Several national programs for geological disposal of higher activity radioactive waste will initiate siting via a call for volunteers. This is probably furthest advanced in Japan (NUMO) and the UK (NDA RWMD). In both these cases, rather than focusing on a reference repository concept, a catalogue of options has been developed. This requires a process to tailor design options to sites and compare particular design/site combinations as part of the site selection process. As has been noted in the longer-running Japanese program, the assessment of options must be as realistic as possible, rather than conservative, as in past nominating approaches to siting. Past demonstration work has focused on feasibility, showing that it is possible to implement a design. Determining practicality, which incorporates many more complex factors, is more difficult. Extensive desk studies can form a basis for such analysis but these must be rigorously tested, which makes full-scale tests in Underground Rock Laboratories (URLs) an essential component of any program. This paper highlights the importance of such demonstration projects and illustrates how they are vital in both improving understanding of the engineered barrier system and stakeholder engagement.

In Japan, uplift/erosion scenarios must be analysed even if they occur far in the future, as no assessment cut-off times have yet been defined. For this purpose, an argumentation method is developed to allow sensible scenarios to be constructed. The consequences of erosion of the repository may be better estimated in terms of radionuclide fluxes and these compared with those of naturally occurring radionuclides. This paper discusses procedures to derive relevant conceptual models and resultant analyses in a credible manner, which illustrates the effectiveness and robustness of the HLW disposal system.

Most designs of disposal facilities for higher toxicity radioactive wastes were developed for generic feasibility assessment and hence tend to be rather simple in terms of layout and conservative in the choice of engineered barrier materials. Recently, there has been a trend to reassess such designs in the light of moves towards implementation, where robustness in terms of post closure safety needs to be balanced against the requirements to assure safety, quality, and economic practicality during operation, to minimise environmental impact and to gain public acceptance. Studies to date have, however, tended to focus predominantly on variants of layout and engineered barrier geometry. Evaluation of alternative materials has received less consideration, despite the huge advances in materials science over the last couple of decades and likely further developments in the time until repositories become operational. This paper will examine the constraints that led to the choice of the most common materials selected for barriers within “wet” host rocks and examine the extent to which performance could be optimised by use of alternative materials.

In preparation for actual repository implementation, designs that could be tailored to specific sites need to be considered. This requires a paradigm shift in design philosophy, moving away from a single reference design towards a flexible and systematic “catalogue” of engineered barrier components and associated repository infrastructure. To support this development, novel methodology for both developing and comparing repository design options for Japanese vitrified high-level radioactive waste (HLW) has been examined. This paper will outline the background of the project and, in particular, some novel approaches examined for facilitating optimisation of practical repository designs. It is complemented by a companion paper - Matsumoto et al: Application of Formal Knowledge Engineering Approaches to Develop A Design Catalogue for A Japanese HLW Repository -, which describes the repository design process in more detail and illustrates results of first dry runs of the methodology.

The volunteer siting of disposal facilities for vitrified high-level radioactive waste (HLW) and TRU waste in Japan results in a particular need for flexibility to allow repositories to be tailored to specific sites. Instead of a single reference concept, a “catalogue” of designs for individual repository components is being developed, which can then be combined to form optimum solutions for particular site boundary conditions. As highlighted in a companion paper – Makino et al: Supporting Development of Practical Designs for a Japanese HLW Repository –, which puts the repository design process in the context of the Japanese HLW programme, the complexity of this work justifies use of advanced Knowledge Engineering approaches. In this paper three components are described – development of the knowledge base, assessing tacit system understanding and production of innovative solutions to resolve conflicts between requirements.

