boom Clay formation. Research has reinforced confidence in clay as a natural barrier, and confirms that deep disposal in poorly indurated clay remains a conceivable option.

Up to now the methodological research and development work has helped establish a significant level of confidence in:

• The properties of the Boom Clay as a natural barrier (low permeability at various scales, diffusion-controlled migration of solutes, high retention properties, ultrafiltration of organic matters and colloids, limited variability of the migration parameters over the thickness of the formation, lateral litho-stratigraphic continuity of the formation).

• The durability of the glass as a waste matrix (over 100 years in the presence of Boom Clay).

• The possibility of excavating the necessary underground facilities without disturbing significantly the barrier properties of the host formation.

• The methodology used to assess long-term radiological safety.

The work confirms the favourable results of assessments, especially regarding the key contribution to long-term safety made by the host formation. Hence the importance of knowing and controlling the disturbances induced in the clay and that may potentially affect its barrier properties. These disturbances may be generated by construction and operation of the repository (EDZ, oxidation) and by the presence of the waste and other repository materials. It also confirms the primary role in the radiological impact of the moderately-retarded or non-retarded radionuclides (129I, 36Cl, 79Se, 126Sn, 14C). The knowledge of the migration properties of these radionuclides in the clay can be regarded as generally adequate, even though the chemistry of tin and selenium needs to be refined.

The Boom Clay clearly represents the most dominant barrier and contributor to long-term safety. It should be noted that one of the basic hypotheses of the safety approach is that a perfect containment of the waste is ensured during the thermal phase of the repository (several hundreds years for vitrified waste or several thousands years for spent fuels) by an austenitic stainless steel overpack. This containment avoids considering numerous phenomena linked with waste matrix degradation and radionuclide migration under a strong thermal gradient.

UNDERGROUND RESEARCH

The primary aim of the Praclay project (Preliminary demonstration test for clay disposal) which began in 1995 and is due to continue until 2015, is to demonstrate the technical and economic feasibility of a design for the deep disposal of high-level heat-emitting vitrified waste in the Boom Clay. This project, which is centred on a direct demonstration of feasibility rather than on a long-term safety assessment by indirect means, has two objectives: to assert with confidence the technical feasibility of the options selected for design, given current technologies, and to corroborate the R&D results. The results will be used to tighten up the operating ranges of the different components of the disposal system and will provide valuable information to improve the design.

Its particular aims can be summed up as follows:

• To demonstrate the technical and economic viability of using the industrial excavation techniques proposed for an actual repository to excavate a gallery which is similar to the disposal galleries in the reference design, except for its length. The connecting gallery will be excavated by a tunnel boring machine equipped with a segment erector.

• To design and construct the intersection of a main gallery and disposal gallery.

• To establish, fit out and fill a disposal gallery (called the Praclay gallery) similar to the galleries proposed in the reference design, except for length. Except for the vitrified waste, which will be represented by electrical heater elements, the materials and techniques used should be identical as possible to those proposed in the reference design.

• To confirm and improve the knowledge and understanding of the reference design.

To prepare for the in situ test, a full-scale (in the radial axis) mock-up of the proposed underground gallery, consisting of the disposal tube and backfill material, was constructed on the surface (19962002). This mockup (known as Ophelie) has all the instruments necessary for testing the experimental underground facilities.

It should demonstrate that it is possible to place the backfill material and to verify that installing the instrumentation system, on the disposal tube, in the backfill material and on the lining, does not create any significant disturbance. It will also allow the behaviour of the backfill material under actual repository conditions of temperature and pressure to be studied, along with its interaction with interstitial water which simulates the groundwater present at depth in the Boom Clay at Mol. Finally it will verify the strength and reliability of the test instruments and equipment before they are used underground. This Praclay experiment will run over a decade. It will focus notably on the theraland excavation disturbances to the host rock as well as on-plug emplacement and performances. Due to current revision of the reference design, the experimental setup and planning are not yet fully defined.