Ian G. McKinley

Development and testing of radionuclide transport models for fractured rock: examples from the Nagra/JNC Radionuclide Migration Programme in the Grimsel Test Site, Switzerland

The joint Swiss National Co-operative for the Disposal of Radioactive Waste (Nagra)/Japan Nuclear Cycle Development Institute (JNC) Radionuclide Migration Programme has now been on-going for over a decade in Nagras Grimsel Test Site (GTS). The main aim of the programme has been the direct testing of radionuclide transport models in as realistic manner as possible. Although it will never be possible to fully test these models due to the large time and distance scales involved, tests of the model assumptions in scaled down but otherwise realistic conditions will contribute to developing confidence in the predictive power of the models. In this paper, the Nagra/JNC approach is highlighted with examples from a large programme of field, laboratory and natural analogue studies based around the GTS. The successes and failures are discussed as in the general approach to the thorough testing of predictive transport codes which will be used in repository performance assessment (PA). Some of the work is still on-going and this represents the first presentation of a unique set of results and conclusions.

Verfication of matrix diffusion by means of natural decay series disequilibria in a profile across a water-conducting fracture in granitic rock

Data from a rock drillcore, taken perpendicular to a water-bearing fracture in crystalline rock, clearly indicate water-rock interactions in and around the fracture. Although there is evidence of microfracturing at some distance from the main fracture, and therefore potential advective flow across the entire shear zone, simple calculations indicate that transport of Ra-226 in the vicinity of the main fracture can be described simply by matrix diffusion. This is a useful simplification for safety assessment studies and the consequences to the estimated radionuclide retardation in the vicinity of the fracture are discussed with respect to the probably over-conservative assumptions made in Nagras safety assessment model base case.

The Geomicrobiology of Nuclear Waste Disposal

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.

A review of research carried out in European rock laboratories

Research and development projects to support geological disposal options for nuclear waste have been carried out in underground research laboratories (URLs) for more than three decades and an extensive literature exists on the work carried out in these facilities. In this paper we focus on classification of URLs in terms of the particular orientation of their work programs. In addition, the evolution of URL work programs with time is overviewed with the particular aim of identifying trends in the types of experimental studies which are included.

A Natural Analogue Study of Radionuclide Migration in Clays

The use of sediment laid down in a Scottish loch during the Flandrain marine transgression as a source term for a natural analogue study of elemental migration in clays over a timescale of 10/sup 3/ - 10/sup 4/ y has been investigated. Depth profiles of over 20 elements have been measured which illustrate varying post depositional behavior 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. 5 references, 1 figure, 1 table.

Laboratory and modelling studies of microbial activity in the near field of a HLW repository

Microbial effects are one of the possible perturbations to the expected performance of a deep geological repository which must be examined as part of a comprehensive safety analysis. Recent literature surveys (eg (1, 2)) and reconnaissance 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 sterilization. Work is now focussed on examining the constraints on microbial populations and their influence on repository performance. In this paper, laboratory studies are described which examine the tolerance to extreme conditions of important groups of micro-organisms which have been identified in relevant geological formations. This work is put into context by using simple models of near field processes. These allow the extent of possible microbial processes to be compared with purely inorganic effects.

Radionuclide Retardation During Transport Through Fractured Granite

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.