Bentonite plays a significant barrier role in many radioactive waste repository designs, where it has been chosen due to its favourable properties such as plasticity, swelling capacity, colloid filtration, low hydraulic conductivity and its stability in relevant geological environments. However, bentonite is unstable at high pH meaning that it could lose its favourable properties if interacted with hyperalkaline leachates from concrete construction materials (e.g. tunnel liners, grouts, etc.), seals and plugs and/or cementitious wastes in a repository. This fact has forced several national programmes to assess alternative construction and sealing materials such as low alkali cements. Recently, it has been assumed that the lower pH (typically pH 10-11) leachates of such cements will degrade bentonite to a much lesser degree than ‘standard’ OPC-based cement leachates (generally with an initial pH>13).

To date, few laboratory or in situ URL (underground rock laboratory) data are available to support the use of low alkali cements in conjunction with bentonites, partly because of the very slow kinetics involved. Consequently, a new project has focussed on finding an appropriate natural analogue site to provide long-term supporting data which will avoid the kinetic constraints of laboratory and URL experiments. Early results have identified an initial, very promising site at Mangatarem in the Philippines, where a quarry excavating bentonite and zeolites is found in the sedimentary carapace of the Zambales ophiolite. In the immediate vicinity of the quarry, ophiolite-derived hyperalkaline groundwaters are present and further field work (including geophysics surveys and borehole drilling) are now being planned to assess regional bentonite/hyperalkaline groundwater interaction. This paper presents an overview of the current status of the project and assesses the relevance of the study to improving understanding of low-alkali cement leachate/bentonite interaction.

The search for greater public acceptance for radioactive waste disposal has meant that repository planning increasingly includes monitoring, institutional control and flexibility with respect to retrieval and reversability. However, the fundamental repository designs are generally unchanged. This paper describes an alternative – the Cavern Extended Storage concept – which aims to incorporate requirements for flexibility and choice for future generations into a deep geological disposal concept that provides a much safer option than extended surface storage.

The Grimsel Test Site, which has been in operation since 1984, is known in the radioactive waste management community for the scope and quality of the experiments and projects performed over the last two decades. This paper will overview Nagra's commitments and the current plans for future activities in association with Grimsel, which are being developed considering time scales of the order of a decade or more.

The joint Nagra/PNC Radionuclide Migration Programme has been running for over ten years in Nagra‘s Grimsel Test Site in the central Swiss Alps. The programme is specifically aimed at the further development of conceptual models of radionuclide transport in the geosphere, rigorously testing the applicability of current transport codes to quantify radionuclide migration in situ and assessing how successfully laboratory sorption data (specifically, Kd values) may be extrapolated to in situ conditions to predict radionuclide retardation in the geosphere [1]. A large series of field tracer migration experiments was carried out in a hydrologically well characterised water-bearing, complex fracture (or shear zone), increasing in complexity from simple, nonsorbing fluoresceine (a fluorescent dye), 3H, 3,4He, 82Br and 123I through weakly sorbing 22.24Na,85Sr and 86Rb to a final, long-term experiment with moderately sorbing 134,137Cs. The radionuclides were injected into a dipole flow field where the flowpath length, dipole width or shape and groundwater flow velocity were all varied. After a considerable learning period, generally good fits could be obtained between transport code predictions and subsequent field tracer breakthrough curves, suggesting that the transport codes tested were a reasonable representation of in situ conditions.

In parallel to studies of disposal of vitrified high-level waste from reprocessing, projects have been initiated to examine options for direct disposal of spent fuel in Switzerland. The basic concept involves in-tunnel emplacement of encapsulated spent fuel in a deep repository which is backfilled with compacted bentonite. Two possible host rocks are considered - crystalline basement and Opalinus Clay. This paper reports the results of a thermal analysis which was carried out to evaluate constraints on repository layout set by the desire to limit temperatures experienced by the bentonite backfill.

The Project Gewähr 1985 (PG'85) safety analysis considered the practicality of HLW disposal in the crystalline bedrock of Northern Switzerland. More recently, an alternative disposal option in the argillaceous sediments overlying the crystalline basement has been examined. This paper examines some aspects of the performance of the near-field engineered barriers in an argillaceous environment, with specific comparison to the PG'85 crystalline case. Two phenomena are considered in particular detail: oxidation of the host rock during the operational phase and the related failure mode of the steel tunnel liner.

