The Use of Natural Uranium Decay Series Studies in Support of Understanding Redox Conditions at Potential Radioactive Wa
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The Use of Natural Uranium Decay Series Studies in Support of Understanding Redox Conditions at Potential Radioactive Waste Disposal Sites Eva-Lena Tullborg1, John A.T. Smellie2 and Angus B. MacKenzie3. 1 Terralogica AB, Gråbo Sweden, 2 Conterra AB, Stockholm, Sweden 3 SURRC, East Kilbride, UK ABSTRACT Uranium is a redox-sensitive element which occurs naturally in the bedrock and groundwaters in Fennoscandia, and can be used, in conjunction with its decay series descendants, to indicate groundwater redox conditions. Not only can contemporary conditions be characterised, but evidence of past changes (e.g. during the last glaciation some 10 ka ago) can be preserved in minerals which coat the fracture walls along groundwater pathways. Uranium decay series data for groundwaters and solid fracture phases from the site of the Äspö Hard Rock Laboratory and its near vicinity are presented and discussed in the context of redox conditions. INTRODUCTION One of the most important criteria to ensure the long-term stability of a deep geological radioactive waste disposal system is maintenance of reducing hydrochemical conditions over the lifespan of the repository and the behaviour of redox sensitive elements (e.g. Fe, U, Ce) under present and past bedrock conditions is of great interest in this context. Of particular relevance to Fennoscandian radioactive waste disposal programmes is the potential for deep penetration of oxidising, sub-glacial water during periods of glaciation. In this context, a study has been carried out at the Äspö Hard Rock Laboratory (HRL) that has attempted to provide evidence of largescale redox behaviour under both present and past (up to 1Ma) hydrochemical conditions. Natural decay series analyses were performed on groundwater samples and solid fracture mineral phases from different depths and hydraulic environments in the bedrock of the Äspö area. Care was taken to differentiate data representing: a) groundwater conditions prior to the HRL construction, and b) groundwater conditions during and subsequent to HRL construction. SAMPLES STUDIED Groundwater samples were collected at different depths and times from isolated sections of boreholes drilled in and around the Äspö site both prior (9 boreholes) and subsequent (19 boreholes) to the HRL excavations. Solid samples comprised: a) different filling materials from hydraulically active fractures (present studies), and b) an exposed bedrock surface sample [1]. Natural decay series analyses of the groundwaters and fracture filling phases were performed by ion exchange separations followed by α-spectrometry. The solid samples consisted either of gouge (in which case the 800 m. Because the groundwater samples are fracture-specific and represent a wide range of physical, hydraulic and hydrochemical properties, depth trends in Figure 1 should be treated with caution.
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