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.

Rapid thermal annealing (RTA) has been shown to be a useful technique for improving the epitaxial quality of CaF2 films grown on Si(100) by molecular beam epitaxy. We have modified the annealing procedure to prevent crack formation, while still maintaining high film quality. Our work also shows RTA to be useful for improving the crystalline quality, chemical stability, and electrical characteristics of CaF2 films grown on Si(111). After RTA, the crystallinity of CaF2 films on Si(111), as measured by electron channeling, is indistinguishable from bulk single crystal CaF2, and the interface state density is reduced by nearly two orders of magnitude from that found in as-grown films.

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.

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.

We show that post anneals of short duration at high temperature can markedly improve the quality of CaF2 films grown by molecular beam epitaxy (MBE) on Si (100). Anneals at 1100°C for 20 sec in an Ar ambient improved χmin, the ratio of backscattered 1.8 MeV He4 ions in the aligned to random direction, from as-grown values of .07 to .26, to post post-anneal values of .03 to .045. This is the best χmin yet reported for the CaF2:Si system. The post-annealed films also show improved resistance to chemical etching and mechanical stress, and increased dielectric breakdown voltages.

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.

The Maqarin site, Jordan is being studied as a natural analogue of a cementitious radioactive waste repository. The microbiology has been studied and diverse microbial populations capable of tolerating alkaline pH were detected at all sampling localities. Dissolved organic carbon was identified as the potentially most important reductant with sulphate identified as the main oxidant, both supplying energy for microbial life. Calculations on upper limits of microbial numbers were made with a microbiology code (MGSE) using existing information but the results are overestimates when compared with field observations. This indicates that the model is very conservative and that more information on, for example, carbon sources is required.

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.

The molecular signature of dissolved organic matter (DOM) in groundwaters can be used as a tool when investigating the palaeohydrological response of groundwater systems in relation to changes in recharge environment, and also for examining groundwater compartmentalisation, mixing and transport at underground repositories for radioactive waste. The DOM in groundwaters from two compartmentalised bodies of groundwater of distinctly different origin within the Äspö Underground Research Laboratory (URL), Sweden and in Baltic seawater has been isolated using tangential flow ultrafiltration (TUF) and diafiltration. Recoveries of DOM ranged from 34.7 to 0.1 mg/L with substantial differences in the concentrations of the groundwaters collected only 120 m apart. Analysis by infrared spectroscopy (IR) and pyrolysisgas chromatography-mass spectrometry (Py-GC-MS) of the isolated DOM revealed that the groundwaters contained abundant alkylphenols which may represent heavily decomposed proteins or lignins originating from biopolymers contained within soils. The difference in the distribution and relative abundance of major pyrolysis products groups such as alkyphenols confirmed that the groundwater and Baltic seawater DOM samples were chemically distinct indicating minimal infiltration of marine groundwater derived by recharge from the Baltic or earlier Littorina Sea within the two compartmentalised groundwater bodies.

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.