The Osamu Utsumi uranium mine, Poços de Caldas, Brazil is a roll front uranium deposit currently under investigation in an international natural analogue project involving Brazil, Sweden, Switzerland, the United Kingdom and the United States. This is a multi-disciplinary study (see the paper at this meeting by Smellie et al (1)), one component of which is an examination of the movement of natural series radionuclides across redox fronts. The processes at such fronts are analogous to those which could occur at radiolytic redox fronts found in the near- or far-field of deep repositories for HLW or spent fuel. Microbiological investigations have been carried out to determine the presence and activity of microbes known to be important in the uranium biogeochemical cycle; to define their importance in the movement of uranium across the redox front and to assess the significance of these observations to relevant safety assessment issues. This paper will summarise the results of these findings.

This paper describes the programme for an in-situ radionuclide migration experiment in fractured crystalline rock and presents the first results of site characterisation work. This study is particularly notable for its extensive hydrologic/geochemical support programme and the use of radiotracers identified as important in safety assessment studies.

Microbial effects are one of the possible perturbations to the expect-ed performance of a deep geological repository which must be examined as part of a comprehensive safety analysis. Recent literature surveys (eg [1, 2] ) and reconnaisance sampling studies [3, 4] have concluded that: a) microbial contamination of a repository is inevitable b) even for high level nuclear waste, conditions in the near-field are insufficiently extreme to ensure complete sterilisation.

The use of sediment laid down in a Scottish loch during the Flandrian marine transgression as a source term for a “natural analogue” study of elemental migration in clays over a timescale of 103 − 104 y has been investigated. Depth profiles of over 20 elements have been measured which illustrate varying post depositional behaviour ranging from effectively total immobility to complete redistribution throughout the sediment profile. These results are contrasted with laboratory batch sorption data for a range of nuclides on samples of this sediment.

The geomicrobiology of high-level nuclear waste disposal is a new field in the area of nuclear waste management. Until recently the likelihood of microbial contamination in a deep repository had not been considered, but possible biogeochemical effects of repository materials, radionuclide transport and groundwater chemistry must now be realistically assessed.

This work was funded by the UK Department of the Environment and the Commission of the European Communities.

Experimental and theoretical data are used to compare the effect of three possible leach mechanisms for borosilicate glass waste buried in a granite host-rock on the release and subsequent migration of 135Cs. Protracted release episodes and variations of up to an order of magnitude in groundwater transport times and five orders in output concentrations are possible.

In several countries low permeability crystalline rocks (e.g. granites) are under consideration as potential hosts for radioactive waste repositories. In such formations groundwater flow occurs predominantly in specific fractures rather than being a general porous flow through the entire rock matrix. By considering fractures to be simple parallel plates various authors have demonstrated the potential importance of diffusion into dead-end pores and the rock matrix itself (‘matrix diffusion’) as a mechanism for the retention of migrating radionuclides. Complementing these theoretical studies, several insitu migration experiments are planned in single fissures in crysalline rocks in Sweden, the U.K. and the U.S.A. The ‘parallel plate’ approximation to a single fissure is, however, acknowledged to be a gross simplification of any real case where “flowing” fractures are expected to be either filled or coated with secondary minerals, formed by hydrothermal alteration of fracture surfaces In the evaluation of net radio-nuclide retardation, therefore, the effect of sorption onto such secondary minerals must be carefully considered.

Much effort is currently centred on the construction of elaborate computer programs to model the release to the biosphere of radionuclides from proposed high level radioactive waste repositories in deep geological formations. Although the lack or poor quality of background data is often emphasised, it is generally considered that by examining a wide range of possible values of the important parameters involved, likely processes are assessed or their omission and possible relevence is acknowledged.